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REVIEW & INTERPRETATION

Spontaneous Hybridization between Bread Wheat (Triticum aestivum L.) and Its Wild Relatives in Europe

^a Instituto de Ecología, Universidad Nacional Autónoma de México (UNAM), A.P. 70-725, 04510 México D.F., México
^b ENSA, UMR-DGPC, 2 place Viala, 34060 Montpellier, France

^* Corresponding author (zaharieva{at}ecologia.unam.mx)

	ABSTRACT

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 
In Europe, wild wheat relatives of the Triticum–Aegilops complex grow in sympatry with cultivated bread wheat (Triticum aestivum L.) and spontaneous hybridization is known to occur. With the development of transgenic wheat, an understanding of the likelihood and occurrence of hybridization and introgression between wheat and its relatives is needed for use in risk assessment. To assess the probability of wheat to wild relative gene introgression, the distribution and biology of wheat wild relatives and their genome affinity and crossability with bread wheat were reviewed. Twelve of the 22 known Aegilops species, as well as one wild Triticum species, T. monococcum L. subsp. aegilopoides (Link) Thell., are known to occur in Europe near or within wheat cultivation. Five tetraploid species, Ae. cylindrica Host., Ae. triuncialis L., Ae. geniculata Roth., Ae. neglecta Req. ex Berthol., and Ae. biuncialis Vis., have wide distribution in most European countries. Bread wheat, wild Aegilops species, and Triticum species are predominantly autogamous (except Ae. speltoides Tausch, typically allogamous), but outcrossing among species is possible depending on species sympatry, concurrent flowering, and sexual compatibility. Spontaneous hybridization with wheat was reported for most of the tetraploid Aegilops species. The probability of gene transfer and gene retention in hybrid progenies is, however, higher when a gene is located on a shared genome, particularly on the D genome shared with Ae. cylindrica and Ae. ventricosa Tausch. Case-by-case and region-by-region assessments are needed to evaluate the risk associated with production and competitiveness of hybrids and their progeny.

Abbreviations: GISH, genomic in situ hybridization • GM, genetically modified • RFLP, restriction fragment length polymorphism • SSR, simple sequence repeat

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 
HYBRIDIZATION and introgression are natural biological processes that can occur among closely related species. For instance, it has long been recognized that cultivated plants naturally hybridize with their wild relatives (de Candolle, 1882). The major crops of wheat, rice (Oryza sativa L.), maize (Zea mays L.), soybean [Glycine max (L.) Merr.], cotton (Gossypium hirsutum L.), sorghum [Sorghum bicolor (L.) Moench], bean (Phaseolus vulgaris L.), sunflower (Helianthus annuus L.), and sugarcane (Saccharum officinarum L.) all have wild relatives to which they can cross (Ellstrand et al., 1999). Wild or weedy relatives may be able to acquire genes from commercial cultivars when they co-occur, have overlapping flowering periods, or share genomes and when no strong reproductive barrier exists that could prevent hybridization and introgression (Ellstrand et al., 1999). Until recently, there has been little interest in gene transfer from crops to their wild relatives. However, with the advent of genetically modified (GM) crops there has been renewed interest in understanding the implications of gene transfer from a cultivated crop to a wild relative.

Wheat is the most widely grown crop in the world and GM cultivars of wheat are in the process of development (Waines and Hegde, 2003). Wheat is cultivated on approximately 23 million ha in Europe (FAO, 2004). Natural hybridization between wheat and some of its wild relatives is known to occur in Europe, in some cases resulting in viable seeds (van Slageren, 1994). Gene transfer requires sympatry of the cultivated and wild species, synchronous pollen emission by the donor and stigma receptivity of the recipient species, and viability of the cross progenies.

The objective of this review is to summarize the scientific literature regarding the hybridization between bread wheat and related species in Europe as a first step to assess the likelihood of introgression of a gene from GM cultivated wheat into its wild relatives. This paper examines (i) the distribution, biology, and flowering phenology of wild wheat relatives (particularly those of genus Aegilops) in Europe, (ii) the genome affinity and crossability of bread wheat and its wild relatives, and (iii) the probability of spontaneous hybridization and gene transfer between wheat and its relatives.

	Taxonomy of Wheat Wild Relatives Present in Europe

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 
Wild relatives of cultivated wheat in Europe that may hybridize with wheat belong to the Triticum and Aegilops genera, family Poaceae Barnhart, subfamily Pooideae, tribe Triticeae Dumort, subtribe Triticinae Griseb (van Slageren, 1994). The Aegilops species have been described and classified by Linnaeus (1753), Jaubert and Spach (1851), Boissier (1884), Zhukovsky (1928), and Eig (1929). Taxonomic approaches to classification based on cytogenetics were developed by Kihara (1940, 1954) and Sears (1948). The latest revision of the classification of the Aegilops genus was made by van Slageren (1994) and represents a synthesis of the nomenclature and the development of clear keys to identify the species. The classification developed by van Slageren will be used in this paper. Currently, there is no consensus on how to classify Triticum sensu stricto, therefore the classification elaborated by van Slageren (1994) will be also used.

Twelve of the twenty-two Aegilops species are present in Europe (van Slageren, 1994). These include Ae. biuncialis, Ae. caudata L., Ae. columnaris Zhuk., Ae. comosa Sibth. and Sm., Ae. cylindrica, Ae. geniculata, Ae. neglecta, Ae. speltoides, Ae. triuncialis, Ae. umbellulata Zhuk., Ae. uniaristata Vis., and Ae. ventricosa (Table 1). Three Aegilops species, Ae. peregrina (Hack.) Maire and Weiller, Ae. bicornis (Forssk.) Jaub and Sp., and Ae. kotschyi Boiss., are present in Cyprus but not in continental Europe and will not be considered. One wild Triticum, T. monococcum subsp. aegilopoides is also considered in this review. The scientific names, synonyms, and common names of the species, according to van Slageren (1994), are presented in Table 1.

	Biology of Wheat-Related Species

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

View larger version (195K): [in this window] [in a new window]   Fig. 1. Tuft of Aegilops geniculata (Montpellier, South of France).

View larger version (124K): [in this window] [in a new window]   Fig. 2. Aegilops plant morphology (e.g., Aegilops geniculata, Bulgaria).

View larger version (121K): [in this window] [in a new window]   Fig. 3. Aegilops spike morphology (e.g., Aegilops geniculata, France).

View larger version (209K): [in this window] [in a new window]   Fig. 4. Triticum monococcum subsp. aegilopoides in a grassland in South of Bulgaria.

	Distribution, Ecology, and Sympatry with Bread Wheat in Europe

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 
The Aegilops species occurring in Europe belong to the Mediterranean group of Triticeae (Sankary, 1990) and are mainly distributed in countries characterized by hot and dry summers and winter rainfall. These species grow in Europe at altitudes up to 1200 m (van Slageren, 1994). An understanding of the distribution of the different wheat relatives in a given country is useful in assessing the potential for hybridization and subsequent introgression. The ecogeographical distribution of the Aegilops species has been described in detail by Sankary (1990) and van Slageren (1994), and of T. monococcum subsp. aegilopoides by Perrino et al. (1996). Triticum monococcum subsp. aegilopoides is present and abundant in southeastern Europe. The presence and abundance of the Aegilops species in Europe are summarized in Table 3.

View this table: [in this window] [in a new window]   Table 3. Distribution and abundance by country of wild Triticum and Aegilops species in Europe (van Slageren, 1994; Perrino et al., 1996). (Data only refer to countries where at least one Aegilops species is native. Austria, Belgium, Czech Republic, Germany, Netherlands, Slovakia, and Sweden where Ae. cylindica is adventive are not included).

The tetraploid species are more widely distributed than the diploids. In Europe, Ae. cylindrica is present in Bulgaria, Hungary, Macedonia, Romania, Serbia-Montenegro, Croatia, Greece, and Slovenia. It has been introduced in Italy, France, Germany, and Switzerland and is adventive in many other European countries such as Austria, Belgium, Czech Republic, Netherlands, Spain, and Sweden (van Slageren, 1994). Aegilops ventricosa occurs in the Iberian peninsula, southern France, Croatia (van Slageren, 1994), and Italy (Larghetti et al., 1992). Aegilops geniculata, Ae. triuncialis, Ae. biuncialis, and Ae. neglecta occur throughout southern Europe. Aegilops neglecta is frequent in southern France, while Ae. biuncialis is particularly frequent in south and southeastern Europe. Aegilops ventricosa, Ae. triuncialis, and Ae. neglecta are adventive in Belgium, Germany, and Switzerland (van Slageren, 1994). Aegilops columnaris is present only in Greece (van Slageren, 1994). The countries where Aegilops species naturally occur have a Mediterranean climate. Among those countries, southeastern Europe (Balkan region) appears richer in Aegilops species compared to the western Mediterranean countries. Greece and Bulgaria have the greatest number of native Aegilops species in Europe (11 and 9 species, respectively) (Zaharieva, 1993; van Slageren, 1994). Some tetraploid species, such as Ae. geniculata, Ae. triuncialis, Ae. biuncialis, Ae. neglecta, and Ae. cylindrica, are native or adventive in a great number of European countries (Table 3).

The abundance of Aegilops species considerably varies among countries. Tetraploid species are more abundant than diploid species. Aegilops triuncialis, Ae. geniculata, Ae. neglecta, and Ae. biuncialis are the most abundant in the South of Europe, with Ae. geniculata being abundant mainly in islands and coastal regions (Sankary, 1990). Information on the abundance of Aegilops species in Slovenia, Croatia, Bosnia, Macedonia, Romania, and Serbia-Montenegro is insufficient. Van Slageren (1994) suggests a widespread distribution of Aegilops species in these countries. In Greece, the diploid species Ae. caudata, Ae. comosa, Ae. uniaristata, and Ae. umbellulata are abundant, while these are scarce or absent in most other European countries.

Most Aegilops species grow in dry and disturbed places such as field edges, roadsides, and sometimes in grassland vegetation, but Ae. caudata, Ae. umbellulata, Ae. ventricosa, and Ae. biuncialis are also found in field margins, and Ae. comosa, Ae. speltoides, Ae. cylindrica, Ae. geniculata, Ae. triuncialis, and Ae. neglecta grow near or within cultivated areas (van Slageren, 1994). Triticum monococcum subsp. aegilopoides occurs along roadsides and on the borders of cultivated fields.

All Aegilops species are colonizers, able to rapidly invade new territories (van Slageren, 1994). Aegilops biuncialis, Ae. cylindrica, Ae. geniculata, and Ae. triuncialis are the best colonizers and develop large stands (van Slageren, 1994). Although invasive, Aegilops species are not considered as weeds in Europe (Hanf, 1982). However, some Aegilops species (Ae. cylindrica, Ae. triuncialis, and Ae. geniculata) introduced in the USA at the end of the 19th century (Johnston and Parker, 1929) are now troublesome weeds (Barkley, 1986; Donald and Ogg, 1991; Hitchkock, 1951).

The reported distribution and ecology information indicates that wheat and some of its wild relative species grow in proximity in Europe. Wheat is cultivated in all European countries where Aegilops species are present and occupies an average of 11% of the total cultivated area (FAO, 2004). Where wheat and a wild relative co-occur there is the possibility for outcrossing if the other requirements are met (e.g., overlapping flowering periods and no genetic barriers).

	Synchronous Flowering with Bread Wheat

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 
Sympatry is not sufficient for two species to cross—the species must also have similar phenological characteristics and overlapping flowering periods. All Aegilops and Triticum species are annual, probably as a result of their adaptation to the seasonal rainfall of the Mediterranean region (Calder, 1966). As wheat, Aegilops species were divided by Tanaka (1954) into three groups, that is, winter, spring, and intermediate types, according to their needs in vernalization, defined by Chouard (1960) as the acquisition or acceleration of the ability to flower by chilling treatment. Most European Aegilops species, as well as T. monococcum subsp. aegilopoides, have a winter growth habit, thus requiring vernalization for flower induction. Aegilops columnaris and Ae. geniculata are characterized as having intermediate (or facultative) growth habit. In Ae. geniculata, normal flowering was observed in greenhouse planting or spring sowing, that is, without vernalization. However, in those conditions plants produced less productive tillers and seeds (Zaharieva, personal observation, 2002).

The flowering time of Aegilops and T. monococcum subsp. aegilopoides in Europe is from April–May until June–July, depending on the species and their ecogeographical location (van Slageren, 1994) (Table 2), while cultivated wheat flowers between the beginning of May and the end of June (Azzi, 1954). Aegilops and bread wheat spikes require from 3 to 5 d to complete flowering (Peterson, 1965; Boguslavski, 1978a), but the duration of the whole plant flowering is longer in Aegilops than in wheat, due to asynchroneous tillering (van Slageren, 1994). This trait enables wild species to escape environmental stress during flowering and ensure seed production. The continuous flowering period of Aegilops species extends the time that cross-pollination and hybrid production with bread wheat could occur.

	Pollination Mode, Glume Opening, and Pollen Movement

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 
The success of gene flow depends on pollination mode (Hamrick et al., 1979; Govindaraju, 1988), flower structure, and pollen dispersal (Waines and Hegde, 2003). Bread wheat as well as T. monococcum subsp. aegilopoides and most Aegilops species are considered to be autogamous (self fertilized) (Peterson, 1965; Hammer, 1980). Pollen grains mature before the floret opens. Most of the pollen falling within the flower becomes lodged on the stigma. However, D'Souza (1970) and Beri and Anand (1971) reported that 30 to 80% of the pollen can be shed outside the flower where it may be carried by wind. Occasionally, some pollen grains from one floret can reach the stigma of a different floret (or spike). Although not well documented, the rate of out-crossing of autogamous Aegilops species should be similar to wheat (Boguslavski, 1979). In wheat, outcrossing rates of up to 10% (depending of cultivar and year) were observed on some varieties (Enjalbert et al., 1998). However, outcrossing rates of 0 to 2% are commonly accepted. Among the Aegilops species present in Europe, Ae. speltoides is considered by Hammer (1980) as typically allogamous (cross fertilized) and Ae. caudata as facultative allogamous. In Ae. speltoides, stamen growth begins at blooming and the anthers disperse most pollen out of the floret. When the floret closes, from 200 to 500 pollen grains can be found on the stigma of selfing species, and very few or no pollen grains in cross-pollinating species (Boguslavski, 1979). Few or no seed were produced on Ae. caudata and Ae. speltoides when the spikes were isolated from receiving pollen from other plants.

Cross-fertilization and potential gene transfer depend, in part, on the degree and duration of glumes opening at anthesis, anther size, and pollen production and viability (Hucl, 1996; Waines and Hegde, 2003). In Aegilops species, glumes, lemmas, and paleas open at flowering (Boguslavski, 1978a) (Fig. 5 ). Florets open in the morning, between 0600 and 1000–1100 h. In cultivated wheat, floret opening reaches its peak between 0900 and 1100–1200 h (Virmani and Edwards, 1983) and the duration that a floret remains open ranges from 8 to 60 min (de Vries, 1971).

View larger version (244K): [in this window] [in a new window]   Fig. 5. Florets opening in (A) Aegilops geniculata and (B) Ae. speltoides.

There are few published studies on pollen dispersal in Triticeae and most are on cultivated wheat. Pollen load in the air at a given time is a function of anther extrusion and the amount of pollen produced by an anther (Joppa et al., 1968). Pollen-mediated gene flow in wheat has been reviewed elsewhere (Waines and Hegde, 2003). Pollen viability was observed to range between 15 and 30 min depending on environmental conditions (de Vries, 1971), a duration long enough to allow cross-fertilization with a nearby wild relative (Hegde and Waines, 2004). Studies indicate that wheat pollen can move up to 60 m (Khan et al., 1973). However, Hucl and Matus-Cádiz (2001) reported that gene flow rate decreased with greater distance from the pollen source and highly depended on the wheat genotype and the wind direction. There is no published information on pollen transport in Aegilops species.

	Genome Constitution of Wild Wheat Relatives and Bread Wheat

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 
Aegilops and Triticum belong to a complex of wild and domesticated species whose allopolyploid members evolved via hybrid speciation (Kimber and Sears, 1987). Kihara (1919) and Sax (1922) showed that in the genus Triticum there are three different genomes, each composed of seven chromosomes. Kihara (1937) and Sears (1941) used karyological studies on different Aegilops species to demonstrate that their base chromosome number (n = 7) was the same as in wheat. They also demonstrated that the Aegilops genus can be divided, as the Triticum genus, into three ploidy levels (i.e., diploid 2n = 2x = 14, tetraploid 2n = 4x = 28 and hexaploid 2n = 6x = 42). The Aegilops species present in Europe are primarily diploid or tetraploid, with the exception of Ae. neglecta which also exists in a hexaploid form. The ploidy level and genome formulae of the selected species according to Waines and Barnhart (1992) are presented in Table 1.

Bread wheat, T. aestivum (2x = 6x = 42, BBAADD) is an allohexaploid with B, A, and D genomes. These genomes are chromosome sets (seven chromosome pairs for each genome) originated from different species and combined together during the evolution of wheat (Feldman et al., 1995). Two out of the three wheat genomes (B and D) came from diploid Aegilops species, while the A genome originated from a diploid Triticum ancestor, T. urartu Tum. ex Gandil. (MacFadden and Sears, 1946; Dvorak, 1988). The wheat D genome originated from Ae. tauschii Coss. (Pathak, 1940). This has been confirmed by the synthesis of hexaploid wheats from crosses between this species and tetraploid wheats (Kihara, 1944; MacFadden and Sears, 1946), by cytological analysis (Gill and Kimber, 1974), and by RFLP (restriction fragment length polymorphism) analysis (Rayburn and Gill, 1987). Conversely, the origin of the B genome is still very controversial as different diploid Aegilops species of the Sitopsis section (S genome species) have been proposed as potential donors, including Ae. bicornis (Sears, 1956), Ae. sharonensis (Kushnir and Halloran, 1981), Ae. longissima (Tsunewaki and Ogihara, 1983), Ae. searsii (Kerby and Kuspira, 1988), and Ae. speltoides (Chen and Gill, 1983; Bahrman et al., 1988; Dvorak and Zhang, 1990; Fernández-Calvín and Orellana, 1990).

Wheat possess 21 pairs of homologous chromosomes. The term homologous refers to a pair of chromosomes within a genome that contain identical linear order of gene sites (or loci) along their lengths and show synapsis during pachynema (Singh, 2003). These homologous chromosomes are classified into seven homeologous groups, each group containing one pair of chromosomes from the B, A, and D genomes (Sears, 1954). Homeologous group 1, for example, contains the pairs 1B, 1A, and 1D. The 21 wheat chromosomes can be readily identified by heterochromatic banding (Gill et al., 1991) or in situ hybridization patterns using repetitive DNA probes (Pederson and Langridge, 1997).

Since the different diploid species of the Triticinae subtribe have been derived from a common ancestor, their chromosomes still retain some degree of similarity, as shown by comparative mapping (Devos and Gale, 2000). A closer examination of synteny among hexaploid wheat homeologous chromosomes by southern hybridization of expressed sequence tags with deletion stocks, however, revealed numerous synteny perturbations with translocations, deletions, inversions, and duplications (Tikhonov et al., 1999; Keller and Feuillet, 2000). Most of these rearrangements originated during the evolution of the diploid ancestors of hexaploid wheat (Akhunov et al., 2003).

The five European Aegilops diploid species Ae. caudata, Ae. comosa, Ae. speltoides, Ae. umbellulata, and Ae. uniaristata have the C, M, S, U, and N genomes, respectively (Waines and Barnhart, 1992). The tetraploid species Ae. cylindrica, Ae. ventricosa, Ae. geniculata, and Ae. triuncialis have the CD, DN, MU, and UC genomes, respectively, while Ae. biuncialis, Ae. columnaris, and Ae. neglecta have the UM genome formula. The hexaploid form of Ae. neglecta has UMN genome composition. The polyploid species of Triticum and Aegilops constitute a classic example of evolution through amphiploidy. The diploid species are at the origin of the tetraploid species, which consequently have common genomes with their diploid ancestors (Table 1). For example, it has been postulated by Kimber et al. (1988) that Ae. geniculata (MU genome) is an amphiploid between Ae. comosa (M genome) and Ae. umbellulata (U genome). According to their genome Kimber (1988) distinguished three clusters among the polyploid Aegilops and Triticum species. In each cluster polyploid species have in common a genome called "pivotal" (A, U, or D) which has changed little throughout the allopolyploidisation and speciation process (Zohary and Feldman, 1962), while their second (or third) genomes (M, N, C, and S) have been modified. The polyploids in group A, for example, share the genome of the common diploid ancestor T. urartu, those in group D share the genome of Ae. tauschii, and those in group U share the genome of Ae. umbellulata. The presence of the pivotal genome may facilitate introgression among the species belonging to a given cluster. This genetic structure (one common genome and one or two different ones) may explain the comparatively high rate of successful spontaneous hybridization (and hence gene flow) between the polyploids (Kimber and Yen, 1988). Hybridization is facilitated by the shared genome, which acts as a buffer, ensuring some fertility in the resulting hybrids. The different genomes, which are brought together from different parents, can exchange genetic material and form a new mixed genome.

	Barriers to Hybridization Between Wheat and Its Wild Relatives

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 
Hybridization between wheat and its wild relatives, with the goal of introgression of useful traits into the cultivated species, has been the subject of considerable work (Sharma and Gill, 1983; Mujeeb-Kazi and Kimber, 1985; Mujeeb-Kazi, 1993, 1995). The three most important obstacles to wide hybridization are (i) incompatibility between parent species, (ii) nonviability of F1 hybrids, and (iii) sterility of F1 hybrids or their progeny (Sharma and Gill, 1983; Baum et al., 1992).

Cross incompatibility preventing hybridization arises when a pollen grain does not germinate, the pollen tube does not reach the ovary, or the male gamete does not fuse with the female gamete. When wheat is used as the female parent the success of the cross highly depends on the wheat cultivar (Ye et al., 1997).

The nonviability of F1 hybrids can be caused by the lack of pairing between genomes of the parental species. Homeologous chromosomes are normally prevented from pairing by two major pairing homeologous genes, Ph1 located on the long arm of chromosome 5B (Riley and Chapman, 1958) and Ph2 (Mello-Sampayo, 1971), and some minor genes (Sears, 1976). Consequently, only homologous partners pair in tetraploid and hexaploid wheats, what ensures diploid-like pairing and disomic inheritance. When Ph gene is deleted or its action neutralized, most Triticeae chromosomes can be induced to pair with their wheat homeologues (Wang, 1990, 1995; Jauhar et al., 1999). Riley et al. (1961) reported a genetic system in Ae. speltoides that can counteract the presence of Ph gene. Aegilops speltoides chromosomes can induce or promote meiotic chromosome pairing and the consequent genetic recombination between related chromosomes from different genomes (homeologous chromosomes) in hybrids with tetraploid and hexaploid wheats.

When chromosomes do not pair in the F1 hybrid, gametes receive different numbers of chromosomes leading, in general, to sterility. In this case, hybrid sterility can be overcome by chromosome doubling or backcrossing to one of the parents (Mujeeb Kazi, 1995). The addition of an entire genome from a wild species (i.e., the production of a synthetic amphiploid) has been an attractive approach for many wheat geneticists. Tschermak and Bleier (1926) were the first to obtain an amphiploid species as a result of the spontaneous doubling of the chromosome number of a wheat hybrid produced by crossing T. turgidum dicoccoides with the wild grass Ae. geniculata. The amphiploids obtained are usually fully fertile. The production of such amphiploids, in which the adaptive hybrid combinations are stabilized by polyploidy and the segregation of parental characters is prevented, opens up many possibilities for the production of new crops. A good example of a successful synthetic amphiploid level is the manmade crop triticale (xTriticosecale Wittm.). Bread wheat arising from hybridization of tetraploid wheat and Ae. tauschii is a good example of success of natural allopolyploidy. Chromosome doubling during meiosis is believed to contribute significantly to the widespread occurrence of polyploids in nature (Harlan and de Wet, 1975; Veilleux, 1985; Jauhar, 1993). In a recent study David et al. (2004) reported a high frequency (estimated at 10^–6) of spontaneous amphiploidy between T. turgidum L. subsp. durum (Desf.) Husn. and Ae. geniculata in sympatric populations. By using genomic in situ hybridization, they showed that fertile amphiploids had arisen through unreduced gametes and that some of them carried wheat–Aegilops geniculata recombinant chromosomes. The authors concluded that amphiploids can provide a route for gene flow, including that of transgenes, to the wild.

During the last two decades, molecular data have provided new insights into polyploid evolution, leading to significant progress in understanding the mechanism and evolutionary aspects of polyploidy. Ozkan et al. (2001) studied the rate and time of elimination of several low-copy DNA sequences, existing at diploid level, in F1 hybrids and newly formed allopolyploids of Aegilops and Triticum. They found that rapid elimination of genome and chromosome specific sequences is a general phenomenon in newly synthesized allopolyploids. Dvorak et al. (2004) suggested a role of this phenomenon in augmenting the differentiation of homeologous chromosomes at the polyploid level, thereby providing the physical basis for the diploid-like meiotic behavior of newly formed allopolyploids. The resultant strict bivalent pairing prevents intergenomic recombination and brings about higher fertility and permanent heterosis between homoeoalleles, thus fostering the successful establishment of the newly formed allopolyploid species in nature. It was assumed that these genomic changes may have contributed to the successful establishment of newly formed allopolyploids as new species (Sasakuma et al., 1995; Feldman et al., 1997; Liu et al., 1998a, 1998b; Ozkan et al., 2001). In nature, amphiploids can also serve as an effective bridge over the interspecific and intergeneric cross breeding barriers.

Finally, another barrier to hybridization is the reduced development of embryo and/or endosperm. Hybrids with normal embryo and reduced endosperm were reported in crosses between wheat and Ae. umbellulata by Mujeeb-Kazi and Kimber (1985). The nonviability of F1 hybrids also can be caused by incompatibility between the genome(s) of one species and cytoplasm of the other. For example, reduced seed viability induced by cytoplasmic male sterility was observed by Maan (1979) in T. monococcum subsp. aegilopoides x T. aestivum hybrids.

	Crossability and Genome Affinity between Bread Wheat and Its Wild Relatives

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 
Hybrids between wheat and different Aegilops species have been produced and described (Kihara, 1937; Knobloch, 1968; Kimber and Abu Bakar, 1979; Sharma and Gill, 1983). An important database on pairing during meiosis in hybrids between wheat and its relatives was developed by Kimber and Abu Bakar (1979) and was used for assessing genome affinity and determining species relationships (Kimber et al., 1981; Mujeeb-Kazi and Kimber, 1985; Gill and Chen, 1987). Chromosome pairing is used to describe the occurrence of one or more chiasmata (points of attachment between homologous chromosomes resulting from crossing over) between two or more entire chromosomes or between two chromosome arms (Kimber et al., 1981). The lack of chromosome association during meiosis results in the formation of two univalents. Association of two pairing chromosomes results in the formation of a rod bivalents (pairing in only one arm) or ring bivalents (pairing in two arms). The evidence for the high chromosome homology is given by the formation of seven bivalents (complete chromosome pairing). Chromosome pairing can be evaluated by assessing the mean chromosomal associations per cell (Kimber et al., 1981). Based on chromosome pairing information, numerical methods of assessing genome affinity or similarity of the B, A, and D genomes of T. aestivum to related genomes of other species have been developed (Kimber and Hulse, 1978; Driscoll et al., 1979; Alonso and Kimber, 1981; Kimber and Alonso, 1981; Kimber et al., 1981).

Genomic affinity of individual chromosomes can also be determined by sequential banding and genomic in situ hybridization (Jiang and Gill, 1993, 1994b). Staining techniques were used to analyze the substructures of cereal chromosomes, and to develop a cytogenetic karyotype of wheat (Gill and Kimber, 1974; Gill et al., 1991). Nonisotopic methods of mapping DNA sequences in situ on chromosomes were used to construct a molecular karyotype of wheat (Rayburn and Gill, 1985; Jiang and Gill, 1994a). These methods have greatly facilitated cytogenetic analysis in wheat and related species, especially the analysis of alien transfers (Friebe et al., 1991, 1996). More recently, molecular markers such as RFLP (restriction fragment length polymorphism) or microsatellites (SSRs, simple sequence repeats) have been used for identification of alien chromosomes in wheat background and estimation of genome homeology (Zhang et al., 1998; Iqbal et al., 2000; Zaharieva et al., 2003b).

Data obtained from these different approaches allowed a description and better understanding of the relationships between the Aegilops and wheat genomes. The D genomes of Ae. cylindrica (CD) and Ae. ventricosa (DN) show complete homology with the D genome of T. aestivum and the D genome of the donor Ae. tauschii (Abu Bakar and Kimber, 1982; McGuire and Dvorak, 1982; Kimber and Zhao, 1983). These genomes have apparently changed little during their evolution in the polyploid species (Zhao and Kimber, 1984). A high-density genetic linkage map of the Ae. tauschii genome constructed by Boiko et al. (1999) demonstrated that most of the markers shared between the Ae. tauschii and T. aestivum physical maps were colinear. However, the discrepancy in the order of five markers on the 3D of Ae. tauschii genetic map versus the 3D T. aestivum physical map indicated a possible inversion. C-banding analysis of Ae. cylindrica genomic constitution made by Linc et al. (1999) showed that banding patterns of the D- and C-genome chromosomes of Ae. cylindrica are similar to those of D- and C-genome chromosomes of the diploid progenitor species Ae. tauschii and Ae. caudata, respectively. Genomic in situ hybridization analysis detected intergenomic translocation in three of the five Ae. cylindrica accessions. The A genome of T. monococcum subsp. aegilopoides has close homology to the A genome of T. aestivum (Kimber et al., 1981). However, the affinity between A genomes of bread wheat and other cultivated polyploid wheats (e.g., durum wheat, BA genome) is higher than between the A genome of wheat and T. monococcum subsp. aegilopoides, probably as a consequence of evolutionary divergence (Kimber et al., 1981). Hybrids between T. monococcum and its wild form subsp. aegilopoides are fully fertile, but hybrids between T. monococcum and T. urartu are fully sterile, although their chromosomes pair as ring bivalents indicating identical genomic constitution (Johnson and Dhaliwal, 1976). Those who wanted to improve T. monococcum by crosses with 4x and 6x wheats were disappointed as such transfers are impossible (Sharma and Waines, 1981). There has been some differentiation between the A genome of diploid and polyploid wheats as evidenced from the reduced level of pairing, or absence of pairing in the case of chromosome 4A (Gill and Chen, 1987) and the amount of C-heterochromatin (Friebe and Gill, 1996). The S genome of Ae. speltoides is closely related to the B genome of wheat (Friebe et al., 2000) and meiotic analysis data on hybrids between T. aestivum and Ae. speltoides have shown preferential pairing between A and D, and between B and S chromosomes (Maestra and Naranjo, 1998).

By analyzing the chromosome pairing of the diploid hybrids Ae. comosa x Ae. uniaristata (MN), Ae. umbellulata x Ae. tauschii (UD), and Ae. uniaristata x Ae. tauschii (ND), Cuñado and Santos (1999) found closer relationships between M and N, and U and D genomes than between N and D genomes (MN > UD > ND). Iqbal et al. (2000) using SSR marker analysis have demonstrated the close homeology of the N genome of Ae. uniaristata and the D genome of wheat. By comparing Ae. umbellulata and hexaploid wheat maps, Zhang et al. (1998) confirmed the homeology between Ae. umbellulata U and wheat D genome chromosomes. Based on the pairing affinities between Ae. geniculata (MU) and wheat genomes (B, A, and D), Fernández-Calvín and Orellana (1992) revealed that the wheat A and D genome chromosomes more frequently associated with M and U genome chromosomes of Ae. geniculata than did the B genome chromosomes. Similarity between U and M genome of Ae. geniculata and D genome of hexaploid wheat was also observed by Zaharieva et al. (2003b) using wheat D genome microsatellites. These experimental results suggest that the relationships between the bread wheat D genome and most of the Aegilops genomes are closer, compared to the A and B genomes.

Results of chromosome pairing (chromosomal associations and chiasmata formation per cell during meiosis) and crossability rate (seed set produced by pollinated florets, expressed as percentage) between bread wheat and different Aegilops species, are summarized in Table 4. In cross combinations, the first mentioned species (or cultivar) is the female parent and the second one the male parent. Normal cross combination refers to the case when bread wheat is used as the female parent and the reciprocal cross to the case when Aegilops or wild Triticum is used as the female parent. These data provide an estimation of the probability for the given wild species to cross with wheat. They have been obtained, however, by manual pollination techniques and sometimes using embryo rescue, leading to overestimation of crossability rate in comparison with hybridization in natural conditions. Crosses such as diploid x hexaploid or tetraploid x hexaploid reduced the fertility of the F₁ generation substantially. Hybrid seed viability was low in crosses involving the diploid Aegilops species, either used as male or female parent, while the seed viability of hybrids involving tetraploid species was more than 50% (Bochev, 1993). After backcrossing with the wheat parent, crossability rate increases (Meissner, 1991; Zaharieva et al., 2003a). Higher seed set was obtained when Aegilops species was used as the female parent. The fact that both species share a genome facilitates the production of hybrids and successful backcrossing. Chromosome pairing in these hybrids is generally high, allowing recombination and introgression events. When the two parents do not share a genome, chromosome pairing is low and crossability is likely to depend on the similarity between the specific genomes. Although wheat x Aegilops hybridization depends to a large extent on how closely related the Aegilops species is to wheat, considerable genetic variation for crossability exists within wheat cultivars and wild relative species (Sharma and Gill, 1983; Farooq et al., 1989; Bochev, 1993).

	Spontaneous Hybridization (Wild Relatives x Bread Wheat)

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 
The occurrence of hybridization under natural conditions in Europe has been documented. Ae. geniculata x bread wheat spontaneous hybrids were reported first by Requien in 1825 (van Slageren, 1994). Natural hybrids between Ae. neglecta and wheat and between Ae. triuncialis and wheat were described in South of France by Godron (1854) and in Spain by Lange (1860), respectively. Van Slageren (1994) listed natural hybrids between bread wheat and Ae. columnaris (Italy), Ae. cylindrica (Hungary), Ae. neglecta (France, Greece, Italy), Ae. triuncialis (France, Spain), and Ae. ventricosa (France). Recently, Guadagnuolo et al. (2001) reported that the mean hybridization rate between Ae. cylindrica and wheat under field conditions in Switzerland was 3%. The documented natural hybrids were limited to crosses between tetraploid Aegilops spp. and wheat and were observed between roads and adjacent wheat fields, or next to the fields where wheat had been cultivated the previous year. These Aegilops x Triticum hybrids were vigorous and usually attained a height of 50 to 90 cm. The observed tough rachis and keeled glumes were thought to come from the wheat parent. Because of the though rachis, the spike disarticulation was whole-spike type. Spike morphological traits were inherited from the Aegilops parent (Fig. 6 ).

View larger version (140K): [in this window] [in a new window]   Fig. 6. Aegilops geniculata x Triticum aestivum natural hybrid in the edge of wheat field (Montpellier, South of France).

The first Aegilops cylindrica x wheat hybrid was described in 1917 by the Hungarian botanist von Degen on the St. Andrew Danub island as a new species he called Aegilops sancti-andreae Deg. (van Slageren, 1994). This late description is quite surprising, since Ae. cylindrica x wheat is the natural hybrid most frequently encountered by van Slageren (1994) during his collecting. Plants fitting the description of Ae. sancti-andreae were later found by Rajhathy (1960) at the same location, along wheat fields in places where Aegilops cylindrica was growing close by, and were identified as Ae. cylindrica x wheat hybrids. This has been confirmed by a chromosome count of 2n = 35. Of 300 plants, six seeds were found, providing partially fertile plants (originated from spontaneous backcrosses with wheat as revealed by cytological examinations) and fertile hybrids were found in the field borders confirming continuous spontaneous crossing. Natural hybrids (0.5%) between male sterile lines of the bread wheat ‘Bezostaya 1’ and Ae. cylindrica were reported in Bulgaria by Gotsov and Panayotov (1972). Manual pollination of the same lines resulted in similar rates of hybrid plants (0.7%) as the natural pollination. A higher rate (12%) of hybrid seed production was obtained by the same authors in a reciprocal cross (i.e., with Ae. cylindrica as the mother plant). According to van Slageren (1994), the sterility of most hybrids is caused primarily by the lack of viable pollen, so that willful pollination with the Triticum parent may restore fertility and lead to further backcross generations that increasingly resemble wheat.

Boguslavski (1978b) conducted a 6-yr study on spontaneous hybridization between species of the Aegilops and Triticum genus at the Vavilov Institute experimental station in Daghestan. Twenty-two Aegilops species accessions were cultivated together with different wheat cultivars. Aegilops x wheat hybrids were mainly observed with tetraploid Aegilops species (Ae. cylindrica, Ae. biuncialis, Ae. neglecta, and Ae. triuncialis). However, a few hybrids were found with the diploid species Ae. speltoides. The number of hybrids obtained was highly year and species dependant, and hybrid fertility ranged from 0.2 to 6%. The fertility of the progeny of these hybrids ranged from 6 to 48%.

More recently, viable seeds on Ae. cylindrica x bread wheat hybrid plants grown under field conditions in USA were reported (Mallory-Smith et al., 1996; Seefeldt et al., 1998; Snyder et al., 2000). Zemetra et al. (1998) and Wang et al. (2001) reported that experimental wheat x jointed goatgrass hybrids were male sterile but had about 1 to 2% female fertility. Snyder et al. (2000) observed that the mean hybrid fertility ranged from 2 to 4% and that viable seeds can be produced on hybrids with either jointed goatgrass or wheat as a pollen donor. What determines the preponderant pollen donor under field conditions is, however, still unclear and further assessments are needed to address this issue. A low rate of backcrosses to jointed goatgrass has been reported by Morrison et al. (2002). Zemetra et al. (1998) also found a low fertility in backcrossed plants (2%). However, the fertility rate increased with each successive backcross, both with Ae. cylindrica (15.1% for the BC1, 37.4% for the BC2) and bread wheat (4.8% for the BC1, 13.7% for the BC2, 55.8% for the BC3). Wang et al. (2001) showed that, on average, a BC1 plant had 2% male fertility and 4% female fertility. These findings suggest that BC1 and/or BC2 plants could serve as either the male or female parent in subsequent crosses. When jointed goatgrass was the recurrent parent, chromosome number decreased with each successive backcross, approaching the 28-chromosome number of jointed goatgrass. After two backcrosses and one or two selfings, individuals with chromosome numbers ranging from 29 to 34 were observed (Morrison et al., 2002).

Kroiss (2001) noted the presence of 14 wheat D genome–specific microsatellite markers in wheat x jointed goatgrass BC1, BC2, and BC2S1 derivatives. This indicated that if a gene is present on the D genome of wheat it has the potential to be retained in the jointed goatgrass backcrosses after self-fertility is restored (Zemetra et al., 2002). In a recent review on hybridization and introgression between bread wheat and jointed goatgrass in USA, Hegde and Waines (2004) estimated that the probability of recovering BC2 seed with jointed goatgrass as a recurrent parent would be approximately one plant out of 1.54 million plants if a transgene is present on D genome of wheat. It is expected that chromosomes of unshared genomes (in this case B and A) will be lost with each subsequent generation of selfing. However, cytological studies (Wang et al., 2001) showed that some B or A genome chromosomes can be retained in advanced derivatives. In addition, Wang et al. (2000) observed a translocation involving wheat B or A, and Ae. cylindrica C genome in BC2S2 plant. However, it was not retained in the following generation, indicating that gene retention by translocation may not occur at a sufficiently high frequency between unshared genomes. As a consequence, the potential of retention of a wheat gene is reduced when located on a unshared genome (B or A), compared to that of a gene of the shared D genome (Zemetra et al., 2002). The same authors considered that further assessments of the effect of selection pressure (herbicide treatment in this case) on gene(s) retention on shared and unshared genomes are warranted. Unfortunately, similar assessments are still lacking with other Aegilops species. On the basis of the findings to date, the risk of introgression of wheat genes into jointed goatgrass populations may be tied to (i) the potential for fertility restoration, (ii) the overall numbers of hybrid plants that potentially can produce seed, and (iii) the viability of the seed as well as its ability to successfully establish. Selection pressures in the form of the wheat harvest and wild-type dispersal biology will play a role in the movement of wheat genes into jointed goatgrass.

	Conclusion and Future Prospects

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 
The transfer of a transgene from genetically modified crops to related species could take place during hybridization. The formation of hybrids between crops and their wild relatives depends on a number of factors including the proximity of the parental plants, their mating system (outcrossing rate), and the synchroneous flowering and genetic compatibility (ploidy level, genome similarity, and chromosome pairing) of the crop and the wild species. In Europe these factors are present as shown by the occurrence of spontaneous hybridization, even at relatively low rates.

In Europe, Aegilops species and the wild emmer wheat T. monococcum subsp. aegilopoides occur in sympatry with cultivated wheat. Tetraploid species sharing the U genome (Ae. geniculata, Ae. triuncialis, Ae. biuncialis, and Ae. neglecta) are present in many European countries and are locally abundant. Among the Aegilops species carrying the D genome, Ae. cylindrica has wide distribution either as a native or introduced plant and shows weedy behavior more than most species of the genus, occupying large stands after recent disturbances. An overview on the distribution patterns of wild wheat relatives and bread wheat cultivation areas shows that some countries, such as those of southeastern Europe (e.g., Greece and Bulgaria) may have a higher likelihood of interspecies hybridization and gene transfer from wheat to its wild relatives, because of the presence and abundance of related species, and environmental conditions favoring outcrossing.

Aegilops species have synchroneous flowering with bread wheat. Aegilops speltoides is allogamous, while the remaining species are predominantly autogamous. However, in these species outcrossing may be able to reach levels similar to other autogamous species like wheat. Moreover, the high and asynchroneous tillering of Aegilops species extends the time that cross-pollination and hybrid production with bread wheat can occur. Depending on the species, more information may be needed concerning some floral biology traits of wheat wild relatives (as pollen dispersal and outcrossing rate) and crop x weed hybridization as a function of distance. Attention should focus on the tetraploid species which are more abundant and were found to intercross more frequently with cultivated wheat.

Spontaneous hybridization was reported for most of the tetraploid Aegilops species: Ae. cylindrica, Ae. geniculata, Ae. neglecta, Ae. triuncialis, Ae. columnaris, and Ae. ventricosa. Conversely, very few or no natural hybrids were found with the diploid species Ae. speltoides and T. monococcum subsp. aegilopoides, respectively. Although most natural hybrids between tetraploid Aegilops and wheat were found to be sterile, viable seeds were occasionally produced. Hybrid seed set shows great variation, with higher values being generally found when an Aegilops species was the female parent. Moreover, hybrids sterility may be overcome by backcrossing with a parent or by chromosome doubling. The close relationship between the bread wheat D genome and most of the constitutive genomes of the tetraploid Aegilops, when compared to the B and A genomes, suggest a higher probability of transfer when the trait is located on a D genome chromosome of bread wheat. Gene introgression is expected to occur more frequently in Ae. cylindrica and Ae. ventricosa through recombination of homologous D genome chromosomes. Specific wheat D genome molecular markers were already found in hybrids between jointed goatgrass, Ae. cylindrica, and their recurrent backcross derivatives, indicating that a gene present on D genome of wheat has the potential to be retained in the jointed goatgrass hybrid progenies, whereas the gene on the B or A genome is less likely to be retained. Consequently, as shown by studies performed in Europe and the USA, an assessment should be made on the consequences of pollen-mediated gene flow to jointed goatgrass Ae. cylindrica, since it is a native or adventive in most European countries, and grows near or within wheat fields.

When hybridization can occur, an assessment is needed of whether the transgene could persist in the wild population across further generations, and whether the transgene confers a selective advantage to the hybrid and/or the wild relatives. Case-by-case analysis is needed to assess if a transgene can enhance the fitness of a recipient wild plants and if they can become more widespread. Since wild populations of Aegilops already possess potential weediness traits, the consequence of adding one or few adaptive traits, such as resistance to disease, pests, cold, or drought, would need to be assessed. Therefore, comparison of fitness-related traits between hybrids and backcross progenies of wheat varieties and wild Aegilops populations, and their respective parent lines should be considered to assess possible selective advantages.

Since wild wheat relatives occur in Europe and natural hybrids are reported in European countries, in situ studies may aid in predicting the occurrence of wheat to wild relative species introgression events and estimate the recombination under natural conditions. Wheat to wild relatives gene flow cannot be considered to have a zero probability, given this is the natural progression involved in the evolution of wheat. Therefore, case-by-case and region-by-region assessments are needed to estimate, depending on the species and conditions, the impact to agriculture and natural areas associated with the production of genetically modified bread wheat x wild relative hybrids and their progenies.

	ACKNOWLEDGMENTS

 
Thanks are due to Monsanto Co. for supporting this study and helpful discussion and comments on the manuscript. We also would like to acknowledge the useful suggestions of the two anonymous reviewers.

Received for publication January 9, 2005.

	REFERENCES

TOP ABSTRACT INTRODUCTION Taxonomy of Wheat Wild... Biology of Wheat-Related Species Distribution, Ecology, and... Synchronous Flowering with Bread... Pollination Mode, Glume Opening,... Genome Constitution of Wild... Barriers to Hybridization... Crossability and Genome Affinity... Spontaneous Hybridization (Wild... Conclusion and Future Prospects REFERENCES

 

• Abu Bakar, M., and G. Kimber. 1982. Chromosome pairing regulators in the former genus Aegilops. Z. Pflanzenzuecht. 89:130–138.
• Akhunov, E.D., A.R. Akhunov, A.M. Linkiewicz, J. Dubcovsky, and D. Hummel. 2003. Synteny perturbations between wheat homoeologous chromosomes by locus duplications and deletions correlate with recombination rates along chromosome arms. Proc. Natl. Acad. Sci. USA 100:10836–10841.[Abstract/Free Full Text]
• Alonso, L.C., and G. Kimber. 1981. The analysis of meiosis in hybrids. II. Triploid hybrids. Can. J. Genet. Cytol. 23:221–234.
• Azzi, G. 1954. Ecologie agricole. (In French.) Baillère. Paris.
• Bahrman, N., M. Zivy, and H. Thiellement. 1988. Genetic relationships in the Sitopsis section of Triticum and the origin of the B genome of polyploidy wheats. Heredity 61:474–480.
• Bai, D., G.J. Scoles, and D.R. Knott. 1994. Transfer of leaf rust and stem rust resistance genes from Triticum triaristatum to durum and bread wheats and their molecular cytogenetic localization. Genome 37:410–418.[Medline]
• Barkley, T.M. 1986. Flora of the Great Plains. Univ. of Kansas Press, Lawrence, KS.
• Baum, M., E.S. Lagudah, and R. Apples. 1992. Wide crosses in cereals. Annu. Rev. Physiol. Plant. Mol. Biol. 43:117–143.[CrossRef][Web of Science]
• Beri, S.M., and S.C. Anand. 1971. Factors affecting pollen shedding capacity in wheat. Euphytica 20:327–332.[CrossRef]
• Bijral, J.S., and T.R. Sharma. 1995. Chromosome pairing in Triticum boeoticum x T. aestivum hybrids. Annu. Wheat Newsl. 41:123–124.
• Boguslavski, R.L. 1978a. Tzvetenie vidov Aegilops v usloviyah yujnogo Dagestana. (In Russian.) Bull. VIR 78:10–12.
• Boguslavski, R.L. 1978b. Spontannaia guibridizatzia u vidov roda Aegilops L. v usloviyah yujnogo Dagestana. (In Russian.) Bull. VIR 84:65–68.
• Boguslavski, R.L. 1979. Harakter tzvetenia I opilenia vidov roda Aegilops L. v usloviyah yujnogo Dagestana. (In Russian.) Bull. VIR 89:58–62.
• Bochev, B. 1993. Cytogenetic studies of wheat T. aestivum L. (In Bulgarian.) Publishing House of Bulgarian Academy of Sciences. Sofia, Bulgaria.
• Boiko, E.V., K.S. Gill, L.K. Mickelson-Young, S. Nasuda, W.J. Raupp, D.S. Hassawi, J.N. Ziegle, A.K. Fritz, D. Namuth, N.L.V. Lapitan, and B.S. Gill. 1999. A high-density linkage map of Aegilops tauschii, the D-genome progenitor of wheat. Theor. Appl. Genet. 99:16–26.[Medline]
• Boissier, P.E. 1884. Flora orientalis. Vol. 5. H. Georg, Geneva.
• Calder, D.M. 1966. Inflorescence induction and initiation in the Gramineae. p. 59–73. In F.L. Milthorpe and J.D. Ivins (ed.) The growth of cereals and grasses. Butterworths, London.
• Chen, P.D., and B.S. Gill. 1983. The origin of chromosome 4A, and genomes B and G of tetraploid wheats. p. 39–48. In S. Sakamoto (ed.) Proc. 6th Int. Wheat Genet. Symp., Kyoto, Japan. 28 Nov.–3 Dec. 1983. Kyoto Univ., Kyoto, Japan.
• Chouard, P. 1960. Vernalization and its relations to dormancy. Annu. Rev. Plant Physiol. 11:191–238.[Web of Science]
• Claesson, L., M. Kotimaki, and R. von Bothmer. 1990. Crossability and chromosome pairing in some interspecific Triticum hybrids. Hereditas 112:49–55.
• Cuñado, N., and J.L. Santos. 1999. On the diploidization mechanism of the genus Aegilops: Meiotic behaviour of interspecific hybrids. Theor. Appl. Genet. 99:1080–1086.[CrossRef]
• David, J.L., E. Benavente, C. Brès-Patry, J.C. Dusautoir, and M. Echaide. 2004. Are neopolyploids a likely route for a transgene walk in the wild? The Aegilops ovata x Triticum turgidum durum case. Biol. J. Linnean Soc. 82:503–510.[CrossRef]
• de Candolle, A. 1882. L'origine des plantes cultivées. (In French.) Appleton, NY.
• Delipavlov, D. 1983. Key to Bulgarian plants. (In Bulgarian, with English abstract.) Zemizdat, Sofia.
• Devos, K.M., and M.D. Gale. 2000. Genome relationships. The grass model in current research. Plant Cell 12:637–646.[Abstract/Free Full Text]
• de Vries, A.P. 1971. Flowering biology of wheat, particularly in view of hybrid seed production—A review. Euphytica 23:152–170.[CrossRef]
• Donald, W.W., and A.G. Ogg. 1991. Biology and control of jointed goatgrass (Aegilops cylindrica), a review. Weed Technol. 53:3–17.
• Dosba, F., and Y. Cauderon. 1972. A new interspecific hybrid: Triticum aestivum ssp. vulgare x Aegilops ventricosa. Wheat Info. Serv. 35:22–24.
• Driscoll, C.J., L.M. Bieling, and N.L. Darvey. 1979. An analysis of frequency of chromosome configurations in wheat and wheat hybrids. Genetics 91:755–767.[Abstract/Free Full Text]
• D'Souza, V.L. 1970. Investigations concerning the suitability of wheat as pollen-donor for cross-pollination by wind as compared to rye, Triticale and Secalotricum. (In German, with English abstract.). Z. Planzenzuecht. 63:246–269.
• Dvorak, J. 1988. Cytogenetical and molecular inferences about the evolution of wheat. p. 187–192. In T.E. Miller and R.M.D. Koebner (ed.) Proc. 7th Int. Wheat Genet. Symp. Cambridge, UK. 13–18 July 1988. Inst. Plant Sci. Res., Cambridge, UK.
• Dvorak, J., Z.-L. Yang, F.M. You, and M.-C. Luo. 2004. Deletion polymorphism in wheat chromosome regions with contrasting recombination rates. Genetics 168:1665–1675.[Abstract/Free Full Text]
• Dvorak, J., and H.B. Zhang. 1990. Variation in repeated nucleotide sequences sheds light on the phylogeny of the wheat B and G genomes. Proc. Natl. Acad. Sci. USA 87:9640–9644.[Abstract/Free Full Text]
• Eig, A. 1929. Monographisch-kritische Übersicht der Gattung Aegilops. (In German.) Feddes Repert. 55:1–228.
• Ellstrand, N.C., H.C. Prentice, and J.F. Hancock. 1999. Gene flow and introgression from domesticated plants into their wild relatives. Annu. Rev. Ecol. Syst. 30:539–563.[CrossRef][Web of Science]
• Endo, T.R. 1979. Selective gametocidal action of a chromosome of Aegilops cylindrica in a cultivar of common wheat. Wheat Info. Serv. 50:24–28.
• Endo, T.R. 1990. Gametocidal chromosomes and their induction of chromosome mutations in wheat. Jpn. J. Genet. 65:135–152.[CrossRef]
• Endo, T.R. 1996. Allocation of a gametocidal chromosome of Aegilops cylindrica to wheat homoeologous group 2. Genes Genet. Syst. 71:243–246.
• Endo, T.R., and Y. Katayama. 1978. Finding of a selectively retained chromosome of Aegilops caudata L. in common wheat. Wheat Info. Serv. 48:32–35.
• Endo, T.R., and K. Tsunewaki. 1975. Sterility of common wheat with Aegilops triuncialis cytoplasm. J. Hered. 66:13–18.[Free Full Text]
• Enjalbert, J., I. Goldringer, J. David, and P. Brabant. 1998. The relevance of outcrossing for the dynamic management of genetic resources in predominantly selfing Triticum aestivum L. (bread wheat). Genet. Sel. Evol. 30:197–211.
• FAO. 2004. FAOSTAT Agricultural data [Online]. Available at apps.fao.org/page/collections (verified 16 Nov. 2005). FAO, Rome.
• Farooq, S., N. Iqbal, and T.M. Shah. 1989. Intergeneric hybridization for wheat improvement. I. Influence of maternal and paternal genotypes on hybrid production. Cereal Res. Commun. 17:17–22.
• Farooq, S., N. Iqbal, and T.M. Shah. 1990a. Intergeneric hybridization for wheat improvement. II. Utilization of the ph1b mutant for direct alien introgression into cultivated wheat and production of backcross seed. Cereal Res. Commun. 18:21–26.
• Farooq, S., T.M. Shah, and N. Iqbal. 1990b. Variation in crossability among intergeneric hybrids of wheat and salt tolerant accessions of three Aegilops species. Cereal Res. Commun. 18:335–338.
• Feldman, M., B. Liu, G. Segal, S. Abbo, A.A. Levy, and J.M. Vega. 1997. Rapid elimination of low-copy DNA sequences in polyploid wheat: A possible mechanism for differentiation of homoeologous chromosomes. Genetics 147:1381–1387.[Abstract]
• Feldman, M., F.G.H. Lupton, and T. Miller. 1995. Wheats. p. 184–192. In J. Smartt and N.W. Simmonds (ed.) Evolution of crop plants. Longman Group, London.
• Fernández-Calvín, B., and J. Orellana. 1990. High molecular weight glutenin subunit variation in the Sitopsis section of Aegilops. Implications for the origin of the B genome of wheat. Heredity 65:455–463.
• Fernández-Calvín, B., and J. Orellana. 1992. Relationships between pairing frequencies and genome affinity estimations in Aegilops ovata x Triticum aestivum hybrid plants. Heredity 68:165–172.
• Friebe, B., and B.S. Gill. 1996. Chromosome banding and genome analysis in diploid and cultivated polyploid wheats. p. 39–60. In P.P. Jauhar (ed.) Methods of genome analysis in plants. CRC Press, Boca Raton, FL.
• Friebe, B., J. Jiang, W.J. Raupp, R.A. McIntosh, and B.S. Gill. 1996. Characterization of wheat-alien translocations conferring resistance to diseases and pests: Current status. Euphytica 91:59–87.[CrossRef][Web of Science]
• Friebe, B., Y. Mukai, H.S. Dhaliwal, T.J. Martin, and B.S. Gill. 1991. Identification of alien chromatin specifying resistance to wheat streak mosaic virus and greenbug in wheat germplasm by C-banding and in situ hybridization. Theor. Appl. Genet. 81:381–389.[Web of Science]
• Friebe, B., L.L. Qi, S. Nasuda, P. Zhang, N.A. Tuleen, and B.S. Gill. 2000. Development of a complete set of Triticum aestivum-Aegilops speltoides chromosome addition lines. Theor. Appl. Genet. 101:51–58.[CrossRef]
• Friebe, B., N.A. Tuleen, and B.S. Gill. 1999. Development and identification of a set of Triticum aestivum-Aegilops geniculata chromosome addition lines. Genome 42:374–380.
• Gill, B.S., and G. Kimber. 1974. Giemsa C-banding and the evolution of wheat. Proc. Natl. Acad. Sci. USA 71:4086–4090.[Abstract/Free Full Text]
• Gill, B.S., and P.D. Chen. 1987. Role of cytoplasm-specific introgression in the evolution of the polyploid wheats. Proc. Natl. Acad. Sci. USA 84:6800–6804.[Abstract/Free Full Text]
• Gill, B.S., B. Friebe, and T.R. Endo. 1991. Standard karyotype and nomenclature system for description of chromosome bands and structural aberrations in wheat (Triticum aestivum). Genome 34:830–839.
• Godron, D.A. 1854. De la fécondation naturelle et artificielle des Aegilops par le Triticum. (In French.) Am. Sci. Nat. 3:215–222.
• Gotsov, K., and I. Panayotov. 1972. Natural hybridization of male sterile lines of common wheat x Ae. cylindrica Host. Wheat Info. Serv. 33–34:20–21.
• Govindaraju, D.R. 1988. A note on the relationship between out-crossing rate and gene flow in plants. Heredity 61:401–404.[CrossRef][Web of Science]
• Guadagnuolo, R., D. Savova-Bianchi, and F. Felber. 2001. Gene flow from wheat (Triticum aestivum L.) to jointed goatgrass (Aegilops cylindrica Host.), as revealed by RAPD and microsatellite markers. Theor. Appl. Genet. 103:1–8.[CrossRef]
• Guzy, M.R., B. Ehdaie, and J.G. Waines. 1989. Yield and its components in diploid, tetraploid and hexaploid wheats in diverse environments. Ann. Bot. (London) 64:635–642.[Abstract/Free Full Text]
• Hammer, K. 1980. Vorarbeiten zur monographischen Darstellung von Wildpflanzen-sortimenten: Aegilops L. (In German.) Kulturpflanze 28:33–180.[Medline]
• Hamrick, J.L., Y.B. Linhart, and J.B. Mitton. 1979. Relationships between life history characteristics and electrophoretically detectable genetic variation in plants. Annu. Rev. Ecol. Syst. 10:173–200.[CrossRef][Web of Science]
• Hanf, M. 1982. Les adventices d'Europe. (In French.) BASF, Ludwigshafen, Germany.
• Harlan, J.R., and J.M.J. de Wet. 1975. On Ø. Winge and a prayer: The origins of polyploidy. Bot. Rev. 41:361–390.[CrossRef]
• Hegde, S.G., and J.G. Waines. 2004. Hybridization and introgression between bread wheat and wild and weedy relatives in North America. Crop Sci. 44:1145–1155.[Abstract/Free Full Text]
• Hitchkock, A.S. 1951. Manual of the grasses in the United States. 2nd ed. Misc. publ. 200. USDA, Washington, DC.
• Hucl, P. 1996. Out-crossing rates for 10 Canadian spring wheat cultivars. Can. J. Plant Sci. 76:423–427.
• Hucl, P., and M. Matus-Cádiz. 2001. Isolation distances for minimizing out-crossing in spring wheat. Crop Sci. 41:1348–1351.[Abstract/Free Full Text]
• Iqbal, N., S.M. Reader, P.D.S. Caligari, and T.E. Miller. 2000. Characterisation of Aegilops uniaristata chromosomes by comparative DNA marker analysis and repetitive DNA sequence in situ hybridisation. Theor. Appl. Genet. 101:1173–1179.[CrossRef]
• Jaubert, H.F., and E. Spach. 1851. Illustrationes plantarum orientalum. Vol. 4. (In Latin.) Roret, Paris.
• Jauhar, P.P. 1993. Cytogenetics of the Festuca-Lolium complex: Relevance to breeding. Springer-Verlag, Heidelberg, Germany.
• Jauhar, P.P., A.B. Almouslem, T.S. Peterson, and L.R. Joppa. 1999. Inter- and intragenomic chromosome pairing in haploids of durum wheat. J. Hered. 90:437–445.[Abstract/Free Full Text]
• Jiang, J., and B.S. Gill. 1993. Sequential chromosome banding and in situ hybridization. Genome 36:792–795.[Medline]
• Jiang, J., and B.S. Gill. 1994a. Nonisotopic in situ hybridization and plant genome mapping: The first 10 years. Genome 37:717–725.[Medline]
• Jiang, J., and B.S. Gill. 1994b. Different species-specific chromosome translocations in Triticum timopheevii and T. turgidum support diphyletic origin of polyploid wheats. Chrom. Res. 2:59–64.[Medline]
• Johnson, B.L., and H.S. Dhaliwal. 1976. Reproductive isolation of Triticum boeoticum and Triticum urartu and the origin of the tetraploid wheat. Am. J. Bot. 63:1088–1094.[CrossRef][Web of Science]
• Johnston, C.O., and J.H. Parker. 1929. Aegilops cylindrica Host, a wheat field weed in Kansas. Trans. Kans. Acad. Sci. 32:80–84.
• Joppa, L.R., F.H. MacNeal, and M.A. Berg. 1968. Pollen production and pollen shedding of hard red spring (Triticum aestivum L. em. Thell.) and durum (T. durum Desf.) wheats. Crop Sci. 13:223–226.
• Keller, B., and C. Feuillet. 2000. Colinearity and gene density in grass genomes. Trends Plant Sci. 5:246–251.[CrossRef][Web of Science][Medline]
• Kerby, K., and J. Kuspira. 1988. Cytological evidence bearing on the origin of the B genome in polyploid wheats. Genome 30:36–43.
• Khan, M.N., E.G. Heyne, and A.L. Arp. 1973. Pollen distribution and seed set on Triticum aestivum L. Crop Sci. 13:223–226.[Abstract/Free Full Text]
• Kihara, H. 1919. Über cytologische Studien bei einigen Getreidearten. 1. Spezies-Bastarde des Weizen and Weizenroggen-Bastard. (In German.) Bot. Mag. (Tokyo) 32:17–38.
• Kihara, H. 1937. Genomanalyse bei Triticum und Aegilops. VII. Kurze Übersicht über die Ergebnisse der Jahre 1934–36. (In German.) Mem. Coll. Agric. Kyoto Imp. Univ. 41:1–61.
• Kihara, H. 1940. Verwandtschaft der Aegilops-Arten im Lichte der Genomanalyse. Ein Überblick. (In German.) Der Züchter 12:49–62.
• Kihara, H. 1944. Die Entdeckung der DD-Analysatoren beim Weizen. (In German.) Agric. Hort. Tokyo 19:889–890.
• Kihara, H. 1954. Considerations on the evolution and distribution of Aegilops species based on the analyser-method. Cytologia (Tokyo) 19:336–357.
• Kimber, G. 1988. Evolutionary patterns in the wheat group. p. 47–51. In T.E. Miller and R.M.D. Koebner (ed.) Proc. 7th Int. Wheat Genet. Symp., Cambridge, England. 13–18 July 1988. Inst. of Plant Sci. Res., Cambridge, UK.
• Kimber, G., and M. Abu Bakar. 1979. Wheat hybrid information systems. Cereal Res. Commun. 7:257–260.
• Kimber, G., and L.C. Alonso. 1981. The analysis of meiosis in hybrids. III. Tetraploid hybrids. Can. J. Genet. Cytol. 23:235–254.[Medline]
• Kimber, G., L.C. Alonso, and P.J. Salle. 1981. The analysis of meiosis in hybrids. I. Aneuploid hybrids. Can. J. Genet. Cytol. 23:209–219.[Medline]
• Kimber, G., and M.M. Hulse. 1978. The analysis of chromosome pairing in hybrids and evolution of wheat. p. 63–72. In S. Ramanujam (ed.) Proc. 5th Int. Wheat Genet. Symp., New Delhi, India. 23–28 Feb. 1978. Indian Soc. Genet. Plant Breed., New Delhi, India.
• Kimber, G., P.J. Sallee, and M.M. Feiner. 1988. The interspecific and evolutionary relationships of Triticum ovatum. Genome 30:218–221.
• Kimber, G., and E.R. Sears. 1987. Evolution in the genus Triticum and the origin of the cultivated wheats. p. 154–164. In E.G. Heyne (ed.) Wheat and wheat improvement. ASA, Madison, WI.
• Kimber, G., and Y. Yen. 1988. Analysis of pivotal-differential evolutionary patterns. Proc. Natl. Acad. Sci. USA 85:9106–9108.[Abstract/Free Full Text]
• Kimber, G., and Y.H. Zhao. 1983. The D genome of the Triticeae. Can. J. Genet. Cytol. 25:581–589.
• Knobloch, J.W. 1968. A check-list of crosses in Gramineae. Michigan State Univ., Lansing.
• Kota, R.S., and J. Dvorak. 1988. Genomic instability in wheat induced by chromosome 6B^S of Triticum speltoides. Genetics 120:1085–1094.[Abstract/Free Full Text]
• Kroiss, L.J. 2001. Retention of wheat alleles in imidazolinone-resistant wheat x jointed goatgrass recurrent backcross generations. M.S. thesis. Oregon State Univ., Corvallis, OR.
• Kushnir, U., and G.M. Halloran. 1981. Evidence for Aegilops sharonensis Eig as the donor of the B genome of wheats. Genetics 99:495–512.[Abstract/Free Full Text]
• Lange, J.M.C. 1860. Pugillus plantarum imprimis hispanicarum. Vol. 1. (In Latin.) Extr. Vid. Medd. Naturh. For. København. Bianco Luno, Hauniae, Copenhagen.
• Larghetti, G., S. Infantino, G. Figliuolo, S. Cifarelli, P.M. Spagnoletti-Zeuli, and P. Perrino. 1992. Wild genetic resources of wheat in southern Italy. Plant Genet. Res. Newsl. 88:74–75.
• Linc, G., B.R. Friebe, R.G. Kynast, M. Molnar-Lang, B. Köszegi, J. Sutka, and B.S. Gill. 1999. Molecular cytogenetic analysis of Aegilops cylindrica Host. Genome 42:497–503.[Medline]
• Linnaeus, C. 1753. Species plantarum. Vol. 2. (In Latin.)
• Liu, B., J.M. Vega, and M. Feldman. 1998b. Rapid genomic changes in newly synthesized amphiploids of Triticum and Aegilops. II. Changes in low-copy coding DNA sequences. Genome 41:535–542.[Medline]
• Liu, B., J.M. Vega, G. Segal, S. Abbo, M. Rodova, and M. Feldman. 1998a. Rapid genomic changes in newly synthesized amphiploids of Triticum and Aegilops. I. Changes in low-copy non-coding DNA sequences. Genome 41:272–277.
• Logojan, A.A., and M. Molnár-Láng. 2000. Production of Triticum aestivum-Aegilops biuncialis chromosome additions. Cereal Res. Commun. 28:221–228.
• Maan, S.S. 1979. Specificity of nucleo-cytoplasmic interactions in Triticum and Aegilops species (a review). Wheat Info. Serv. 50:71–79.
• MacFadden, E.S., and E.R. Sears. 1946. The origin of Triticum spelta and its free-threshing hexaploid relatives. J. Hered. 37:81–89.[Free Full Text]
• Maestra, B., and T. Naranjo. 1998. Homoeologous relationships of Aegilops speltoides to bread wheat. Theor. Appl. Genet. 97:181–186.[CrossRef][Web of Science]
• Mallory-Smith, C.A., J. Hansen, and R.S. Zemetra. 1996. Gene transfer between wheat and Aegilops cylindrica. p. 441–445. In Proc. 2nd Int. Weed Control Congress, Copenhagen. 25–28 June 1996. Dep. of Weed Control and Pesticide Ecology, Copenhagen.
• Marañon, T. 1987. Ecologia del polimorphismo somatico de semillas y la sinaptospermia en Aegilops neglecta Req. ex Bertol. (In Spanish.) Anales Jardín Bot. Madrid 44:97–107.
• McGuire, P., and J. Dvorak. 1982. Genetic regulations of heterogenetic chromosome pairing in polyploid species of the genus Triticum sensu lato. Can. J. Genet. Cytol. 24:57–82.
• Meissner, C. 1991. Incorporation of disease resistance from the genus Aegilops into Triticum aestivum. p. 392–397. In I. Panayotov and S. Pavlova (ed.) Wheat breeding prospects and future approaches. Agric. Acad., Albena, Bulgaria.
• Mello-Sampayo, T. 1971. Genetic regulation of meiotic chromosome pairing by chromosome 3D of Triticum aestivum. Nature New Biol. 230:22–23.[Web of Science][Medline]
• Morishita, D.W. 1996. Biology of jointed goatgrass. p. 7–9. In B. Jenks (ed.) Pacific Northwest Jointed Goatgrass Conf., Univ. of Nebraska, Lincoln. 21–29 Feb. 1996. Univ. of Nebraska, Lincoln, NE.
• Morrison, L.A., O. Riera-Lizarazu, M.I. Vales, L. Cremieux, R.S. Zemetra, J. Hansen, and C. Mallory-Smith. 2002. Genetic diversity and gene flow in the crop-weed complex of wheat (Triticum aestivum L.) and jointed goatgrass (Aegilops cylindrica Host). p. 125–130. In P. Hernandez, M.T. Moreno, J.I. Cubero, and A. Martin (ed.) Triticeae IV, Proc. of the 4th Int. Triticeae Symp., Cordoba, Spain. 10–12 Sept. 2002. Consejería de Agricultura y Pesca, Junta de Andalucia, Cordoba, Spain.
• Mujeeb-Kazi, A. 1993. Interspecific and intergeneric hybridization in the Triticeae for wheat improvement. p. 95–102. In A.B. Damania (ed.) Biodiversity and wheat improvement. John Wiley & Sons, Chichester, UK.
• Mujeeb-Kazi, A. 1995. Utilizing wild grass biodiversity in wheat improvement: 15 years of wide crosses research at CIMMYT. CIMMYT Res. Rep. No. 2. CIMMYT, Mexico D.F.
• Mujeeb-Kazi, A., and G. Kimber. 1985. The production, cytology and practicality of wide hybrids in the Triticeae. Cereal Res. Commun. 13:111–124.
• Nasuda, S., B. Friebe, and B.S. Gill. 1998. Gametocidal genes induce chromosome breakage in the interphase prior to the first mitotic cell division of the male gametophyte in wheat. Genetics 149:1115–1124.[Abstract/Free Full Text]
• Ozgen, M. 1983. Hybrid seed set in wheat x Aegilops crosses. Wheat Info. Serv. 56:9–11.
• Ozkan, A., A. Levy, and M. Feldman. 2001. Allopolyploidy-induced rapid genome evolution in the wheat (Aegilops–Triticum) group. Plant Cell 13:1735–1747.[Abstract/Free Full Text]
• Pathak, N. 1940. Studies in the cytology of cereals. J. Genet. 39:437–467.
• Pederson, C., and P. Langridge. 1997. Identification of the entire chromosome complement of bread wheat by two-colour FISH. Genome 40:589–593.[Medline]
• Perrino, P., G. Laghetti, L.F. D'Antuono, M. Al Ajlouni, M. Kanbertay, A.T. Szabo, and K. Hammer. 1996. Ecogeographical distribution of hulled wheat species. p. 102–120. In S. Padulosi et al. (ed.) Hulled wheats. Promoting the conservation and use of underutilized and neglected crops. IPGRI, Rome.
• Peterson, R. 1965. Wheat. Botany, cultivation, and utilization. Leonard Hill Books, London, UK, and Interscience Publishers Inc., NY.
• Rajhathy, T. 1960. Continuous spontaneous crosses between Aegilops cylindrica and Triticum aestivum. Wheat Info. Syst. 11:20.
• Rayburn, A.L., and B.S. Gill. 1985. Use of biotin-labelled probes to map specific DNA sequence on wheat chromosomes. J. Hered. 76:78–81.[Abstract/Free Full Text]
• Rayburn, A.L., and B.S. Gill. 1987. Molecular analysis of the D genome of the Triticeae. Theor. Appl. Genet. 73:358–388.
• Riley, R., and V. Chapman. 1958. Genetic control of cytologically diploid behaviour of hexaploid wheat. Nature (London) 182:713–715.[CrossRef]
• Riley, R., G. Kimber, and V. Chapman. 1961. The origin of the genetic control of the diploid-like meiotic behaviour of polyploid wheat. J. Hered. 52:22–26.[Free Full Text]
• Sankary, M.N. 1990. Ecogeographical survey of Aegilops in Syria. p. 147–160. In J.P. Srivastava and A.B. Damania (ed.) Wheat genetic resources: Meeting diverse needs. John Wiley & Sons, Chichester, UK.
• Sasakuma, T., Y. Ogihara, and H. Tsujimoto. 1995. Genome rearrangement of repetitive sequences in processes of hybridization and amphiploidization in Triticinae. p. 563–566. In Z. Li and Z. Xin (ed.) Proc. 8th Int. Wheat Genet. Symp. 20–25 July 1993. Agricultural Scientech Press, Beijing.
• Sax, K. 1922. Sterility in wheat hybrids. II. Chromosome behavior in partially sterile hybrids. Genetics 7:513–550.[Free Full Text]
• Sears, E.R. 1941. Amphiploids in the seven-chromosome Triticineae. Missouri Agric. Exp. Stn. Res. Bull. 336. Univ. of Missouri, Columbia, MO.
• Sears, E.R. 1948. The cytology and genetics of the wheats and their relatives. Adv. Genet. 2:239–270.
• Sears, E.R. 1954. The aneuploids of common wheat. Missouri Agric. Exp. Stn. Res. Bull. 572. Univ. of Missouri, Columbia, MO.
• Sears, E.R. 1956. The B genome of Triticum. Wheat Info. Serv. 4:8–10.
• Sears, E.R. 1976. Genetic control of chromosome pairing in wheat. Annu. Rev. Genet. 10:31–51.[CrossRef][Web of Science][Medline]
• Seefeldt, S.S., R.S. Zemetra, F.L. Young, and S.S. Jones. 1998. Production of herbicide-resistant jointed goatgrass (Aegilops cylindrica) x wheat (Triticum aestivum) hybrids in the field by natural hybridization. Weed Sci. 46:632–634.
• Sharma, H.C., and B.S. Gill. 1983. Current status of wide hybridization in wheat. Euphytica 32:17–31.[CrossRef][Web of Science]
• Sharma, H.C., and J.G. Waines. 1981. Attempted gene transfer from tetraploids to diploids in Triticum. Can. J. Genet. Cytol. 23:639–645.
• Singh, H., H.S. Dhaliwal, J. Kaur, and K.S. Gill. 1993. Rust resistance and chromosome pairing in Triticum x Aegilops crosses. Wheat Info. Serv. 76:23–26.
• Singh, J.S., and T.R. Sharma. 1997. Morpho-cytogenetics of Triticum aestivum L. x Aegilops speltoides Tausch. Hybrids. Wheat Info. Serv. 84:51–52.
• Singh, R.M. 2003. Plant cytogenetics. CRC Press, Boca Raton, FL.
• Snyder, J., C. Mallory-Smith, S. Balter, J. Hansen, and R.S. Zemetra. 2000. Seed production on Triticum aestivum by Aegilops cylindrica hybrids in the field. Weed Sci. 48:588–593.[CrossRef]
• Tanaka, M. 1954. Spring and winter growing habit of Aegilops and Triticum. Wheat Info. Serv. 1:18.
• Tikhonov, A.P., P.J. Sanmiguel, Y. Nakajima, N.M. Gorenstein, J.L. Bennetzen, and Z. Avramova. 1999. Colinearity and its exceptions in orthologous adh regions of maize and sorghum. Proc. Natl. Acad. Sci. USA 96:7409–7414.[Abstract/Free Full Text]
• Tschermak, E.v., and H. Bleier. 1926. Über fruchtbare Aegilops-Weizenbastarde. (In German.) Berichte der Deutschen Botanischen Gesellschaft 44:110–132.
• Tsujimoto, H., and K. Tsunewaki. 1983. Genetic analyses on a gametocidal gene originated from Aegilops aucheri. p. 1077–1081. In S. Sakamoto (ed.) Proc. of the 6th Int. Wheat Genet. Symp., Kyoto, Japan. 28 Nov.–3 Dec. 1983. Kyoto Univ., Kyoto, Japan.
• Tsujimoto, H., and K. Tsunewaki. 1988. Gametocidal genes in wheat and its relatives. III. Chromosome location and effects of two Aegilops speltoides-derived gametocidal genes in common wheat. Genome 30:239–244.
• Tsunewaki, K., and V. Ogihara. 1983. The molecular basis of genetic diversity among cytoplasms of Triticum and Aegilops species. II. On the origin of the polyploidy wheat cytoplasms as suggested by chloroplast DNA restriction fragment patterns. Genetics 109:155–171.
• van Slageren, M.W. 1994. Wild wheats: A monograph of Aegilops L. and Amblyopyrum (Jaub and Spach) Eig (Poaceae). Agricultural Univ., Wageningen, The Netherlands and Int. Cent. for Agric. Res. in Dry Areas, Aleppo, Syria.
• Veilleux, R. 1985. Diploid and polyploid gametes in crop plants: Mechanisms of formation and utilization in plant breeding. Plant Breed. Rev. 3:253–288.
• Virmani, S.S., and I.B. Edwards. 1983. Current status and future prospects for breeding hybrid rice and wheat. Adv. Agron. 36:145–214.
• Waines, J.G., and D. Barnhart. 1992. Biosystematic research in Aegilops and Triticum. Hereditas 116:207–212.
• Waines, J.G., and S.G. Hegde. 2003. Intraspecific gene flow in bread wheat as affected by reproductive biology and pollination ecology of wheat flowers. Crop Sci. 43:451–463.[Abstract/Free Full Text]
• Wang, R.R.-C. 1990. Understanding the effect of the Ph gene of wheat on chromosome pairing and its implications. p. 259–263. In G. Kimber (ed.) Proc. 2nd Int. Symp. Chrom. Eng. in Plants. 13–15 Aug. 1990. Univ. of Missouri, Columbia, MO.
• Wang, R.R.-C. 1995. Intergeneric and interspecific genome analysis in Triticeae. p. 57–64. In Z.S. Li and Z.Y. Xin (ed.) Proc. 8th Int. Wheat Genet. Symp. 20–25 July 1993. Agricultural Scientech Press, Beijing.
• Wang, Z.N., A. Hang, J. Hansen, C. Burton, C.A. Mallory-Smith, and R.S. Zemetra. 2000. Visualization of A and B genome chromosomes in wheat (Triticum aestivum L.) x jointed goatgrass (Aegilops cylindrica Host) backcross progenies. Genome 43:1038–1044.[Medline]
• Wang, Z.N., R.S. Zemetra, J. Hansen, and C.A. Mallory-Smith. 2001. The fertility of wheat x jointed goatgrass hybrid and its backcross progenies. Weed Sci. 49:340–345.[CrossRef]
• Ye, X., H. Xu, Z. Li, L. Du, and L. Zhao. 1997. Crossability analysis of reciprocal hybridizations between common wheat (Triticum aestivum L.) and white rye (Secale cereale L.) and Aegilops ovata L. (In Chinese, with English abstract.) Sciencia Agric. Sinica 30:91–93.
• Zaharieva, M. 1993. Aegilops species in Bulgaria. Their ecogeography and distribution. p. 369–374. In A.B. Damania (ed.) Biodiversity and wheat improvement. John Wiley & Sons, Chichester, UK.
• Zaharieva, M., A. Cortez, V. Rosas, S. Cano, J. Sanchez, L. Juarez, R. Delgado, and A. Mujeeb-Kazi. 2001a. Potential of Aegilops geniculata genetic resources for wheat improvement. Annu. Wheat Newsl. 47:102–103.
• Zaharieva, M., A. Cortez, V. Rosas, S. Cano, J. Sanchez, L. Juarez, R. Delgado, and A. Mujeeb-Kazi. 2003a. Triticum aestivum/Ae. geniculata and Triticum/durum/Ae. geniculata hybrids, BCs and amphiploids production. Annu. Wheat Newsl. 49:69–71.
• Zaharieva, M., E. Gaulin, M. Havaux, E. Acevedo, and P. Monneveux. 2001b. Drought and heat responses in the wild wheat relative Aegilops geniculata Roth: Potential interest for wheat improvement. Crop Sci. 41:1321–1329.[Abstract/Free Full Text]
• Zaharieva, M., K. Suenaga, H.M. William, and A. Mujeeb-Kazi. 2003b. Microsatellite markers for detection Aegilops geniculata Roth. M and U genome chromosomes in wheat background. Annu. Wheat Newsl. 49:75–78.
• Zemetra, R.S., J. Hansen, and C.A. Mallory-Smith. 1998. Potential for gene transfer between wheat (Triticum aestivum) and jointed goatgrass (Aegilops cylindrica). Weed Sci. 46:313–317.
• Zemetra, R.S., C.A. Mallory-Smith, J. Jansen, Z. Wang, A. Snyder, A. Hang, L. Kroiss, O. Liera-Lizarazu, and I. Vales. 2002. The evolution of biological risk program: Gene flow between wheat (Triticum aestivum L.) and jointed goatgrass (Ae. cylindrica Host.). p. 178–187. In Scientific Methods Workshop: Ecological and Agronomic Consequences of Gene Flow from Transgenic Crops to Wild Relatives. 5–6 Mar. 2001. Ohio State Univ., Columbus, OH.
• Zhang, H., J. Jia, M.D. Gale, and K.M. Devos. 1998. Relationships between the chromosomes of Aegilops umbellulata and wheat. Theor. Appl. Genet. 96:69–75.[CrossRef][Web of Science]
• Zhao, Y.H., and G. Kimber. 1984. New hybrids with D genome wheat relatives. Genetics 106:509–515.[Abstract/Free Full Text]
• Zhukovsky, P.M. 1928. Kritiko-systematischeskii obzor vydov roda Aegilops L. (Specierum generis Aegilopis L. revisio critica). (In Russian.) Trudy Prikl. Bot. Genet. Selekc. 18:417–609.
• Zohary, D., and M. Feldman. 1962. Hybridisation between amphiploids and the evolution of polyploids in the wheat (Aegilops-Triticum) group. Evolution 16:44–61.[CrossRef][Web of Science]

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