HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (8) Citing Articles via Google Scholar Google Scholar Articles by Hegde, S. G. Articles by Waines, J. G. Search for Related Content PubMed Articles by Hegde, S. G. Articles by Waines, J. G. Agricola Articles by Hegde, S. G. Articles by Waines, J. G. Related Collections Weed Management Wheat Plant Genetic Resources

PLANT GENETIC RESOURCES

Genetic Diversity in Wild and Weedy Aegilops, Amblyopyrum, and Secale Species—A Preliminary Survey

^a Dep. of Botany and Plant Sciences, University of California, Riverside, CA 92521
^b Genetic Resources Unit, International Center for Agricultural Research in the Dry Areas (ICARDA), P.O. Box 5466, Aleppo, Syria

* Corresponding author (giles.waines{at}ucr.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
The wild and weedy relatives of bread wheat, Triticum aestivum L., are suggested as potential sources of useful alleles for bread wheat improvement. For example, the genus Aegilops L. has contributed two of the three bread wheat genomes. In this study we measured the nature and extent of allozyme variation for 10 isozymes in three diploid and eight polyploid Aegilops, one Amblyopyrum (Jaub. & Spach), and one feral Secale L. species collected from 15 populations in their centers of origin in Syria, Lebanon, Turkey, and California. The predominantly autogamous Aegilops species were, to a large extent, homozygous and homogeneous. The presence of heterozygous genotypes in Aegilops tauschii Coss. (H₀ = 0.0033 ± 0.01) and Ae. crassa Boiss (H₀ = 0.0048 ± 0.08) indicated the possibility of limited facultative outcrossing in these Aegilops species. The obligate outcrossers, Amblyopyrum muticum (Boiss.) Eig (H_e = 0.09 ± 0.16) and Secale cereale L. (H_e = 0.09 - 0.13), showed less than expected genetic variation. The extent and nature of genetic variation were identical between the introduced Californian Ae. cylindrica Host and that which occurs in the Fertile Crescent (H_e = 0.00); however, in Ae. triuncialis L., the introduced Californian population had a higher genetic diversity estimate (H_e = 0.06 ± 0.12) than the population from the Fertile Crescent (H_e = 0.00). The average genetic distance between the polyploid Aegilops species was greater (D = 0.64 ± 0.28) than that observed between the diploid Aegilops species (D = 0.18 ± 0.10), or between the diploid Aegilops and weedy S. cereale (D = 0.52 ± 0.06). We discuss the implication of these findings for germplasm collection and wheat breeding.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
IT IS ESTIMATED that the world needs a billion tons of wheat (Triticum aestivum L.) within the next 25 years from the present production level of 560 million tons, and the extra demand is expected to be fulfilled mainly through conventional breeding (Braun et al., 1998). There is growing concern among wheat breeders that the remaining variability in the bread wheat gene pool is grossly insufficient to address current and future breeding objectives (Rejesus et al., 1996). For that reason it is necessary to broaden the genetic base of wheat, and germplasm accessions most distinct from modern cultivars are predicted to contain the highest number of unexploited potentially useful alleles (Vavilov, 1940).

There have been some efforts to evaluate, document, and use landraces, primitive forms, wild relatives, as well as other members of the Triticeae for breeding purposes (Knott, 1989). For example, Mujeeb-Kazi et al. (1996) generated additional genetic diversity in hexaploid wheat (BBAADD) [genomic formula according to Waines and Barnhart (1992)] by producing synthetic wheats through hybridization of durum wheat (Triticum turgidum L.), the donor of the B and A genomes with Ae. tauschii Coss., the donor of the D genome (Kihara, 1944). Moreover, breeders have achieved commercial benefits from similar distant hybridization by transferring field-tested stem rust and leaf rust resistant genes from Ae. triuncialis L. (UUCC), a wild relative, to wheat through backcrossing and embryo rescue (Knott, 1989).

Besides agronomic benefits, genetic diversity studies in Aegilops are also important to discover the basis of weediness in some of the Aegilops species. For instance, at least two of the seven introduced Aegilops species in the United States are troublesome weeds. Jointed goatgrass (Ae. cylindrica Host) is a noxious annual weed that is particularly troublesome in winter wheat-growing areas of the western United States. This weed is a strong competitor of winter wheat costing approximately $45 million annually in reduced wheat yield and quality in the USA (Donald and Ogg, 1991). Similarly, barbed goatgrass, Ae. triuncialis, is considered a noxious weed of rangelands in Northern and Central California. Therefore, understanding the genetic structure of these troublesome weedy populations may facilitate designing new ways to control them, especially when most other conventional methods of weed control have failed to contain their spread (Donald and Ogg, 1991).

The primary gene pool of wheat comprises four wild species, Triticum monococcum L. ssp. aegilopoides (Link) Boiss., T. urartu Tumanian ex Gandilyan, T. turgidum L. ssp. dicoccoides (Körn ex. Asch. & Graebn.) Thell., and T. timopheevii (Zhuk.) Zhuk, and their domesticated forms in the genus Triticum L. The genera Aegilops (B/G and D genomes) and Amblyopyrum constitute most of the secondary gene pool of wheat (Harlan and de Wet, 1971). Studies of variation in nuclear DNA (Dvorak and Zhang, 1990) strongly support the idea that genomes B/G were derived from an S-genome species of the section Sitopsis, most likely related to Ae. speltoides Tausch, whereas Ae. tauschii contributed the D genome to bread wheat.

Aegilops is characterized as a Mediterranean–Western Asiatic element and its center of diversity follows the central part of the Fertile Crescent arc in West Asia. Amblyopyrum is a Western Asiatic element of limited distribution. This monospecific genus occurs in central Anatolia and Armenia. The ecological distribution of the two genera is very similar, with species occurring frequently along roadsides, edges of cultivation, dry hillsides, and grassy steppes, where it tolerates disturbance (van Slageren, 1994).

Genetic diversity studies (Nevo et al., 1982; Smith-Huerta et al., 1989; Medlinger and Zohary, 1995; Hegde et al., 2000) of wild wheat populations have dealt mainly with diploid and tetraploids, and the goatgrass Ae. speltoides. Medlinger and Zohary (1995) observed higher genetic diversity estimates of five Aegilops species in section Sitopsis than the average value reported for annual grasses using allozyme electrophoresis (Hamrick and Godt, 1990). A genetic diversity study on wild Triticum species (Nevo et al., 1982) from the same area reported relationships between population genetic variability and environmental heterogeneity, however, this association was not observed by Medlinger and Zohary (1995) in Aegilops.

The genus Aegilops contains 22 species comprising both diploids and polyploids, that originated from the center of origin (van Slageren, 1994). The primary objective of this study was to understand (i) the extent and pattern of genetic diversity among diploid and polyploid wild species of Aegilops; and (ii) the process of evolution in these species. In addition, populations of Ae. cylindrica and Ae. triuncialis from California were included to compare the genetic structure of these introduced weedy species to original populations collected from their centers of origin.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
We report the allozyme variation in natural populations of diploid and polyploid Aegilops, and Amblyopyrum species collected from Syria, Lebanon, Turkey, and California. For comparison Secale cereale (rye), was included as an outgroup. Rye, like wheat and goatgrass, is a native of the Mediterranean region. A weedy form of rye found in Southern California occurs along roadsides, in pastures and fields, and disturbed areas at higher elevations (Beauchamp, 1986).

Sampling
One spike was randomly collected from each of 30 or more plants in each population representing Aegilops species from all sections except Comopyrum (Table 1).

The populations were scored for the following isozymes: acid phosphatase (Acp), isocitrate dehydrogenase (Idh), malate dehydrogenase (Mdh), phosphoglucose isomerase (Pgi), 6-phosphogluconate dehydrogenase (6Pgd), phosphoglucomutase (Pgm), shikimate dehydrogenase (Skdh), and triosephosphate isomerase (Tpi). The staining procedure, the number of loci per enzyme, and their alleles were inferred from past electrophoretic studies of Triticum and Aegilops (Nevo et al., 1982; Smith-Huerta et al., 1989; Medlinger and Zohary, 1995). In each locus, alleles were alphabetically designated with ‘a’ being that producing the band of slowest mobility. Likewise, multiple isozyme loci were represented with ‘1’ being the slowest. On the basis of bandwidth criterion, we were not able to distinguish the specific contribution of the individual genome or homeologous locus. Therefore, for convenience, depending on the examined enzyme, we designated the two genomes of the tetraploid species as slow (S) and fast (F) genomes on the basis of their mobility (Nevo et al., 1982). This assignment may not accurately assign enzyme bands to the products of the distinct genomes of the polyploid because each genome may possess a locus or alleles that are capable of migrating slower or faster on the gel. For that reason, we did not make any attempt to associate the ‘S’ and ‘F’ genomes with the conventional genomes of the tetraploid or hexaploid Aegilops.

Genetic Diversity Parameters
The following genetic variability parameters were calculated at the population or between species levels; alleles per locus (A), percent polymorphic loci (P), observed heterozygosity (H_o), and expected heterozygosity (He = Hardy-Weinberg expected heterozygosity, Nei, 1987). The mean number of alleles per locus was determined by summing all the alleles observed within a population(s) or between species and dividing this sum by the number of loci (Hamrick and Godt, 1990). Percent polymorphic loci was calculated by dividing the number of loci polymorphic within a population or between species by the total number of loci analyzed and expressed as a percentage. The observed heterozygosity per locus was calculated by dividing the total number of heterozygous loci within a population(s) or between species by the total number of loci. Genetic diversity was calculated for each locus (including monomorphic and polymorphic loci) by: He = 1 - x²_i. For the within-population values x_i is the frequency of the i^th allele in each population, whereas for the between populations or species values, x_i is the mean frequency of the i^th allele pooled across all populations or species. Mean genetic diversity at the individual population or between populations or species level was obtained by averaging H_e over all loci (Hamrick and Godt, 1990). The linkage disequilibrium between loci was estimated (Weir, 1979). The gene flow "Nm" was indirectly estimated from Fst values (Slatkin and Barton, 1989). The genetic identity (I) and genetic distance (D) values were calculated between populations within a species and between species (Nei, 1987). The genetic distance values (D) were used to generate an unweighted pair-group clustering based on the arithmetic averages (UPGMA) phenogram (Felsenstein, 1994).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Genetic Variability
Although polyploids have more than one genome, alleles for six isozymes appeared to have come from only one of the genomes. The remaining four isozymes, Pgi-1, Pgm-1, Tpi-1, and Tpi-2, had allelic contributions from more than one genome (homeologous loci), as the band width of polyploids were substantially wider than their diploid control plants. The hexaploid species Ae. vavilovii (Zhuk.) Chennav. exhibited tetraploid-like isozyme expression by exhibiting only one slow and one fast isozyme band on the gel.

The majority of diploid and polyploid populations were homozygous and homogeneous, especially among the Aegilops species (Tables 2 and 3). Overall, 20% of the loci in diploids and 6% of the loci in polyploids were polymorphic, and 30% of the diploid and 50% of the polyploid populations were homozygous for all ten loci studied. Populations of Ae. tauschii (Rasafa, pop. 2), Ae. searsii Feldman & Kislev ex Hammer, Ae. cylindrica, Ae. biuncialis Vis. and Ae. vavilovii were monomorphic for all the loci examined. Among diploid Aegilops and Amblyopyrum species, Ae. caudata L. and Amblyopyrum muticum (Boiss.) Eig were polymorphic, but for a different set of isozymes. Aegilops caudata loci Idh, Mdh-1, Mdh-2 and Pgd were polymorphic, while loci Pgd, Pgi, Pgm, Tpi-1 and Tpi-2 were polymorphic in Am. muticum. Between two populations of Ae. tauschii (Tables 1 and 2) the Rasafa population was completely monomorphic, while the other population, 28 km away, was polymorphic for the Pgd locus. Within Ae. triuncialis (Table 3), the Turkey population was polymorphic for Pgi-1F and the California population was polymorphic for Pgd-2, Pgm-1S, and Pgm-1F.

In our study, both populations of Ae. tauschii, Ae. crassa, and one of Ae. cylindrica and Ae. triuncialis were collected from large original populations from their center of origin. Contrary to our expectation, allelic variation did not vary between the original and the introduced populations of Ae. cylindrica (Pop. Nos. 10 and 11; Table 4). On the other hand, allelic variation was higher in the introduced Californian population of Ae. triuncialis compared with the population from the center of origin. These observations imply that stochastic events may not be the major factors shaping the observed population/species genetic structure in the case of Ae. tauschii and Ae. cylindrica and natural selection and adaptation might be a causal factor for the observed results. The strongest empirical evidence for the selection-adaptation hypothesis (Hegde et al., 2000) was provided by Nevo et al. (1982) for a tetraploid wild wheat species. Using a variety of genetic markers, they demonstrated that in wild tetraploid wheat, T. dicoccoides (Körn, ex Asch. & Graebn.), genetic diversity is at least partly adaptive and differentiated primarily by ecological factors, such as soil, moisture stress, and microclimatic conditions. Between species observed genetic variation in our study could also be the result of fine-scale adaptation to the ecological diversity of the region. On a global scale, the ecology of the species in our study may differ depending on their geographic locations, as all of these species occur in dry and disturbed habitats of the Mediterranean region.

Low within-species genetic variation was observed in polyploid Aegilops species compared with diploid Aegilops species. Nevo et al. (1982) and Hegde et al. (2000) observed similar low genetic diversity in wild tetraploid wheats. Galili and Feldman (1983) attributed the lack of genetic variation in polyploids to gene silencing, which is described as an evolutionary response of the species to nullify the deleterious effects of large-scale genome duplications. On the contrary, the experimental analysis of isozyme loci from different genomes indicated that few loci have been silenced in hexaploid wheat (Hart, 1983). The polyploid Aegilops species exhibited substantially more between-species genetic diversity than the diploid Aegilops. In addition to the ecological diversity and selection-adaptation argument, the higher estimates of between-species genetic variation in polyploid Aegilops species could also be due to the presence of diverse genome combinations (Table 1). There are few reported incidences of genome modification in polyploid Aegilops species compared with the original diploid progenitor genomes (Badaeva et al., 1998). In addition, it has been shown that, sometimes, isozyme polymorphism in polyploids arises by the association of polypeptides encoded by genes located in homeologous chromosomes (Hart, 1983).

The allelic polymorphism for a locus sometimes can arise as a consequence of its genetic linkage with another polymorphic locus. The genetic linkage between loci is invoked to explain polymorphism that persists in inbreeders (Jain, 1983). We calculated pairwise linkage estimates according to Weir (1979) between polymorphic loci in all possible combinations for the polymorphic populations of three presumed inbreeders Ae. tauschii, Ae. crassa and Ae. triuncialis. Of these three species, only the Ae. triuncialis population from California exhibited significant linkage (p < 0.05) between the two alleles of the locus Pgm-1S and the two alleles of the locus Pgm-1F. The locus Pgd-2, the only remaining polymorphic locus in this population, did not show any linkage effect with other loci. Therefore, part of the genetic variability in the California Ae. triuncialis population could be the result of linkage between polymorphic loci.

The obligate outcrossers, Am. muticum and S. cereale, had less than expected genetic variation despite their ability to generate heterozygous individuals. Genetic diversity estimates of S. cereale from Southern California populations (H_e = 0.09 - 0.13) were lower than reported earlier for this species from Northern California and Southern Oregon (H_e = 0.29 - 0.35) (Sun and Corke, 1992). If reduced genetic variation is the causal factor for the colonizing success of many introduced species, as suggested for plants (Allard, 1965) or empirically shown for an ant species (Tsutsui et al., 2000), then the lower genetic variation observed for the weedy S. cereale could be viewed as an evolutionary strategy of this species to enhance its colonization success. Alternatively, it may mean that the populations tested were already similar in origin, thus reducing the variation and in most annuals, their life form (growth habit) supports the rapid colonization specifically of disturbed sites.

Genetic Distance and Species Relationship
Diploid species of Aegilops were genetically more related to each other than to polyploid species (Table 5). The average genetic distance among diploid Aegilops species was 0.18 ± 0.10; between diploid Aegilops species and Am. muticum was 0.33 ± 0.07; between diploid Aegilops species and S. cereale was 0.52 ± 0.06; and among polyploid Aegilops species was 0.64 ± 0.28. Current or historical gene flow is one of the major factors influencing genetic differentiation between related species. The experimental evidence (Zohary and Feldman, 1962) indicates that tetraploid Aegilops species are loosely interconnected through occasional hybridization and gene-flow and diploid Aegilops species are more distinct among themselves. This is also supported by the observation of more natural hybrids among polyploid Aegilops species than among diploid Aegilops species (van Slageren, 1994). However, we did not observe significant gene flow among diploid (Nm = 0.06) or polyploid Aegilops species (Nm = 0.02). Therefore, the higher degree of genetic differentiation among polyploid Aegilops species might be the result of genome modification (Badaeva, 1998) or the consequence of homeologous chromosome interactions (Hart, 1983).

View larger version (20K): [in this window] [in a new window]   Fig. 1. UPGMA phenogram based on genetic distances for populations of Aegilops, Amblyopyrum, and Secale species (refer to Table 1 for the population descriptions).

Our genetic diversity study in wild and weedy populations of Aegilops, Amblyopyrum, and Secale species suggests that the majority of Aegilops species were genetically less diverse than other annual grasses, while allogamous Am. muticum and S. cereale exhibited less than expected genetic diversity. Overall genetic diversity within individual polyploid Aegilops species is less compared to diploid Aegilops species, yet the polyploids are genetically more diverse from each other than the diploids. Whenever more than one population of a species was analyzed, genetic diversity was found to be spatially distributed between populations within a species. Therefore, for germplasm collection, it may be more profitable to sample a large number of populations of each species from diverse ecological sites to maximize genetic variation in the collection. Aegilops species are excellent model organisms to address several systematic and evolutionary questions because of their diverse genome compositions, different ploidy levels, and ready availability of extant natural populations from their center of origin.

	ACKNOWLEDGMENTS

 
We thank Alptekin Karagöz, Simon Khairella, Mark Nesbitt, and Andrew Sanders for help with collections and Norman Ellstrand for comments and suggestions. J.G.W. received support from NCISE-IARC Pilot Linkage Program during germplasm collection.

Received for publication November 13, 2000.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

• Allard, R.W. 1965. Genetic systems associated with colonizing ability in predominantly self-pollinated species. p. 49–76. In H.G. Baker and G.L. Stebbins (ed.) The genetics of colonizing species. Academic Press, Inc., New York.
• Badaeva, E.D., B. Friebe, S.A. Zoshchuk, A.V. Zelenin, and B.S. Gill. 1998. Genome differentiation in diploid and polyploid Aegilops species. p. 61–64. In A.E. Slinkard (ed.) Proc. 9th Intl. Wheat Genet. Symp., Vol. 1, Saskatoon, Canada. 2–7 Aug. 1998. University Extension Press, Saskatoon, SK, Canada.
• Beauchamp, R. 1986. A flora of San Diego County, California. Sweetwater River Press, National City, CA.
• Braun, H.J., T.S. Payne, A.I. Morgounov, M. van Ginkel, and S. Rajaram. 1998. The challenge: one billion tons of wheat by 2020. p. 33–40. In A.E. Slinkard (ed.) Proc. 9th Intl. Wheat Genet. Symp., Vol. 1, Saskatoon, Canada. 2–7 Aug. 1998. University Extension Press, Saskatoon, SK, Canada.
• Donald, W.W., and A.G. Ogg. 1991. Biology and control of jointed goatgrass (Aegilops cylindrica), a review. Weed Tech. 5:3–17.
• 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. 87:9640–9644.[Abstract/Free Full Text]
• Felsenstein, J. 1994. PHYLIP (Phylogeny Inference Package) Version 3.5c. Department of Genetics, University of Washington, Seattle, WA.
• Galili, G., and M. Feldman. 1983. Diploidization of endosperm protein genes in polyploid wheats. p. 1119–1123. In S. Sakamoto (ed.) Proc. 6th Intl. Wheat Genet. Symp., Kyoto, Japan. 28 Nov.–3 Dec. 1983. Plant Germ-Plasm Institute, Kyoto University, Japan.
• Hamrick, J.L., and M.J.W. Godt. 1990. Allozyme diversity in plant species. p. 43–65. In A.H.D. Brown et al. (ed.) Plant population genetics, breeding, and genetic resources. Sinauer Assoc., Inc., Sunderland, MA.
• Harlan, J.R., and J.M.J. de Wet. 1971. Toward a rational classification of cultivated plants. Taxon 20:509–517.
• Hart, G.E. 1983. Hexaploid wheat (Triticum aestivum L. em Thell). p. 35–56. In S.D. Tanksley and T.J. Orton (ed.) Isozyme in plant genetics and breeding, Part B. Elsevier, Amsterdam, the Netherlands.
• Hegde, S.G., J. Valkoun, and J.G. Waines. 2000. Genetic diversity in wild wheats and goatgrass. Theor. Appl. Genet. 101:309–316.
• Jain, S. 1983. Genetic characteristics of populations. p. 240–258. In H.A. Mooney and M. Gordon (ed.) Disturbance and ecosystems: components of response. Springer-Verlag, Berlin.
• Kihara, H. 1944. Discovery of the DD-analyser, one of the ancestors of Vulgare wheats. (In Japanese). Agric. Hort. (Tokyo) 19:13–14.
• Kihara, H. 1954. Considerations on the evolution and distribution of Aegilops species based on the analyser-method. Cytologia 19: 336–357.
• Knott, D.R. 1989. The wheat rusts—Breeding for resistance (Monographs on Theor. Appl. Genet. 12). Springler-Verlag, Berlin.
• Medlinger, S., and D. Zohary. 1995. The extent and structure of genetic variation in species of the Sitopsis group of Aegilops. Heredity 74:616–627.
• Mujeeb-Kazi, A., V. Rosas, and S. Roldan. 1996. Conservation of genetic variation of Triticum tauschii (Coss.) Schmalh. (Aegilops squarrosa auct. non L.) in synthetic hexaploid wheats (T. turgidum L. s.lat. x T. tauschii; 2n=6x=42, AABBDD) and its potential utilization for wheat improvement. Genet. Resour. Crop Evol. 43: 129–134.
• Nei, M. 1987. Molecular evolutionary genetics. Columbia University Press, New York.
• Nevo, E., E. Goldberg, A. Beiles, A.H.D. Brown, and D. Zohary. 1982. Genetic diversity and environmental associations of wild wheat Triticum dicoccoides in Israel. Theor. Appl. Genet. 78:260–264.
• Rejesus, R.M., M. van Ginkel, and M. Smale. 1996. Wheat breeder's perspectives of genetic diversity and germplasm use. Wheat Special Rept. 40. CIMMYT, Mexico D.F.
• Seberg, O., and G. Petersen. 1998. A critical review of concepts and methods used in classical genome analysis. Bot. Rev. 64:372–417.
• Slatkin, M., and N.H. Barton. 1989. A comparison of three indirect methods for estimating average levels of gene flow. Evolution 43: 1349–1368.
• Smith-Huerta, N.L., A.J. Huerta, D. Barnhart, and J.G. Waines. 1989. Genetic diversity in wild diploid wheats Triticum monococcum var. boeoticum and T. urartu (Poaceae). Theor. Appl. Genet. 78:260–264.
• Soltis, D.E., and P.S. Soltis. 1989. Isozymes in plant biology. Chapman and Hall, London.
• Spearman, C. 1904. The proof and measurement of association between two things. Am. J. Psych. 15:72–101.
• Sun, M., and H. Corke. 1992. Population genetics of colonizing success of weedy rye in Northern California. Theor. Appl. Genet. 82: 321–329.
• Tsutsui, N.D., A.V. Suarez, D.A. Holway, and T.J. Case. 2000. Reduced genetic variation and the success of an invasive species. Proc. Natl. Acad. Sci. 97:5948–5953.[Abstract/Free Full Text]
• van Slageren, M.W. 1994. Wild Wheats: a monograph of Aegilops L. and Amblyopyrum (Jaub. & Spach) Eig (Poaceae). Wageningen Agricultural University Papers 94–7, Wageningen, the Netherlands.
• Vavilov, N.I. 1940. The new systematics of cultivated plants. p. 549–566. In J. Huxley (ed.) The New Systematics. The Clarendon Press, Oxford, UK.
• Waines, J.G., and D. Barnhart. 1992. Biosystematic research in Aegilops and Triticum. Hereditas 116:207–212.
• Weir, B. 1979. Inferences about linkage disequilibrium. Biometrics 35:235–254.[Web of Science][Medline]
• Zohary, D. 1965. Colonizer species in the wheat group. p. 403–423. In H.G. Baker and G.L. Stebbins (ed.) The genetics of colonizing species. Academic Press, Inc., New York.
• Zohary, D., and M. Feldman. 1962. Hybridization between amphidiploids and the evolution of polyploids in the wheat (Aegilops–Triticum) group. Evolution 16:44–61.

This article has been cited by other articles:

				R. Baalbaki, N. Hajj-Hassan, and R. Zurayk Aegilops Species from Semiarid Areas of Lebanon: Variation in Quantitative Attributes under Water Stress Crop Sci., February 24, 2006; 46(2): 799 - 806. [Abstract] [Full Text] [PDF]

				K. Yamane and T. Kawahara Intra- and interspecific phylogenetic relationships among diploid Triticum-Aegilops species (Poaceae) based on base-pair substitutions, indels, and microsatellites in chloroplast noncoding sequences Am. J. Botany, November 1, 2005; 92(11): 1887 - 1898. [Abstract] [Full Text] [PDF]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (8) Citing Articles via Google Scholar Google Scholar Articles by Hegde, S. G. Articles by Waines, J. G. Search for Related Content PubMed Articles by Hegde, S. G. Articles by Waines, J. G. Agricola Articles by Hegde, S. G. Articles by Waines, J. G. Related Collections Weed Management Wheat Plant Genetic Resources