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CELL BIOLOGY & MOLECULAR GENETICS

Seedset on Synthetic Haploids of Durum Wheat

Cytological and Molecular Investigations

^a USDA-ARS, Northern Crop Science Lab., Fargo, ND 58105 USA
^b Dep. of Botany, Faculty of Sciences, Univ. of Aleppo, P.O. Box 12252, Aleppo, Syria

pjauhar{at}badlands.nodak.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Because of their great importance as cytogenetic and breeding tools, haploids have been produced in several crop plants, including wheat (Triticum aestivum L.). Reports of seedset on haploid plants are very rare. Earlier, we produced 142 haploids (2n = 2x = 14; AB genomes) of seven commercial durum wheat (Triticum turgidum L.) cultivars (Cappelli, Durox, Langdon, Lloyd, Medora, Monroe, and Renville) by crossing them with maize (Zea mays L.). Of these, we studied 101 haploids. Some haploids from each of the cultivars set seed without colchicine treatment or cross pollination. The cytological basis of this interesting phenomenon was studied. Because all cultivars have the homoeologous pairing suppresser Ph1, their haploids formed mostly univalents and had irregular meiosis. Yet, viable seed was formed on some haploids. The seedset varied with the genotype. Langdon, with a mean of 2.75 seeds per haploid, was the highest yielder. These seeds gave rise to normal disomic (2n = 4x = 28; AABB) plants. The seeds had viable embryos formed by fusion of unreduced male and female gametes with 14 chromosomes each. The unreduced gametes were formed by two closely related first division restitution mechanisms resulting in meiotic non-reduction: (i) complete failure of movement of univalents at anaphase I, followed by normal second (equational) division, and (ii) anaphase I movement of all univalents to one pole. Thus, formation of these gametes bypassed the reductional division but occurred by normal equational division. It is hypothesized that lack of pairing may be a prerequisite for the occurrence of meiotic restitution and hence chromosome doubling. Fluorescent GISH (genomic in situ hybridization) analyses of somatic and meiotic chromosomes of the haploid-derived plants showed the complete duplication of both the A- and B-genome chromosomes. Fertility of the derived disomics and the presence of two doses of the marker chromosome involving the 4A·7B translocation, an evolutionary landmark of durum wheat, further corroborated the precise duplication of all chromosomes. We found that the distal segment translocated from chromosome 7B constitutes approximately 24% of the long arm of 4A.

Abbreviations: DH, doubled haploid • FDR, first division restitution • FISH, fluorescent in situ hybridization • FITC, fluorescein isothiocyanate • GISH, genomic in situ hybridization • PI, propidium iodide • PMCs, pollen mother cells • SDR, second division restitution

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
HAPLOID PLANTS have half the somatic chromosome number and are powerful cytogenetic and breeding tools. A large number of haploids have been produced in cereal crops including wheat (Kasha et al., 1990; Jauhar et al., 1991; Riera-Lizarazu and Mujeeb-Kazi, 1993). They can be employed in several areas of fundamental research in genetics and cytogenetics, e.g., isolation of aneuploids (Sears, 1954) and elucidation of genetic control of chromosome pairing (Sears, 1976; Jauhar et al., 1991, 1999). The haploid-derived homozygous lines provide a rapid means of achieving homozygosity, thereby accelerating breeding programs (Baenziger, 1996; Khush and Virmani, 1996). By crossing durum wheat (T. turgidum L., 2n = 4x = 28; AABB) with maize, we produced euhaploids (2n = 2x = 14; AB genomes) of seven commercial cultivars (Cappelli, Durox, Langdon, Lloyd, Medora, Monroe, and Renville) of durum wheat (Almouslem et al., 1998) and studied both inter- and intragenomic chromosome pairing (Jauhar et al., 1999). Although the corresponding (homoeologous) chromosomes of the A and B genomes are capable of pairing with one another, a homoeologous pairing suppresser gene, Ph1 (located in the long arm of chromosome 5B), suppresses homoeologous pairing (Sears, 1976). Consequently, only homologous partners pair in tetraploid durum wheat, which ensures diploid-like pairing and disomic inheritance. Even when each chromosome in the durum haploids is present in a single dose, Ph1 does not permit pairing among homoeologues. However, in the absence of Ph1 extensive homoeologous pairing takes place (Jauhar et al., 1999).

In this study, we investigated 101 Ph1-haploids derived from seven commercial durum cultivars. Although all haploids formed mostly 14 univalents, as expected, and showed very irregular meiosis, at least some haploids in each of the seven durum cultivars produced viable seed without colchicine treatment or cross pollination. Reports of seedset on haploid plants are very rare. We investigated the meiotic basis of this interesting phenomenon. Employing fluorescent in situ hybridization (FISH) or [fluorescent genomic in situ hybridization (GISH)], we studied the chromosomal composition of the seed-derived disomic plants. Cytological and molecular investigations on the causes of seedset in synthetic durum haploids are described and the breeding implications of seedset are briefly discussed.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Haploids of seven durum cultivars, viz., Cappelli, Durox, Langdon, Lloyd, Medora, Monroe, and Renville, which we produced earlier via hybridization with maize (Almouslem et al., 1998) were studied cytogenetically. During the course of these studies, we observed seedset on some haploids of each of these cultivars. The seeds were harvested for further studies.

Production of Plants from Seeds
Thirty-eight of the healthier looking seeds (Cappelli, 1; Durox, 6; Langdon, 7; Lloyd, 3; Medora, 7; Monroe, 4; and Renville, 10) from the durum haploid plants were germinated in petri dishes on moist filter paper. The seedlings were planted in a greenhouse [20–23°C, with an 8-h dark period, a 16-h light period (supplemental lighting in conjunction with natural lighting)] in 14-cm-diam pots containing Sunshine Mix No. 1 (Sun Gro Horticulture, Bellevue, WA) and fertilized 7 d later with Osmocote Plus (15-9-12) (Scotts-Sierra Hort. Prod., Marysville, OH) and raised to maturity.

Chromosome Studies by Conventional Techniques
Somatic chromosomes were studied from root-tips taken from the sprouted seed according to the techniques described earlier (Jauhar, 1993; Almouslem et al., 1998). For meiotic studies, immature spikes at the appropriate stage were fixed in Carnoy's fluid containing 95% (v/v) ethanol:chloroform:glacial acetic acid (6:3:1). Anthers were squashed and chromosomes stained with acetocarmine or carbol fuchsin according to procedures described earlier (Jauhar and Almouslem, 1998).

Fluorescent In Situ Hybridization (FISH) Studies on Somatic and Meiotic Chromosomes
Both somatic and meiotic chromosome spreads were obtained by squashing appropriately fixed root-tips and anthers in 45% (v/v) acetic acid. The chromosome spreads were covered with a glass cover slip and observed under a phase contrast microscope. Chromosome numbers were recorded. Slides containing well-spread metaphase chromosomes were transferred to a -80°C freezer and kept for up to one month, if necessary.

FISH was conducted on well spread chromosome preparations with total genomic Triticum urartu Tumanian DNA as the probe (100 ng/slide, labeled with biotin-14-dATP), and Aegilops speltoides Tausch genomic DNA as the blocker (2000 ng/slide). Hybridization protocols standardized earlier (Jauhar et al., 1999) were followed. Propidium iodide (PI) was used as a counterstain and fluorescein isothiocyanate (FITC)-conjugated avidin DCS was used to detect the probe.

Slides were observed under a Zeiss Axioskop epi-fluorescent microscope with a Zeiss AttoArc 100 watt adjustable UV light source. To visualize the FITC signal of the labeled probe a Zeiss set code 09 excitation filter with a Chroma D535/40 m (Chroma Technology Corp., Brattleboro, VT) emission filter was used. For the PI counterstain a Zeiss set code 00 excitation filter with a Chroma D605/55 m emission filter was used. Images were captured with a SPOT II digital camera (Diagnostic Instruments, Inc., Sterling Hts., MI) connected to an IBM compatible computer using the camera software supplied by the manufacturer. Images were oriented to the proper layout using Paint Shop Pro v. 5 (JASC Software Inc., Minnetonka, MN). Final images were printed with a color printer.

	Results

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Using the maize technique, we earlier produced 101 durum haploids (2n = 2x = 14) (Fig. 1A) with Ph1 (Almouslem et al., 1998) and studied their meiosis in pollen mother cells (PMCs) (Jauhar et al., 1999). These Ph1-haploids formed mostly 14 univalents at metaphase I (average of 1350 PMCs = 0.2 II + 13.6 I, Jauhar et al., 1999), and consequently showed numerous abnormalities at anaphase I and telophase I. It is remarkable that despite the meiotic abnormalities, at least some haploids of each of the durum cultivars produced seed (Fig. 1B; Table 1) . We investigated the cytological basis of this seedset on haploids. Using fluorescent in situ hybridization (FISH), we elucidated the chromosomal constitution of the seed-derived disomic plants. Some interesting meiotic abnormalities that led to seedset are listed below.

View larger version (51K): [in this window] [in a new window]   Fig. 1 (A) Parental durum plant cv. Durox (left); and its haploid (right). (B) Normal durum seed (left); somewhat shriveled seed from a haploid plant (right)

View larger version (52K): [in this window] [in a new window]   Fig. 2 Meiotic stages in durum haploids leading to first division restitution (FDR) nuclei and dyads. (A) All 14 univalents arranged on metaphase plate. (B) All univalents split into chromatids at meta-anaphase I. (C) All divided univalents fail to move to poles and undergo restitution. (D) An FDR nucleus undivided (left), and second (equational) division of an FDR nucleus producing a dyad (right)

View larger version (82K): [in this window] [in a new window]   Fig. 3 Unequal chromosome separation at anaphase I in durum haploids. (A) 7:7 separation. (B) 6:8 separation. (C) 5:9 separation. (D) 4:10 separation. (E) 3:11 separation. (F) 2:12 separation. (G) 1:13 separation. (H) 0:14 separation, where all chromosomes move to one pole. (I) Telophase I with all chromosomes on one pole

View larger version (12K): [in this window] [in a new window]   Fig. 4 Fluorescent in situ hybridization (FISH) study on random anaphase movement of univalents to the poles in durum haploids. (A) Propidium iodide (PI)-stained chromosomes. (B) Same cell as in A after FISH, showing 4:3 distribution of A-genome chromosomes and 3:4 distribution of B-genome chromosomes. (C) PI-stained chromosomes. (D) Same cell as in C after FISH, showing 2:5 distribution of A-genome chromosomes and 5:2 distribution of B-genome chromosomes

The seed on haploid plants was generally shriveled, compared to the plump seed on normal durum cultivars (Fig. 1B). Shriveling was due to poor development of endosperm, presumably caused by the haploid nature of the plant; less nutrition was available for endosperm formation. However, the seeds were viable. The 38 seeds selected from the seven cultivars showed 100% germination in petri dishes under laboratory conditions. It is possible that with double fertilization, the gametes that produced the endosperm might not be balanced and may have led to shriveling.

Production of Disomic (2n = 28) Plants
Plants obtained from seed from haploids were disomic and normal, with a typical phenotype of the parental durum cultivar from which they were derived. They were vigorous and tillered as well as the parental cultivar and were fully fertile. No height or leaf measurements were taken.

FISH Analysis of Somatic Chromosomes of Seed-Derived Plantlets
Chromosome counts from root-tips of seed-derived plantlets confirmed their disomic status (Fig. 5A and C) . Fluorescent GISH, using total genomic probe of T. urartu (the A-genome donor), showed the complete duplication of the A-genome and B-genome chromosomes (Fig. 5B and D). The chromosomes involving the 4A·7B translocation, the evolutionary signature of durum wheat, were easily observed (Fig. 5B and D). We found that the distal segment translocated from chromosome 7B constitutes about 24% of the long arm of 4A.

View larger version (106K): [in this window] [in a new window]   Fig. 5 Somatic and meiotic chromosomes of disomic plants derived from seedset on durum haploids. (A) 28 chromosomes counterstained with propidium iodide (PI); (B) Fluorescent in situ hybridization (FISH) analysis of the same cell (hybridized with the A-genome probe) showing 14 brightly lit chromosomes of the A genome; note 14 chromosomes of the B genome faded into the background. Note arrows showing the chromosome 4A·7B translocation. (C) PI-stained 28 chromosomes. (D) The same cell as in C after FISH, showing 14 fluorescing A-genome chromosomes, along with 14 faded B-genome chromosomes. Also note arrows showing the chromosome 4A·7B translocation. (E) 14 bivalents of seed-derived disomic plants; note 13 ring II and 1 rod II. (F) FISH of the same cell as in E (hybridized with the A-genome probe) showing seven fluorescing bivalents of the A genome, and seven bivalents of the B genome faded into the background. This further shows precise duplication of the A- and the B-genome chromosomes

	Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
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), even though mitotic chromosome doubling may also occur spontaneously (Jauhar and Singh, 1969). Meiotic nonreduction and functioning of unreduced male and female gametes leads to chromosome doubling in intergeneric hybrids of grasses (see for reference, Xu and Joppa, 1995). The unreduced gametes generally arise through two types of meiotic restitutions: the first division restitution (FDR), or second division restitution (SDR) (Peloquin et al., 1989a,b; Tai, 1994). Haploids have been produced in numerous species of grasses including cereals: rice (Oryza sativa L., Gosal et al., 1997), wheat (e.g., Jauhar et al., 1991; Riera-Lizarazu and Mujeeb-Kazi, 1993; Baenziger, 1996; Hu, 1997), maize (Büter, 1997), barley (Hordeum vulgare L., Kasha et al., 1990; Pickering and Devaux, 1992; Forster and Powell, 1997), oat (Avena sativa L., Rines et al., 1997), and rye (Secale cereale L., Deimling and Flehinghaus-Roux, 1997). However, reports of seedset on haploids are extremely rare. We observed seedset on haploids of all seven commercial durum wheat cultivars and investigated the cytological basis of this interesting phenomenon. We discovered that two types of meiotic abnormalities in our durum haploids played a major role in producing fertile seeds: (i) FDR, and (ii) anaphase I movement of all chromosomes to one pole.

FDR in pollen mother cells resulted in the inclusion of all 14 univalents (or 28 chromatids) in one nucleus, which on equational division led to the production of unreduced, functional gametes. It is reasonable to assume that this phenomenon also occurred during megasporogenesis and produced unreduced female gametes. And the fusion of unreduced male and female gametes produced normal seed which gave rise to disomic durum plants. FISH analysis showed the complete duplication of the A- and B-genome chromosomes. Fertility of the derived disomics and the presence of two of the marker chromosomes involving the 4A·7B translocation, a typical evolutionary signature of durum wheat, further testified to the precise duplication of all chromosomes. Meiotic nonreduction is of common occurrence in nature. As early as 1930, Aase stated: "Haploidy may be changed to diploidy in the meiotic division through nonreduction of univalents." Meiotic restitution is known to induce chromosome doubling and hence fertility in several interspecific and intergeneric hybrids of grasses (Maan and Sasakuma, 1977; Jauhar, 1993; Xu and Joppa, 1995). It is important to note that in all these wide hybrids, there is very little chromosome pairing, if any. We observed this phenomenon mostly in our durum haploids which had Ph1 and hence no pairing. It is possible that lack of pairing may be a prerequisite for the occurrence of meiotic restitution and hence chromosome doubling.

Seedset was observed on maize and oat haploids; both sets of haploids had little or no chromosome pairing. In a population of 282 maize haploids (monoploids), Chase (1949) found that 139 shed pollen, 68 formed kernels, and 34 yielded self-pollinated progeny. This self-fertility was attributed to spontaneous chromosome doubling in cells giving rise to reproductive tissue (Chase, 1949). Rines and Dahleen (1990) found that haploid oat plants recovered from oat x maize crosses were partially self-fertile with up to 23% seedset. However, they found that many of the plants grown from this seed were aneuploids.

Another interesting meiotic abnormality observed in durum haploids was the movement of all 14 univalents to one pole at anaphase I. This phenomenon has essentially the same consequences as the FDR described above. When all univalents move to one pole, they, in essence, bypass the reductional division of meiosis and then, on equational division, produce unreduced gametes. In spontaneous haploids of pearl millet, Jauhar (1970) and Powell et al. (1975) observed that all seven univalents moved to one pole in some PMCs and possibly contributed to the development of unreduced gametes.

The phenomena described above could have breeding implications. In both phenomena, the diploid chromosome complement is first restored and then the diploid complement undergoes mitotic (equational) division resulting in unreduced gametes (microspores during male meiosis and megaspores during female meiosis). During the equational division, sister chromatids of each chromosome (univalent of durum haploids) move to opposite poles, and therefore the two resultant nuclei are essentially similar to each other and to the parental meiocyte. The haploid-derived homozygous lines or doubled haploids (DH) may prove useful in basic cytogenetic studies, in mapping, and in practical plant breeding. Thus, DH can be employed in developing comprehensive maps and analysis of quantitative trait loci (Hayes et al., 1996; Cadalen et al., 1998). The instant homozygosity derived through chromosome doubling of haploids can accelerate breeding programs (Baenziger, 1996; Khush and Virmani, 1996). To be useful, however, an efficient method of DH production is necessary. Our durum haploids produced a low frequency of DH. However, there was a genotypic variation for DH production in our durum cultivars and it may or may not be possible to select for this trait. It is nevertheless significant that we obtained doubled haploids in all seven commercial cultivars of durum wheat without colchicine treatment or cross pollination.Aase 1930

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the USDA or imply approval to the exclusion of other products that also may be suitable.

Received for publication March 1, 2000.

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