Chromosome Behaviors And Their Controlling Genes In Hybrids Of Wheat With Alien Species

Bread wheat (Triticum aestivum L.,2n=6x=42, AABBDD) is a classic example of allopolyploids, originating from the natural hybridization of T. turgidum L. (2n=28, AABB) and Aegilops tauschii Cosson (2n=14, DD), followed by spontaneous genome doubling via the union of unreduced gametes. T. turgidum genotypes are considered to play an important role on the production of unreduced gametes. However, the underlying genes have not been mapped in wheat. On the other hand, the ph gene system in wheat ensures the cytological diploidization with strict homologous pairing by preventing the homoeologous chromosome pairing among A, B, and D genomes. In previous study, common wheat landrace Kaixian-luohanmai (KL) is believed to harbor a recessive ph-like gene, ph-KL, different from the well-known genes phi and ph2.In the present study, we focused to study the two traits:the production of unreduced gametes and homoeologous chromosome pairing in wheat-alien hybrids. We analyzed the meiotic phenotypes and further mapped a major gene for the production of unreduced gametes. We developed an effective method for development of hexaploid triticale by the hybridization between synthetic hexaploid wheat with rye. Meiotic phenotypic differences on homoeologous chromosome pairing at metaphase I between hybrids of wheat genotypes Chinese Springphlb (CSph1b) and KL with rye were investigated by in situ hybridization techniques. The results were as follows:Observation of meiosis in pollen-mother-cells from T. turgidum xAe. tauschii hybrids indicated that first division restitution, which exhibited prolonged cell division during meiosis I, was responsible for unreduced gamete formation. Correlation analysis indicated that the dyad ratio was positively correlated with the seed set ratio among the three hybrid combinations, indicating high fertility resulted from high frequency dyad formation.Two T. turgidum x Ae. tauschii triploid populations and their corresponding hexaploid populations were used to map the quantitative trait locus (QTL) for unreduced gamete triat. A major QTL for this trait, named QTug.sau-3B, was detected on chromosome 3B in the triploid populations. This QTL may close to Ttam-3B, a collinear homolog of tarn in wheat. QTug.sau-3B is a major locus for formation of unreduced gametes. Although the relationship between QTug.sau-3B and Ttam requires further study, high frequencies of unreduced gametes may be related to reduced expression of Ttam in wheat.We developed an effective method for production of hexaploid triticale via hybridization of synthetic hexaploid wheat (SHW) with rye. The three employed SHW lines were derived from hybridization of T. turgidum with Aegilops tauschii Cosson, and inherited meiotic restitution genes, which can promote the formation of functional gametes in haploid status, from their T. turgidum parents. Although the resulting tetraploid F] hybrids with rye (genome ABDR) produced amphiploids (octoploid triticale) and partial amphiploids, the final hybrid products obtained through fertility selection over several generations were hexaploids. These hexaploids were the result of preferential elimination of D-genome chromosomes. In addition to complete hexaploid triticale with 28 intact A/B and 14 intact R chromosomes, we obtained hexaploid triticales with other chromosome constitutions, including monosomic, substitution, and translocation lines. Chromosomes 2D and 5D from the wild species A. tauschii were incorporated into the hexaploid triticales. Out of eight analyzed stable lines derived from three different SHW-Ll/rye Fi plants, we observed four lines with small-fragment translocations between wheat and rye chromosomes. Rapid production of hexaploid triticales using this method involves two factors:(1) hybridization between hexaploid wheat with a meiotic restitution gene(s) and rye and (2) selection for good fertility during F3 and subsequent generations.Meiotic phenotypic differences on homoeologous chromosome pairing at metaphase I between hybrids of wheat genotypes Chinese Spring ph1b (CSph1b) and KL with rye were studied by genomic in situ hybridization (GISH). The frequency of wheat-wheat associations was higher in CSph1b×rye than in KL×rye. However, frequencies of wheat-rye associations and rye-rye associations were higher in KL×rye than in CSph1bx×ye. These differences may be the result of different mechanisms of control between the ph-like gene(s) controlling homoeologous chromosome pairing in KL and phlb. Wheat-wheat associations were much more frequent than wheat-rye pairing in both hybriods. This may be caused by lower overall affinity, or homoeology, between wheat and rye chromosomes than between wheat chromosomes.Sequential FISH by using probes pTa-k229+pScD15887 and pSc119.2+pTa535 was used to identify the paired chromosomes in KL-rye F1 hybrids. We found that 80.78% paired chromosomes occurred between A-and D-chromosomes, similar to previously reported in CSph1b-rye hybrid (80.05%). Pairing level of B-R chromosomes was higher than A-R and D-R. Since out of 109 paired chromosomes,95% occurred within homoeologous groups, ph-KL is confirmed as aph gene that affects homoeologous pairing.