Genetic Differentiation Within Aegilops Tauschii And Chromosome Variation In Synthetic Hexaploid Wheat

The D genome of common wheat（Triticum aestivum,2n=6x=42,AABBDD）is provided by Aegilops tauschii（2n=2x=14,DD）,which shows higher genetic diversity than common wheat.Based on spike morphology of conventional taxon,Ae.tauschii has been subdivided into ssp.tauschii（cylindrical spikelets）and ssp.strangulata（moniliform spikes bearing quadrate spikelets）.However,there is a poor match between this whole plant morphology-based subdivision and genetic relationships inferred from genotyping and the most apparent contradiction is encountered with accessions,which own spike morphology characteristic between classical ssp.tauschii and ssp.strangulata accessions.The nascent allohexaploid arisen from the whole genome doubling of polyhaploid hybrids between tetraploid wheat and diploid Ae.tauschii is called as synthetic hexaploid wheat（SHW）,which is an important bridage marterial to introgress gene from the wheat donor species into wheat and a model to stduy the genetic variation during the speciation of common wheat.In present study,we re-subdivided the intraspecific relationship within Ae.tauschii by combining morphological data,FISH（fluorescent in situ hybridization）karyotype and a previously assembled phylogeny based on marker genotyping.At the same time,the chromosome variation and genetic characteristics of synthetic wheat and its hybrids were analyzed by using genetic map and FISH technologies.The main results are as follows:1.Morphological reclassification of natural 405 accessions of Ae.tauschii from17 regions was carried out.Six spike morphological types were detected in Ae.tauschii,among them,S type and T type were typical spike morphological types of the two subspecies,belonging to L2 and L1 in phylogeny,respectively,and S1-S4were intermediate types belong to L2 in phylogeny.The distribution of sites hybridizing to the two probes oligo-p Ta-535 and（CTT）₁₀ classified the 31 Ae.tauschii accessions into two clades,designated D^tand D^s,which corresponded perfectly with the previously assembled phylogeny based on marker genotype.The D^tcluster was populated exclusively by ssp.tauschii accessions,while the D^s cluster harbored both ssp.strangulata and morphologically intermediate accessions.Therefore,it is proposed that ssp.tauschii is restricted to carriers of the D^tkaryotype in which their spikelets are regularly spaced along the rachis.Accessions classified as Ae.tauschii ssp.strangulata carry the D^s karyotype in which their spikelets are irregularly spaced.Based on this criterion,forms formerly classified as ssp.tauschii var.meyeri have been re-designated ssp.strangulata var.meyeri.The comparisons of the karyotyping and phylogeny between ssp.strangulata var.meyeri and the bread wheat D genome has suggested that the ssp.strangulata var.meyeri was the most probable D genome donor of bread wheat.2.An integrated genetic map of a recombinant inbred population bred from a cross between a synthetic hexaploid wheat and a commercial Chinese bread wheat cultivar Chuanmai 32（CM32）,based on 28 variable FISH sites and>150,000 single nucleotide polymorphism（SNP）loci.The majority（20/28）of the variable FISH sites were physically located within a chromosomal region consistent with their genetic location inferred from that of their co-segregating SNP loci.The eight exceptions reflected the presence of either a translocation（1R/1B,1A/7A）or a presumptive intra-chromosomal inversion（4A）.For eight out of nine FISH sites detected on the Chinese Spring karyotype,there was a good match with the reference genome sequence,indicating that the most recent assembly has dealt well with the problem of placing tandem repeats.The integrated genetic map produced for wheat is informative to locate blocks of tandemly repeated DNA and can aid in improving the quality of the genome sequence assembly in regions surrounding these blocks.3.Chromosome number and structural variations were not uncommon in synthetic wheat SHW-L1 and its F₂ hybrids and RILs with common wheat Chuanmai32.Single cross resulted higher frequence of aneuploids compared with SHW-L1.While the chromosome stablity of most progenies obtained by continuous self-pollination selection was improved,some lines still showed a high frequency of chromosome variation.We proposed that the accumulation of chromosome structure variation during self-pollination may lead to the obvious differentiation of chromosome stability among different lines;the aberrant meiosis,including univalent,lagging chromosome,and micronucleus were the possible reasons to cause chromosome variation in SHW and its hybrids with CM32;somatic chromosome variation was also a pathway of forming chromosome variation.The results of present study are helpful to deepen the understanding of the cytogenetic changes of synthetic wheat and its hybrids,and provide theoretical support for the breeding and utilization of SHWs.