Genetic Variations Induced By Distant Hybridization Of Wheat-alien Species

Distant hybridization and allopolyploidization may induce genetic variations. Common wheat represents a textbook species of allopolyploids, originated from crossing Triticum turgidum L. with Aegilops tauschii Cosson, followed by spontaneously chromosome doubling. Wheat has a large number of alien species, including many allopolyploids. These speices with differentiated genetic relationships provide materials for studying on issues of distant hybridization and allopolyploidization. We here analyzed two hybrid models:T. turgidum×Ae. tauschii and common wheatxrye. The two species of the former has a closely genetic relationship and their hybrid can be used as a mimic of common wheat origination and create newly synthetic hexaploid wheat. The two species of common wheat-rye hybridization has a more distant relationship and no species is naturally produced by their genome combination although there is the man-made species, Triticale. In the present study, we investigate the chromosome variations on number and structure caused by the two hybridization modes by using genome in situ hybridization (GISH) and fluorescence in situ hybridization (FISH). We further analyze the effects of distant hybridization on genomic DNA using Simple sequence repeats (SSRs, also known as microsatellites) and DArT markers. The main results are following:In the analyzed nascent hexaploid wheat from T. turgidum x Ae. tauschii hybridization, there are high frequencies of chromosome variations on number contrast with low frequencies of chromosome variations on structure with genotype-dependent, indicating the structural stability. In the analyzed progines from common wheat-rye hybridization, however, both number and structure of chromosomes were frequently observed and indicated the cytological instability, a reflect of the conflict between wheat and rye genomes. The common wheat-rye hybrids finally lead two cytotypes:hexaploid with AABBDD genomes and octaploid with AABBDDRR genomes.SSR are known to be mutational hotspots in genomes. DNA rearrangements have also been reported to accompany allopolyploidization. A study of the effect of allopolyploidization on SSR mutation is therefore important for understanding the origin and evolutionary dynamics of SSRs in allopolyploids. Three synthesized double haploid (SynDH) populations were made from 241 interspecific F1 haploid hybrids between T. turgidum and Ae. tauschii through spontaneous chromosome doubling via unreduced gametes. Mutation events were studied at 160 SSR loci in the S1 generation (the first generation after chromosome doubling) of the three SynDH populations. Of the 148260 SSR alleles investigated in S1 generation, only one mutation (changed number of repeats) was confirmed with a mutation rate of 6.74 c 10-6. This mutation most likely occurred in the respective F1 hybrid. In comparison with previously reported data, our results suggested that allohexaploidization of wheat did not increase SSR mutation rate.We further analyzed 25 F10 lines of D-2-3-4/rye and 13 F10 lines of Chinese Spring/rye using 24 pairs of SSR primers and found a much higher SSR mutation rate than T. turgidum x Ae. tauschii hybrids, confirming the effects of hybrid species. The SSR mutation is 35.53% and caused by the changes of repeat units of SSR. Using 1173 DarT markers, the chromosome substitution and addition lines involved in chromosome 6R and 6A were identified. The common wheat parent D-2-3-4 and Chinese Spring showed a closely genetic distant of 0.05 based on 13061 DarTseq markers. However, the genetic distant was increased to 0.09 between the F10 lines of D-2-3-4/rye and Chinese Spring/rye, indicating an obvious contribution of distant hybridization. These F10 lines were clustered into three branches both for DArT and SNP markers, the lines form Chinese Spring/rye, the lines without rye chromosomes from D-2-3-4/rye, and the lines with rye chromosomes from D-2-3-4/rye. Moreover, some lines were formed in an extra branch, probably because they contained small rye chromosome fragments that was fail to detect.Based on the distributions of SNP on chromosomes, we compared the SNP variation on the two hybrid combinations and found a higher variation in Chinese Spring/rye than D-2-3-4/rye. In both populations, A-genome showed more variations than B-and D-genome and homoeologous 2 showed the highest variations. It is interesting that there had a high frequency of SNP deletions. As most of DArTseq methods are using methylation sensitive RE the polymorphism patterns produced by DArTseq include a component of methylation profiling and therefore are capable of detecting epigenetic variation. Therefore, partial variations may be caused by methylation.