Genetic Recombination Of The Wheat Pentaploidization And Characterization Of The Special Pentaploids Derivatives

Wheat is the main source of energy and nutrients for humans and has become one of the most important food crops in the world.Hexaploid wheat originated from the allopolyploidization between tetraploid wheat and diploid Aegilops tauschii.Due to the genetic evolutionary bottleneck caused by polyploidization,the genetic diversity of the AB genomes in hexaploid wheat was significantly lower than that of the tetraploid donor.Pentaploid hybrids produced by crossing hexaploid wheat with tetraploid wheat are a crucial bridge for continuously introgressing favourable genes from tetraploid wheat into hexaploid wheat after allohexaploidization and promoting the evolution of hexaploid wheat.Therefore,studying the genetic dynamics and contributions of the pentaploid hybrids in the introgression crossing between hexaploid wheat and tetraploid wheat is of great importance in understanding the gene introgression between hexaploid wheat and tetraploid wheat as well as the innovation and utilization of germplasm resources.In the present study,two groups of F₂ wheat populations of pentaploids and hexaploids（SPW1-SHW1 and SPW2-SHW2）were created to compare the differences in genetic recombination of AB genomes in the pentaploid and hexaploid background,and discuss the role that pentaploids played in the introgressive hybridization.Moreover,pentaploids were produced through crossing the amphiploid wheat derived from the cross between durum wheat and Aegilops tauschii with its tetraploid donor.The pentaploids were self-pollinated to produce progenies until the F₇ generation.The genetic constitution of chromosomes of individuals in the whole F₇ population was analyzed through cytogenetic technologies.The inheritance patterns of the D genome was analyzed as well.Besides,two special pentaploid wheat derivatives in the F₇population were identified and evaluated through cytogenetic,kompetitive allele-specific PCR and genome resequencing technologies.The main results are as follows:1.Effects of pentaploidization on the genetic recombination of AB genomes.Two F₂ wheat populations of pentaploids and hexaploids（SPW1-SHW1 and SPW2-SHW2）were used to analyze the genetic differences between pentaploidization and hexaploidization,including distorted segregation of polymorphic SNP loci,genetic maps,and the recombination frequency in AB genomes.Compared to hexaploidization,the recombination frequency between the adjacent and linked SNP markers in the AB genome of the pentaploidization population was significantly increased（P<0.01）and led to 3 to 4times higher recombination in the pentaploidization,according to the constructed genetic maps based upon the genotyping results.The high germination rate in the pentaploids from populations with the high ploidy hexaploid wheat as the female parent suggested a more efficient way to produce pentaploids derivatives.The high proportions of the distorted segregation markers that skewed in favour of the female genotypes/alleles in the pentaploidization populations might associate with the increase of the adaptability and survival of the offspring.The increase of recombination frequency along the whole chromosome plays a key role in creating newly elite allelic combinations and breaking up the genetic drag between desirable and deleterious genes.2.Characterization of the chromosome constitution and D genomes introgression in the F₇ progenies derived from pentaploids.The multicolour fluorescence in situ hybridization（mc-FISH）was used to analyze the F₇ progenies derived from pentaploids,which were produced from the hybridization between synthesized amphiploid hexaploid wheat Lanmai/AT23 and its tetraploid donor Lanmai.The results showed that 94.42%and3.55%of individuals were tetraploid and hexaploid types,respectively,and only 2.03%were aneuploid individuals.Two special germplasms,4D（4B）substitution line YL-443 and2DS-2AS.2AL translocation line YL-429,were identified from tetraploid type individuals.Besides,5.58%of individuals showed‘recessive’introgression of D genomes through the identification of the D genomic specific marker Dgas44,suggesting a homoeologous recombination between the AB genomes and the alien D genomes induced by the pentaploids.3.Characterization of the durum wheat-Ae.tauschii 4D（4B）substitution line with big spike and high resistance to stripe rust.The pentaploids derived line YL-443with a big spike and high resistance to stripe rust was identified as a Lanmai-AT23 4D（4B）disomic substitution line.Compared to tetraploid parent Lanmai,YL-443 significantly increases the spikelet number per spike,florets number per spike,and kernel number per spike,in which the florets number per spike and kernel number per spike increased 36.3%and 75.9%,respectively.YL-443 shows pubescent leaf sheath and leaf auricle,which trait is specifically controlled by the genes in chromosome 4D.Aegilops tauschii AT23 contains the stripe rust resistance gene Yr28 based upon the PCR amplification results.Therefore,it shows high resistance to stripe rust.The developed substitution line can be used for the genetic improvement of durum wheat on yield potential and stripe rust resistance.4.Characterization of the durum wheat-Ae.tauschii 2DS-2AS.2AL translocation line with non-glaucous.The derived line YL-429 was revealed as a translocation line that the small segment close to the telomeric region of the short arm of chromosome 2A in durum wheat Lanmai（glaucous）was replaced by the 0–14 Mb segment of chromosome2D in Aegilops tauschii AT23（non-glaucous）,resulting in a non-glaucous phenotype since this 2DS segment contained wax inhibitor gene Iw2.Gene ontology enrichment analysis indicates that the inserted genes contained in the 2DS segment may be involved in the terpene synthase activity and photosystem II reaction,which need further works to identify.In summary,the pentaploid hybrids that are used as a breeding bridge can significantly increase the genetic recombination of AB chromosomes,break the negative linkage drag,and promote the transfer of elite genes,leading to numerous genetic diversity recombinant individuals.Moreover,the pentaploid hybrids could promote the development of specific D-genome introgressive materials during the selfing process and be used for genetic improvement of durum wheat and bread wheat.