The Effects Of Wheat Ph Genes On Homoeologous Chromosome Recombination

Hexaploid wheat has three pairs of genomes:A,B and D.Each genome has seven chromosomes,each chromosome in a genome has its genetic equivalents in the other two genomes;together such three genetically related chromosomes form a homoeologous group.Therefore,hexaploid wheat has 21 pairs of identical chromosomes,called homologues,and seven sets of homoeologues.Homoeologues originated from a common ancestor,and retain considerable DNA sequence similarity even if they may be different in size and appearance.Because of the considerable DNA sequence similarity,homoeologues have retained the potential for meiotic pairing.However,in wheat only homologues pair in MI of meiosis,regularly forming bivalents and guaranteeing regular segregation of chromosomes.This diploidizing meiotic behavior in polyploid wheat is imposed by Pairing homoeologous（Ph）genes.The pairing control in wheat is a complex gene system which consists of several loci.Based on the phenotype they produce,these genes can be classified into two groups:suppressors and promoters.Under normal conditions this system precisely limits meiotic pairing to homologues,thus enforcing stable diploidization.On the other hand,the restriction to strictly homologous pairing prevents any attempts to transfer desirable genes from related species to wheat by meiotic recombination.Pairing promoters with strong effects,or the absence of suppressors,may break through this barrier.One of the known promoters of the Ph system in wheat is the ph-KL gene（s）originating from a Chinese wheat landrace Kaixianluohanmai.In its presence,increased meiotic homoeologous paring was observed in hybrids of KL×rye and KL×Ae.variabillis,The major gene of the Ph system in wheat is Ph1,located on the long arm of 5B.It is a very strong suppressor of homoeologous pairing.Its removal permits homoeologous pairing and recombination of wheat and related chromosomes.This dissertation focuses on two Ph genes,and the main results are below:1.The characteristics of homoeologous pairing in haploids induced by ph-KL.In the hybrids of KL×rye,the frequencies were A-D（77.88%）、B-D（8.85%）、A-B（6.19%）、W-R（4.42%）and others（2.65%）.The most closely related wheat genomes（A and D）paired with the highest frequency.A haploid of the KL line,generated by the Hordeum bulbosom Method,averaged 3 paired chromosome arms per cell,and the majority were rod bivalents.High temperature reduced the pairing frequency in KL haploid;supplementation with2mmol/L Mg²⁺Hogland solution did not appear to significantly affect meiotic pairing in KL haploids.2.The ph-KL is capable of inducing the recombination of wheat-alien chromosomes in hexaploid.Three populations were generated by crossing KL with substitution lines of 2S^v（2B）,2S^v（2D）and the T2BS.2RL translocation line,followed by a backcross to KL.Plants with single alien chromosomes or the translocation（effectively,double monosomics or translocation heterozygotes）were self-pollinated to produce recombinant populations.For the 2S^v-2B population,seven KASP markers differentiating between 2S^v-2B were developed and used to select recombinant chromosomes.In total,2210 plants were screened,some were also genotyped by the 55K SNP array and by in situ hybridization.Forty six recombinants were identidied,suggesting the frequency of recombination of2.08%.For the 2S^v-2D population,four KASP markers differentiation between 2S^v-2D were developed and one primary recombinants among 792 screened plants were identified,with the recombination frequency of 0.13%.In the 2RL-2BL population,1000 plants were screened by C-banding and no recombinants were identified.3.The physical mapping of gene（s）controlling the late-heading trait and tenacious glume trait in 2S^vchromosome,by using a set of 34 recombinants of 2S^v-2B induced by the ph-KL system.Based on the physical map of chromosome 2B,the late-heading locus originating from chromosome 2S^v（designated as Ppd-2S^v）was located on the short arm at the distance of 46.6Mb from the centromere;the tenacious glume locus from chromosome2S^v（designated as Tg-2S^v）was located on the short arm between 24.6-46.4Mb。4.The major locus of ph-KL is mapped on the long arm of chromosome 3A using bi-parental populations.Three polyhaploid populations with genome constitutions ABDR were generated by crossing Syn-SAU-5、CS or CM32（low pairing parents）with KL（high pairing parent）and test crossed with rye.A QTL analysis of 133 plants of KL/Syn-SAU-5//QL population revealed one major QTL for chromosome pairing located on 3AL.Similarly,a QTL analysis of 79 plants of the KL/CS//QL population also suggested one major QTL located on 3AL.Therefore,one major QTL for the KL effect in wheat is proposed on chromosome arm 3AL,designated as QPh.sicau-3A.This QTL is located in the interval 696-725Mb of the CS reference genome assembly version 1.0.SNPs flanking the locus were identified and processed into KASP markers,and used to genotype the KL/CM32//QL and 2S^v-2B populations.Plants carrying the QPh.sicau-3A locus had higher levels of meiotic chromosome paring than those absent the locus.Therefore,we propose that locus QPh.sicau-3A in KL is responsible for promoting homoeologous pairing.5.The characteristic of meiotic pairing when Ph1 present and absent.Crossover points were analyzed between three wheat and rye homoeologous chromosome arms,and compared to homologous crossovers in a reference wheat population.The results showed that the general patterns of homologous（with Ph1）and homoeologous（with ph1b）crossing over were the same.In all intervals analyzed,homoeologous crossovers fell within the range of frequency distribution of homologous crossovers among individual families of the reference population.No specific DNA sequence characteristics were identified that could be targeted by the Ph1 locus;the only difference between homologous and homoeologous crossing over appears to be in frequency.This study shed a light on the understanding of the mechanism of Ph gene in genetics and functions.It also enriched the theory of chromosome engineering in inducing wheat-alien chromosome recombination.