Meiotic Chromosome Behavior In Newly Synthesized Allotetraploid Wheat With Different Subgenome Combinations,and Its Impact On Fitness

As one of the pillars of food crops and the representative of young allopolyploids,wheat has been used as a typical experimental material for studying polyploid evolution for a long time.Common wheat（Triticum aestivum L.,BBAADD,2n=6X=42）has undergone two sequential allopolyploidization events,which contains three sets of genomes.The first allopolyploidization event occurred between genome BB and AA,while the second one occurred between BBAA and DD.Three diploid ancestral species are known to exist simultaneously in the Eastern Mediterranean and Near Eastern regions.Allotetraploid wheat produced by hybridization and genome doubling of any two of the three diploid species under laboratory conditions can form fertile offspring.However,in the history of wheat evolution,only o species of SSAA（S≈B）leading to formation of allotetraploid wheat（T.turgidum）successfully formed species and survived today.The genome compositions of allotetraploids and the relationship between evolution and speciation during wheat evolution have become one of the important studies.In this study,the early generations of four artificially constructed allotetraploid wheats AT1(S^shS^shA^mA^m),AT2（S¹S¹AA）,AT3（AADD）,and AT4（S^bS^bDD）early generations,together with natural formed wild allotetraploid wheat T.turgidum ssp.dicoccoides accession IW33（BBAA）and cultivated allotetraploid wheat T.turgidum ssp.durum cv.TTR13（BBAA）were used as materials.By using sequential FISH/GISH,we investigated meiotic chromosome behavior,pollen viability,karyotype stability of progeny and seed setting rate to understand the effect of chromosomal behavior during meiosis on phenotypic fitness of allotetraploid wheat with different genome compositions.We found that in the newly synthesized allotetraploid wheats,the stability of meiotic chromosome behavior of the two allotetraploid wheats with genome composition SSAA were higher than the allotetraploid wheats with genome compositions AADD and SSDD.The abnormal meiotic chromosome behaviors observed in the metaphase I（MI）included multivalent,univalent,early separation of bivalent and mismatch bivalent.For subgenome bias,B subgenome showed the highest frequency of chromosome variation,while D subgenome showed the lowest frequency.For chromosome bias,univalent tended to occur in the homeologous 4（4B chromosome in AT1 and AT4,4A chromosome in AT3）,multivalent tended to occur in the homeologous 2（homeologous 4 for AT3）,the mismatch bivalent tended to occur in the homeologous 1 in AT1 and AT2,homeologous 2 and 5 in AT3 and AT4,respectively.The karyotype stability analysis of progenies included the tendency of subgenome and chromosome.The tendency of meiotic chromosome variations was consistent with chromosomal variation in the progenies.For subgenome bias of chromosome number variation,B subgenome showed the highest frequency,followed by A and D subgenome.For chromosome structural variation,B subgenome showed the highest frequency of variation in the SSAA combination,while the variation frequency of D subgenome showed the highest frequency of variation in AT3（AADD）.D subgenome and S subgenome showed similar frequencies in AT4（S^bS^bDD）.For chromosome bias,chromosomal variation occurred respectively in homologous groups 6,5,2,and 4 in AT1(S^shS^shA^mA^m),AT2（S¹S¹AA）,AT3（AADD）and AT4（S^bS^bDD）.Compared the meiotic chromosomal behavior of AT3（AADD）and AT4（S^bS^bDD）with the chromosome variation in the progenies,we found that there was a certain correspondence,which was not observed in AT1(S^shS^shA^mA^m)and AT2（S¹S¹AA）.The abnormal variation of meiotic chromosome behavior had led to reduced pollen fertility and seed setting.The pollen viability of allotetraploid of composition SSAA was higher than the allotetraploid wheat of compositions AADD or SSDD.The pollen viabilities of natural and cultivated wheat were similar to the newly synthesized allotetraploid with genome SSAA.The order of seed setting rates from high to low was:cultivated allotetraploid wheat TTR13（BBAA）,wild allotetraploid wheat IW33（BBAA）,newly synthesized allotetraploid AT2（S¹S¹AA）,AT1(S^shS^shA^mA^m),AT3（AADD）,and AT4（S^bS^bDD）.In summary,cnaturally formed tetraploid wheats were superior than newly synthesized allotetraploid wheat in meiotic chromosome stability,pollen viability,progeny karyotype stability and seed setting rate.Among the newly synthesized allotetraploid wheats,those with genome compositions SSAA were better than those with genomes AADD and SSDD.Our results have provided new information regarding formation and evolution of tetraploid wheat and meiosis of polyploid in general.