Study Of The Meiotic Chromosome Stability In The Early Stage Of Allohexaploid Wheat Formation

Common wheat(Triticum aestivum L.)is one of the three major food crops and undertakes the food security around the world.The evolutionary of common wheat(Triticum aestivum L.,BBAADD,2n = 6X = 42)contained two sequential allopolyploidization events: the first one leading allotetraploidization about 0.5 million years ago and the second one leading allohexaploidization about 8 000 years ago.As a young allopolyploid species,common wheat represented a typical model of speciation via allopolyploidy.Though euploidy was the case in natural hexaploidy wheat,newly synthesized allohexaploid wheat was associated with widespread and persistent whole-chromosome aneuploidy and showed no significant difference of fitness between euploid and aneuploid at least under laboratory conditions.The key issue for resolving the emergent challenges of meiotic instability at the initial stage of hexaploid wheat formation and enhancing its fitness to become a successful species remained poorly understood.We have subjected cohorts progenies of a synthetic allohexaploid wheat in early generation to survive from the simulant conditions of allohexaploid wheat headstream and selected out the tolerant ones with increased euploidy frequency.We combined the meiotic chromosome behavior analysis and transcriptome sequencing to systematically study the issue and find out the related meiosis gene candidates in allopolyploid wheat.First,we subjected progenies of a synthetic hexaploid wheat to heat or salt stress.After obtaining the stress-tolerant individuals,we karyotyped each plant by using sequential FISH/GISH.The result showed significant higher euploidy ratio(~ 80%)in both of the stresstolerant groups compared with the mock control groups(35.5%).To approve the heritability of tolerance and karyotype stability,we continued resistance detection and karyotyping for three successive generations.The result showed that both the stress-tolerance and enhanced karyotype stability phenotypes are transgenerationally heritable(73.68%-93.33%).Second,we conducted meiosis analysis on the progeny plants already three generations away from the abiotic stresses.the result showed that the major clear abnormality in pollen mother cells(PMCs)at metaphase I(MI)was the presence of univalent and bivalent disjunction was detected in a proportion of PMCs;in telophase I(TI)of abnormal PMCs,lagging chromosomes were observed.Both the chromosome and subgenome biases in forming meiotic univalent are significant and largely consistent with their biases towards whole-chromosome aneuploidy detected in somatic cells.Chromosome 4B showed the highest frequency of univalent among the 21 chromosomes.Considering each subgenome as a whole,subgenoeme B showed the highest frequency of univalent formation.The frequency of univalent,early disjunction bivalents and lagging chromosomes were reduced in stress-tolerant groups compared with the mock control group,which indicated that the meiotic chromosome stability was significantly attenuated in progenies of the stress-tolerant plants.In order to investigate the difference of gene expression among stress-tolerant groups and the mock control group,we conducted transcriptome-based comparative analysis of RNAs isolated from leaf and four representative developmental stages of anthers corresponding to pre-meiotic,meiotic I,meiotic II and tri-nucleate pollens.As an additional control in this experiment,the standard laboratory cultivar,Chinse Spring(CS),of natural common wheat(T.aestivum)with default normal meiosis,was also included.Based on the candidate meiosis genes in Arabidopsis thaliana,we proposed a list of meiosis gene candidates in hexaploidy wheat.We identify a set of candidate meiosis genes with altered expression patterns in the stresstolerant plants vs.the control plants,which however are similar to natural common wheat(cv.CS).By performing the VIGS-mediated downregulation of either of two meiosis gene candidates in CS,we found that the frequency of univalent,early disjunction bivalents and lagging chromosomes were increased in the infected plants,which conferred an unstable meiosis phenotype mimicking the synthetic hexaploid wheat.Our results suggest that heritable changes in the expression of some critical meiosis genes may be selected as a spandrel of stress tolerance,which significantly improves meiosis stability in the synthetic hexaploid wheat.There is no immediate fitness advantage by evolving a more stable meiosis per se;however,due to its genetic correlation with another trait which is adaptive under a specific environmental context,a more stabilized meiosis was indirectly selected and maintained.Our results suggest a plausible solution to swiftly attenuate possible emergent meiosis challenges at the initial stages of hexaploid wheat speciation.