Effects Of Aneuploids On Gene Expression In Synthetic Hexaploid Wheat

Common wheat(Triticum aestivum L.,2n = 6x = 42,AABBDD)is a typical allohexaploid species,arisen from the whole genome doubling of polyhaploid hybrids between tetraploid wheat(T.turgidum,AABB)and diploid Ae.tauschii(DD).Modern wheat produces low aneuploid individuals.In contrast,synthetic hexaploid wheat,created by somewhat duplicating the origin of bread wheat,has high frequency of aneuploidy.Interesting,the high resistance to anueploidy in synthetic hexaploid wheat is shared by natural bread wheat.However,the underlined mechanism for the high tolerance to aneuploidy remains largely unknown.In the present study,we identified the karyotypes of inbred progenies of five individual plants that have the knwon chromosome constitutions in the genetic background of synthetic hexaploid wheat(T.turgidum AS313 � Ae.tauschii AS60)using molecular and cytological methods.We then utilized transcriptome sequencing to analyze the global gene expression of aneuploid lines including whole-chromosome aneuploids on 4B and 7B,and segmental aneuploid on 2A,in order to reveal the molecular mechanism of resistance to aneuploidy.The main results are as follows:1.High frequencies of chromosome variations occur in synthetic hexaploid wheat,mostly with perturbations of chromosome number but also some with chromosome structure variation.Out of the karyotyped 150 plants using FISH and GISH,high frequencies of aneuploidy were detected in the selfed progenies of both euploid and aneuploid plants.This phenomeon was consistent with other findings elsewhere for nascent wheat.The main type of aneuploidy is chromosome number variation,ranging from 39-44,but mostly for the change of a copy of a chromosome.Chromosome structure variations included small fragments,chromosome translocations and iso-chromosomes formed by breakage and fusion.Meanwhile,we obatined a unique material with a duplicated fragment deriving from the 73.2-182.3 Mb region of chromosome 2A,designated as Seg T2 A.2.We developed and validated 117 molecular markers specific for chromosomes 4B(57) and 7B(60)of the synthetic hexaploid wheat based on Chinese Spring reference sequence.The validation was performed by using the cytologically confirmed nullisome 4B and 7B materials.3.Monosome and trisome produced subtle effects on phenotype,but the vigor and fertility of nullisome were reduced dramatically compared to euploid.Gaining(Tri4B)and losing(M4B)a single chromosome only caused slight defects on phenotype,with narrower width of third leaf and a lower seed-setting in M4 B,shorter spike length and less spikelets in Tri4 B compared to euploid plant.In another aneuploid,Seg T2 A had longer spikes than euploid,suggesting a potential value in breeding varieties with heavy spike.Compared to Seg T2 A,N7B+Seg T2A(nullisome 7B)showed serious phenotypic defects with significant reductions in vigor and fertility.These results were consistent with the previous reports on Chinese Spring aneuploids,The common feature indicated that certain genetic basis for phenotypic effects of aneuploidy in modern-day wheat has been established in the early stage of origination.4.Different from Chinese Spring,the response of gene expression of synthetic hexaploid wheat to aneuploidy was predominated by cis-effects.In this study,over 90% of differentially expressed genes between euploid and aneuploid were from the dosage varied chromosome.About 80% of differentially expressed genes on the varied chromosome displayed positive linear correlation between gene expression and chromosome dosage.5.Triads in both synthetic hexaploid wheat and bread wheat have similar expression patterns.Most triads(59.8%)were assigned to the balanced category,with similar relative abundance of transcripts from the three homoeologs.Triads with single-homoeolog suppression were less,with 9.6% for A-homoeolog suppression,10.5% for B-homoeolog suppression and 9.6% for D-homoeolog suppression.Triads with single-homoeolog dominance were the least,with 3.2% for A-homoeolog dominance,and 3.5% for both B-and D-homoeolog dominance.Changes in two copies of chromosomes had obvious effects on the total expression of triads,but the changes in a copy exhibited little effects.Taken together,the independently stable expression of homoeologs may be the genetic basis for the high tolerance to aneuploidy in nascent hexaploid wheat.