Resequencing of 145 Landmark Cultivars Reveals Asymmetric Sub-genome Selection and Strong Founder Genotype Effects on Wheat Breeding in China

Controlled pedigrees and the multi-decade timescale of national crop plant breeding programs offer a unique experimental context for examining how selection affects plant genomes.More than 3000 wheat cultivars have been registered,released,and documented since 1949 in China.In this study,a set of 145 elite cultivars selected from historical points of wheat breeding in China were re-sequenced.A total of 43.75 Tb of sequence data were generated with an average read depth of 17.94x for each cultivar,and more than 60.92 million SNPs and 2.54 million InDels were captured,based on the Chinese Spring RefSeq genome v1.0.Seventy years of breeder-driven selection led to dramatic changes in grain yield and related phenotypes,with distinct genomic regions and phenotypes tar-geted by different breeders across the decades.There are very clear instances illustrating how introduced Italian and otherforeign germplasm was integrated into Chinese wheat programs and reshaped the genomic landscape of local modern cultivars.Importantly,the resequencing data also highlighted significant asymmetric breeding selec-tion among the three sub-genomes:this was evident in both the collinear blocks for homeologous chromosomes and among sets of three homeologous genes.Accumulation of more newly assembled genes in newer cultivars implied the potential value of these genes in breeding.Conserved and extended sharing of linkage disequilibrium(LD)blocks was highlighted among pedigree-related cultivars,in which fewer haplotype differences were detected.Fixation or replacement of haplotypes from founder genotypes after generations of breeding was related to their breeding value.Based on the haplotype frequency changes in LD blocks of pedigree-related cultivars,we propose a strategy for evaluating the breeding value of any given line on the basis of the accumulation(pyramiding)of bene-ficial haplotypes.Collectively,our study demonstrates the influence of "founder genotypes" on the output of breeding efforts over many decades and also suggests that founder genotype perspectives are in fact more dy-namic when applied in the context of modern genomics-informed breeding.

The Battle to Sequence the Bread Wheat Genome:A Tale of the Three Kingdoms

In the year 2018,the world witnessed the finale of the race to sequence the genome of the world’s most widely grown crop,the common wheat.Wheat has been known to bear a notoriously large and complicated genome of a polyploidy nature.A decade competition to sequence the wheat genome initiated with a single consortium of multiple countries,taking a conventional strategy similar to that for sequencing Arabidopsis and rice,became ferocious over time as both sequencing technologies and genome assembling methodologies advanced.At different stages,multiple versions of genome sequences of the same variety(e.g.,Chinese Spring)were produced by several groups with their special strategies.Finally,16 years after the rice genome was finished and 9 years after that of maize,the wheat research community now possesses its own reference genome.Armed with these genomics tools,wheat will reestablish itself as a model for polyploid plants in studying the mechanisms of polyploidy evolution,domestication,genetic and epigenetic regulation of homoeolog expression,as well as defining its genetic diversity and breeding on the genome level.The enhanced resolution of the wheat genome should also help accelerate development of wheat cultivars that are more tolerant to biotic and/or abiotic stresses with better quality and higher yield.

Wheat research and breeding in the new era of a high-quality reference genome

The publications of the International Wheat Genome Sequencing Consortium(IWGSC) released in August 2018 are reviewed and placed into the context of developments arising from the availability of the highquality wheat genome assembly.

Centromere repositioning and shifts in wheat evolution

The centromere is the region of a chromosome that directs its separation and plays an important role in cell division and reproduction of organisms.Elucidating the dynamics of centromeres is an alternative strategy for exploring the evolution of wheat.Here,we comprehensively analyzed centromeres from the de novoassembled common wheat cultivar Aikang58(AK58),Chinese Spring(CS),and all sequenced diploid and tetraploid ancestors by chromatin immunoprecipitation sequencing,whole-genome bisulfite sequencing,RNA sequencing,assay for transposase-accessible chromatin using sequencing,and comparative genomics.We found that centromere-associated sequences were concentrated during tetraploidization and hexaploidization.Centromeric repeats of wheat(CRWs)have undergone expansion during wheat evolution,with strong interweaving between the A and B subgenomes post tetraploidization.We found that CENH3 prefers to bind with younger CRWs,as directly supported by immunocolocalization on two chromosomes(1A and 2A)of wild emmer wheat with dicentromeric regions,only one of which bound with CENH3.In a comparison of AK58 with CS,obvious centromere repositioning was detected on chromosomes 1B,3D,and 4D.The active centromeres showed a unique combination of lower CG but higher CHH and CHG methylation levels.We also found that centromeric chromatin was more open than pericentromeric chromatin,with higher levels of gene expression but lower gene density.Frequent introgression between tetraploid and hexaploid wheat also had a strong influence on centromere position on the same chromosome.This study also showed that active wheat centromeres were genetically and epigenetically determined.