Rapeseed (Brassica napus), an important oilseed crop, has adapted to diverse climate zones and latitudes by forming three main ecotype groups, namely winter, semi-winter, and spring types. However, genetic variations underlying the divergence of these ecotypes are largely unknown. Here, we report the global pattern of genetic polymorphisms in rapeseed determined by resequencing a worldwide collection of 991 germplasm accessions. A total of 5.56 and 5.53 million single-nucleotide polymorphisms (SNPs) as well as 1.86 and 1.92 million InDels were identified by mapping reads to the reference genomes of "Darmor-bzh" and "Tapidor," respectively. We generated a map of allelic drift paths that shows splits and mixtures of the main populations, and revealed an asymmetric evolution of the two subgenomes of B. napus by calculating the genetic diversity and linkage disequilibrium parameters. Selective-sweep analysis revealed genetic changes in genes orthologous to those regulating various aspects of plant development and response to stresses. A genome-wide association study identified SNPs in the promoter regions of FLOWERING LOCUS T and FLOWERING LOCUS C orthologs that corresponded to the different rapeseed ecotype groups. Our study provides important insights into the genomic footprints of rapeseed evolution and flowering-time divergence among three ecotype groups, and will facilitate screening of molecular markers for accelerating rapeseed breeding.
Keywords: Brassica napus; GWAS; ecotype divergence; flowering-time trait; genome resequencing; selective sweep.
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