Sinapis genomes provide insights into whole-genome triplication (WGT) and divergence patterns within tribe Brassiceae.

Sinapis alba and S. arvensis are mustard crops within the Brassiceae tribe of the Brassicaceae family, and represent an important genetic resource for crop improvement. We performed de novo assembly of Brassica nigra, S. alba and S. arvensis, and conducted comparative genomics to investigate the pattern of genomic evolution since an ancient whole-genome triplication (WGT) event. Both Sinapis species retained evidence of the Brassiceae WGT ~20.5 million years ago (Mya), with subgenome dominance observed in gene density, gene expression and selective constraint. While S. alba diverged from the ancestor of Brassica and Raphanus at ~ 12.5 Mya, the divergence time of S. arvensis and B. nigra was ~6.5 Mya. S. arvensis and B. nigra had greater collinearity compared with their relationship to either B. rapa or B. oleracea. Two chromosome of S. alba (Sal03 and Sal08) were completely collinear with two ancestral chromosomes proposed in the Ancestral Crucifer Karyotype (ACK) genomic block model, the first time this has been observed in the Brassiceae. These results are consistent with S. alba representing a relatively ancient lineage of the species evolved from the common ancestor of tribe Brassiceae, and suggest that the phylogeny of the Brassica and Sinapis genera requires some revision. Our study provides new insights into the genome evolution and phylogenetic relationships of Brassiceae and provides genomic information for genetic improvement of these plants.