2018 ASHS Annual Conference

Improvement efforts in Brassica oleracea vegetables such as broccoli (B. oleracea var. italica) are often limited by a lack of knowledge of the genetic diversity contained within available germplasm. Here, we employ genotype-by-sequencing on a diverse panel of (N=134) B. oleracea entries including landrace and improved broccoli, landrace and improved cauliflower, kale, Chinese kale, Brussels sprouts, cabbage, collard, kohlrabi, and undomesticated B. oleracea relatives. We identified over 45K high-quality SNPs (mean=91.0/Mbp or 45.5/haploblock) revealing a complex and admixed population structure. Vegetable groups were effectively resolved using principal coordinate analysis. When compared with a pool of landrace broccoli (N=37), all improved broccoli entries (N=26) released after 2000 were shown to be closely related, in higher linkage disequilibrium than landrace broccoli, and largely represented by a single subpopulation indicative of a genetic bottleneck occurring during modern cultivar development. Over 96% of SNPs differentiating improved cauliflower from landrace cauliflower were common to a larger pool of broccoli entries, supporting a “broccoli before cauliflower” domestication model via apparent introgression of broccoli alleles into improved cauliflower germplasm. Fixation index analysis identified several chromosomal regions differentiating improved broccoli and cauliflower entries. Of all the B. oleracea vegetable groups, kale and Brussels sprouts entries shared the greatest similarity with undomesticated B. oleracea wild relatives. Collard and cabbage entries shared high mutual similarity; supporting earlier work indicating cabbage as the primary collard progenitor. Chinese kale entries formed a monophyletic clade but share population structure components with Russian kale entries and the undomesticated B. insularis and B. macrocarpa.