Cabbage head splitting can greatly affect both the quality and commercial value of cabbage (Brassica oleracea). To detect the genetic basis of head-splitting resistance, a genetic map was constructed using an F2 population derived by crossing “748” (head-splitting-resistant inbred line) and “747” (head-splitting-susceptible inbred line). The map spans 830.9cM and comprises 270 markers distributed in nine linkage groups, which correspond to the nine chromosomes of B. oleracea. The average distance between adjacent markers was 3.6cM. A total of six quantitative trait loci (QTLs) conferring resistance to head splittingwere detected in chromosome 2, 4, and 6. Two QTLs, SPL-2-1 and SPL-4-1, on chromosomes 2 and 4, respectively, were detected in the experiments over 2 years, suggesting that these two potential loci were important for governing the head-splitting resistance trait. Markers BRPGM0676 and BRMS137, which were tightly linked with head-splitting resistance, were detected in the conserved QTL SPL-2-1 region using bulked segregant analysis. Synteny analysis showed that SPL-2-1 was anchored to a 3.18Mb genomic region of the B. oleracea genome, homologous to crucifer ancestral karyotype E block in chromosome 1 of Arabidopsis thaliana. Moreover, using a field emission scanning electron microscope, significant differences were observed between the two parental lines in terms of cell structures. Line “747” had thinner cell wall, lower cell density, larger cell size, and anomalous cell wall structure compared with the resistant line “748”. The different cell structures can provide a cytological base for assessing cabbage head splitting.

We investigated the genetic diversity and structure of the 239 fixed lines with 47 simple sequence repeat (SSR) and 109 NGS-generated SNP markers evenly distributed in B. rapa genome. Phylogenetic analysis classified the vegetable fixed lines to four subgroups, with the three types forming a separate and relatively farther cluster. Population structure analysis identified four sub-populations corresponding to geographic origin and morphological traits, and revealed extensive admixture. The vegetable B. rapa fixed lines successfully developed in our study would be valuable materials for multinational B. rapa diversity resources establishment. Understanding the genetic diversity and population structure could be useful for utilization of the representing genetic variation and further genetic and genomic analysis.