Population genomic insights into the domestication of Brassica juncea var. tumida

Brassica juncea var. tumida, commonly known as Zha Cai, is a pickled stem mustard widely cultivated in southern China. Its most distinctive trait is the swollen stem, which serves as the primary economic organ for harvest. However, the origin and domestication history of tumida remain unclear, hindering genetic improvement and molecular breeding efforts. Here, we assembled a chromosome-level genome of the landrace 'YAXY' from Chongqing-the center of tumida diversity-totaling 909.1 Mb with a contig N50 of 4.17 Mb. We also collected and resequenced 203 tumida accessions across southern China. By integrating the 'YAXY' reference genome with population data, we generated the first comprehensive tumida variation dataset, comprising 1.38 million single-nucleotide polymorphisms (SNPs) and 0.27 million insertions and deletions (InDels). Joint analysis of the newly sequenced tumida population and 504 public B. juncea datasets revealed that tumida and leafy types from southern China share a common origin from local oilseed mustard. Tumida domestication was accompanied by a strong genetic bottleneck. Additionally, we conducted genome-wide association studies (GWAS) for 21 agronomic traits and identified candidate genes linked to key domestication traits in tumida. For the swollen stem trait, selective sweep and GWAS analyses jointly identified candidate genes likely involved in lignification. Transcriptome data showed consistent differential expression of BjuA05g15010, the Arabidopsis SAGL1 ortholog, across all swelling stages, suggesting a key role in stem morphogenesis. Collectively, our findings shed light on tumida evolution and provide valuable genomic resources and candidate genes to support genetic research and breeding in B. juncea.