Plant polygalacturonases (PGs) are closely related to cell-separation events during plant growth and development by degrading pectin. Identifying and investigating their diversification of evolution and expression could shed light on research on their function. We conducted sequence, molecular evolution, and gene expression analyses of PG genes in Brassica oleracea. Ninety-nine B. oleracea PGs (BoPGs) were identified and divided into seven clades through phylogenetic analysis. The exon/intron

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... ructures and motifs were conserved within, but divergent between, clades. The second conserved domain (GDDC) may be more closely related to the identification of PGs. There were at least 79 common ancestor PGs between Arabidopsis thaliana and B. oleracea. The event of whole genome triplication and tandem duplication played important roles in the rapid expansion of the BoPG gene family, and gene loss may be an important mechanism in the generation of the diversity of BoPGs. By evaluating the expression in five tissues, we found that most of the expressed BoPGs in clades A, B, and E showed ubiquitous expression characteristics, and the expressed BoPGs in clades C, D, and F were mainly responsible for reproduction development. Most of the paralogous gene pairs (76.2%) exhibited divergent expression patterns, indicating that they may have experienced neofunctionalization or subfunctionalization. The cis-elements analysis showed that up to 96 BoPGs contained the hormone response elements in their promoters. In conclusion, our comparative analysis may provide a valuable data foundation for the further functional analysis of BoPGs during the development of B. oleracea.