Having resequenced 199 Brassica rapa and 119 B. oleracea varieties of leaf-heading types, they found four loci in each that indicated parallel selection.
They also found "strong subgenome parallel selection linked to the domestication of the tuberous morphotypes of turnip (B. rapa) and kohlrabi (B. oleracea)".
They concluded that the plants' diversification into heading and tuber-forming types occurred "through convergent subgenome parallel selection of paralogous genes".
Modern cabbage, cauliflower and kohlrabi varieties all derive from B. oleracea, which still grows wild on the southern English coast, while in Asia a different species, B. rapa, yielded similar crops including Chinese cabbage, pak choi and turnips.
Wageningen UR plant breeding scientist and co-author of the paper Guusje Bonnema said: "These two Brassica species were apparently both relatively easy to domesticate, sometimes into crops that are very alike. There are also Brassica crops in both Europe and Asia which are cultivated for their floral organs, like cauliflower, broccoli, broccoletto and caixin."
The paper suggests that triplication in their genomes made Brassica species more amenable to domestication, since large segments of DNA, and many genes, are present in the genome in triplicate. These so-called paralogous genes ensure that variations of genes can be selected in the offspring without any loss of essential functions.
"Because a cabbage contains three copies of a specific gene, one copy can develop a mutation which makes the leaves fold, for instance, while other copies retain their original function," Bonnema explained.
"Comparing this to other domesticated species without recent genome duplication, we see that these species generally display less different vegetable types. As a result, a mutation in a gene is not as easily selected and will disappear."
He added: "Knowing which gene creates a wider or larger leaf and which gene makes the inner leaves fold inward allows us to select specifically on these properties."
The findings are published in Nature Genetics.