Phylogenetic analysis of a collection of Brassica genomes reveals Chinese flowering cabbage has a closer evolutionary relationship with the diploid progenitor of the allotetraploid species, B. juncea
The Brassicaceae family serves as a useful model for studying polyploidy and chromosome evolution. The evolutionary relationship of six ecologically important Brassica species including three diploid species (B. rapa , B. oleracea , and B. nigra ) and three allotetraploid species (B. napus , B. juncea , andB. carinata ) was well described in a classical U triangle model(Cheng et al., 2016) . To elucidate the evolutionary distance of the current Chinese flowering cabbage genome to other Brassica genomes, we constructed a phylogenetic tree (Fig. 4) for 12 collected Brassica genomes and eight related Brassicaceae species using the coding sequences of 434 single-copy genes that are present in all of the species. The result shows that the three Brassica genome types are clearly separated from each other among the investigated species. The current Chinese flowering cabbage has a AA genome type which is closer to the AA genome of the allotetraploid species, B. juncea , than the AA genome of another B. rapa line, B. rapa var pekinensis in the phylogenetic tree, suggesting Chinese flowering cabbage is evolutionarily closer to the diploid progenitor of the allotetraploid species, Brassica juncea . Also, in the CC genome clade, B. oleracea var capitata was clustered firstly with two B. napus CC genomes and then with B. oleracea var italica , implying B. oleracea var capitata has a CC genome that is closer to the donor of CC genome of the B. napus . Similarly,B. rapa Z1 was clustered firstly with B. napus AA genome and then other AA genomes, pointing to it as being evolutionarily closer to the AA genome progenitor of B. napus .