Impacts of polyploidy on genome evolution in Brassica species
The triangle of U describes the relationship between six species of the Brassica genus, where three lower chromosome number species (Brassica rapa, Brassica oleracea and Brassica nigra) have hybridized in all combinations to generate three allopolyploid species (Brassica napus, Brassica juncea and Brassica carinata). Each of the lower chromosome number species are paleopolyploids having evolved through multiple rounds of genome duplication and subsequent diploidisation. Complete genome sequences have been generated for each of these Brassica species and comparative analyses among them are facilitated by alignment to a common ancestral Brassicaceae genome. The relationship among these species offers a unique opportunity to study the impact of multiple polyploidisation events on the evolution of genome structure, in particular maintenance or fractionation of duplicated gene complements, the potential for homoeologous chromosomal exchange in the allopolyploid species, and the consequent influence of these events on adaptive traits.