Repeat Evolution in Brassica rapa (AA), B. oleracea (CC), and B. napus (AACC) Genomes

The genus Brassica is an important resource for major agricultural products such as oils, vegetable and fodder. The Brassiceae tribe-specific whole-genome triplication that occurred ~15.9 million years ago influenced the speciation and morphological diversification that has been exploited in agriculture, making Brassica an excellent model system for studying polyploidization- mediated evolution. Genome sequencing and comparative genome analysis have revealed conserved structures and uncovered the genome evolution of Brassica species. While chromosome shuffling and asymmetric subgenome gene retention are widely reported in Brassica species, limited information is available about the dynamics of repetitive elements (REs), which are central to epigenetic mechanisms and thus play a pivotal role in plant genome adaptation and evolution. The assembled reference genome sequences of B. rapa (AA) and B. oleracea (CC), and their derived allotetraploid, B. napus (AACC), cover 58%, 86%, and 75% of their respective estimated genome sizes. The remaining non-assembled genome portions vary between these three genome sequences, and the major components remain hidden in each genome. Here, we review the dynamics of the major Brassica repeats that have played roles in speciation of the AA, CC, and AACC genomes. We show that 10 major Brassica repeats appear to occupy more than 50% of each respective unassembled genome sequence, yet represent less than 1% of assembled reference genome sequences. We have estimated their genome proportions using whole-genome Illumina reads and cytogenetic analyses in an attempt to understand the role of these repeats in genome evolution.