Genome Evolution in Brassica before the Whole-Genome Triplication Event

The released genomes of several crucifer species, and especially the sequence of Brassica rapa (Chinese cabbage), has allowed us to re-evaluate the evolutionary history of the mesohexaploid Brassica and Brassiceae genomes. We show that all three B. rapa subgenomes have a similar structure and were derived from seven chromosomes of the ancestral translocation Proto-Calepineae Karyotype (tPCK, n=7). tPCK itself is an evolutionary younger variant of the Proto-Calepineae Karyotype (PCK, n=7) which most likely descended from the older Ancestral Crucifer Karyotype (ACK) with n=8. We propose a two-step merger of three progenitor tPCK genomes to form the mesohexaploid (2n=6x=42) ancestor of the genus Brassica and probably of the whole tribe Brassiceae (47 genera, ca. 238 species). Our data further corroborate the inclusion of Brassiceae and at least five more tribes into the PCK clade whose members descended from a PCK-like ancestor.