Brassica genome sequencing: understanding the consequences of recurrent polyploidizations on genome organization and evolution

Jérémy Just

Polyploidy, the fact for an organism of having two or more distinct chromosome sets, is widespread
in plants and has been a predominant factor in the evolution and success of Angiosperms. Brassica
species (cabbage, turnip, rapeseed, mustards. . . ), for which no complete genome sequence is yet
available, are complex polyploids that underwent additional polyploidization events in comparison
to any completely sequenced plant genome, such as Arabidopsis thaliana, rice, grapevine, papaya. . .
Arabidopsis and Brassica species have diverged about 14 Mya. Their last common ancestor
had already undergone three rounds of duplication or triplication during the last 200 My. Genetic
and comparative analyses of current diploid Brassica species, such as B. rapa (`AA' genome,
10 chromosome pairs) or B. oleracea (`CC', 9 chromosome pairs), revealed marks of a genome
triplication, which happened shortly after the Arabidopsis/Brassica split. More recently, several
independent hybridizations occurred between these diploid species, leading to important crop
species such as rapeseed (Brassica napus, `AACC', 19 chromosome pairs). As a result, B. napus
is the crop species having the most highly duplicated genome, with theoretically 72 accumulated
ancestral genomes.
The ongoing large-scale Brassica rapa sequencing project (http://www.brassica-rapa.org/)
has allowed us to shed light on A. thaliana and Brassica divergent genome evolution and measure
their relative evolution speed. We have also shown that B. rapa genomic regions have undergone a
global size reduction by about 30 % as compared to their A. thaliana orthologous counterparts.
Whole-genome comparative approaches for sequencing several Brassica species genomes, in-
cluding that of Brassica napus which we have recently started, and studying polyploidization and
subsequent diploidization processes at the genus scale will be discussed.