Evolutionary remodeling of chromosomes maps in yeasts
G. Fischer
Unité de Génétique Moléculaire des Levures, Institut
Pasteur, 25, rue du Docteur Roux, 75724 Paris Cedex 15
The Génolevures I program represented the fist
attempt to explored the genetic diversity present within a whole class of
eukaryotes, the hemiascomycetous yeasts (Souciet et al., FEBS Letters, 2000).
The second phase of the Génolevures program lead to the release of 4 complete
genome sequences of species covering the entire evolutionary range present
within this group (Dujon et al., Nature, 2004). Presently, the (nearly)
complete genome sequences of about two dozen yeast species have been reported,
allowing the development of powerful comparative approaches to decipher the
mechanisms of genome evolution. The level of genetic diversity spanned by these
species is often unsuspected. For instance, the average protein divergence of
more than 50% found between Saccharomyces cerevisiae and Yarrowia lipolytica,
an alkane-using yeast, reveals that Hemiascomycetes are molecularly as diverse
as the entire phylum of chordate. We used the complete (or nearly complete)
genome sequences of 11 yeast species covering the entire clade to quantify the
rates of gene order changes in the different branches of the Hemiascomycete
tree. We also identified all inversion events that occurred within synteny
blocks conserved between pairwise comparisons of 6 fully sequenced genomes
corresponding to representative species from the different lineages. We showed
that the rates of macro- and microrearrangements of gene order are correlated
within individual lineages but are highly variable across the different
lineages. The most unstable genomes correspond to yeasts adapted to a
particular host, like pathogens, followed by the ones that have inherited a
whole genome duplication event. We found a tremendous level of chromosomal
reorganization outside of the Saccharomyces Œsensu stricto¹ species.
Chromosomal maps have been intensively shuffled by numerous interchromosomal
rearrangements, even between species that have retained a very high physical
fraction of their genomes within small synteny blocks. Despite this intensive
reshuffling of gene positions, essential genes, which cluster in low
recombination regions in the genome of S. cerevisiae, tend to remain adjacent
during evolution, indicating selective constraints onto gene order
preservation. This work suggests that these constraints might be alleviated by
increased genome redundancy and decreased effective population sizes.