The Saccharomyces Ôsensu strictoÕ group includes yeast species that will mate with one another, but the resulting hybrids are sterile. Retrospective analysis of the genomes of these closely related species revealed the presence of several chromosomal translocations, posing the problem whether such rearrangements play a significant role on speciation processes.
Using a modification of the cre/loxP system it was possible to construct S. cerevisiae strains (ScT) carrying the translocations present in another
sibling species, S. mikatae. To study the impact
that chromosomal rearrangements have on the fertility of the hybrids, the ScT
strains of S. cerevisiae were crossed with S.
mikatae and the viability of their meiotic products
was assessed. Some of these interspecific hybrids produced a large number of
spores that were viable, although extensively aneuploid, providing experimental
evidence that some translocations could intensify the post-zygotic barriers,
once a species has arisen by another route. Engineered S. cerevisiae strains, mimicking the genome
structure present in S. mikatae, were also used in competition experiments to
determine the contribution of reciprocal translocations to the organismÕs
fitness. The vast majority of competition experiments undertaken provided
evidence for positive fitness effects of reciprocal translocations, with the
rearranged strains of S. cerevisiae out-competing
the reference S. cerevisiae strain with no translocation,
especially under glucose limitation.
Although it is always difficult to extrapolate laboratory
results to natural systems, these data indicate that chromosomal rearrangements in Saccharomyces may provide
selective advantages in some conditions, independently of any other genetic
differences.
This is compatible with a model of
adaptive fixation of translocations in the Saccharomyces Ôsensu strictoÕ group of species.
The contribution of individual genes to the fitness of the yeast Saccharomyces cerevisiae was also investigated by means of large-scale competition analysis and population profiling. Bar-coded heterozygous strains, carrying mutations in each and every gene present in the yeast genome (ca 6000 different mutants), were grown together in competition experiments under different nutrient limitations. At different time points, the fraction of every mutant strain in the pool was detected via hybridisation of all the bar-codes to a high density array. These population profiles were compared to that at time zero, allowing the identification of genes whose deletion quantitatively affected the yeast growth.