The
completion of the yeast genome project, coupled with the development of
DNA microarray hybridization technology, opens up unprecedented
opportunities for the study of evolution in action. We have conducted
replicate evolutionary experiments wherein a diploid yeast strain was
propagated asexually for several hundred generations under glucose-limitation.Under these conditions new adaptive mutants sweep through
populations approximately every fifty generations.Evolved strains
demonstrate significantly higher fitness than their common ancestor.
Using microarrays we have analyzed changes in mRNA levels that
distinguish ancestral from evolved strains. Replicate short-term
chemostat monocultures of each strain are highly consistent in their
global patterns of gene expression. By contrast, comparisons of
ancestral to experimentally evolved strains show significant differences
with respect to a limited set of genes. Perhaps more strikingly, these
comparisons reveal that similar changes have occurred within a subset of
genes that encode enzymes of glycolysis and the TCA cycle. Patterns of
gene expression at these loci have enabled us to accurately predict the
outcome of comparisons between strains with respect to yield biomass,
cell number and steady-state levels of specific metabolites. Lineages
derived from a common ancestor may follow independent adaptive
trajectories, yet nevertheless converge on a common physiological
phenotype. Given that a limited number of periodic selection events have
occurred, it appears that regulatory loci responsible for coordinate
changes in expression within specific pathways are key targets of
selection as yeast evolve under nutrient limitation.
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