The fact that changes in transcriptional logic bring about adaptive changes — as might occur when developmental modules are rearranged — is no longer front page news. Now, a study of mating type in two divergent yeast species shows the opposite principle: that phenotypes might stay the same during evolution, while the transcriptional logic is sometimes completely rewired.
The ascomycete yeasts
Candida albicans
and
Saccharomyces cerevisiae
each come in two varieties, a and α, depending on which allele they express at the mating-type (MAT) locus. Each mating type expresses a set of specific genes, which allows mating only between a and (types. In this respect the two yeast species are identical — however, they differ radically in how they regulate the expression of these antigens. In C. albicans, a-type genes are 'off' by default, and are turned 'on' in a-type cells; in S. cerevisiae, a-type genes are 'on' by default, and need to be repressed in α-type cells. The end result is the same — a-type genes are 'on' in a cells and 'off' in (cells — but the two yeasts use opposite means of achieving this pattern of expression. How did this transition take place, in the 200–800 million years since the two species shared a common ancestor? By combining microarrays (see image) and other bench experiments with comparative genomics in modern yeasts, Annie Tsong, Brian Tuch and colleagues have identified the cis and trans elements that were in place at the transition, and have traced the order in which the events took place.
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