Closely related budding yeast species respond to different ecological signals for spore activation.

Spore activation is one of the most important developmental decisions in fungi as it initiates the transition from dormant and stress resistant cells to vegetative cells. Because in many species mating follows spore activation and germination, signals that trigger this developmental transition can also contribute to species reproductive barriers. Here we examine the biochemical signals triggering spore activation in a natural species complex of budding yeast, Saccharomyces paradoxus (lineages SpA, SpB, SpC and SpC*). We first demonstrate that we can quantitatively monitor spore activation in these closely related lineages. Second, we dissect the composition of culture media to identify components necessary and/or sufficient to activate spores in the four lineages. We show that, contrary to expectation, glucose is necessary but not sufficient to trigger spore activation. We also show that two of the North American lineages (SpC and SpC*) diverge from the other North American (SpB) and European (SpA) lineages in terms of germination signal as their spore activation requires inorganic phosphate. Our results show that the way budding yeast interpret environmental conditions during spore activation diverged among closely related and incipient species, which means that it may play a role in their ecological differentiation and reproductive isolation.