Testing the adaptive value of sporulation in budding yeast using experimental evolution

Saccharomyces yeast grow through mitotic cell division, converting resources into biomass. When cells experience starvation, sporulation is initiated and meiosis produces haploid cells inside a protective ascus. The protected spore state does not acquire resources and is partially protected from desiccation, heat, and caustic chemicals. Because cells cannot both be protected and acquire resources simultaneously, committing to sporulation represents a trade-off between current and future reproduction. Recent work has suggested that passaging through insect guts selects for spore formation, as surviving insect ingestion represents a major way that yeasts are vectored to new food sources. We subjected replicate populations from five yeast strains to passaging through insects, and evolved control populations by pipette passaging. We assayed populations for their propensity to sporulate after resource depletion. We found that ancestral domesticated strains produced fewer spores, and all strains evolved increased spore production in response to passaging through flies, but domesticated strains responded less. Exposure to flies led to a more rapid shift to sporulation that was more extreme in wild-derived strains. Our results indicate that insect passaging selects for spore production and suggest that domestication led to genetic canalization of the response to cues in the environment and initiation of sporulation.