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.