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

Saccharomyces yeast can grow through mitotic vegetative cell division while they convert resources in their environment into biomass. When cells encounter specific low nutrient environments, sporulation may be initiated and meiotic division produces 4 haploid cells contained inside a protective ascus. The protected spore state does not acquire new resources but 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 one of the major environmental factors that select for the formation of spores is passaging through insect guts, as this also represents a major way that yeasts are vectored to new food sources. We subjected replicate populations of a panel of 5 yeast strains to repeated, predictable passaging through insects by feeding them to fruit flies (Drosopila melanogaster) and then allowing surviving yeast cell growth in defined media for a fixed period of time. We also evolved control populations using the same predictable growth environments but without being exposed to flies. We assayed populations for their sporulation rate, as measured by the percentage of cells that had sporulated after resource depletion. We found that the strains varied in their ancestral sporulation rate such that domesticated strains had lower sporulation. During evolution, all strains evolved increased sporulation in response to passaging through flies, but domesticated strains evolved to lower final levels of sporulation. We also found that exposure to flies led to an evolved change in the timing of the sporulation response relative to controls, with a more rapid shift to sporulation, and that wild-derived strains showed a more extreme response. We conclude that strains that have lost the ability to access genetic variation for total sporulation rate and the ability to respond to cues in the environment that favor sporulation due to genetic canalization during domestication.