Constitutive sporulation in wild fission yeast enhances insect-mediated survival

Sexual reproduction is generally considered costly for unicellular organisms and is typically induced in yeast only under nutrient limitation. However, wild strains of Schizosaccharomyces japonicus isolated from fruit flies in Japan exhibit constitutive sporulation even under nutrient-rich conditions. Here, we investigated the ecological and evolutionary significance of this unusual phenotype. We demonstrate that spores produced under nutrient-rich conditions survive passage through the digestive tract of Drosophila melanogaster, whereas vegetative cells do not. Although constitutive sporulation reduces competitive growth during laboratory serial passaging, fly-mediated transmission strongly enriches high-sporulation (hc⁹⁰) strains, indicating positive selection by insect predation. Experimental evolution using a chemically mutagenized library further confirmed that passage through fruit flies enriches mutants capable of nutrient-independent sporulation. Whole-genome and genetic analyses revealed that the hc⁹⁰ phenotype is associated with combinations of mutations in six meiotic regulatory genes-byr2, mcs4, ste6, ste11, tor2, and win1-suggesting epistatic interactions rather than single-gene effects. Suppressor mutations arising during laboratory cultivation frequently targeted ste11, reinforcing its central role in regulating this trait. Field surveys showed that genetically diverse hc⁹⁰ strains are widely distributed across Japan from insects, flowers, moss, and soil. Together, our findings indicate that long-term yeast-insect interactions can maintain costly sexual reproduction as an adaptive strategy for survival and dispersal. This study provides direct experimental evidence linking microbial reproductive strategies to ecological selection by animal hosts.