External Seminar "Fruit flies select for nutrient-independent sporulation in wild yeast" by Hironori Niki

Fruit flies select for nutrient-independent sporulation in wild yeast

Speaker: Hironori NIKI, National Institute of Genetics, Japan

Abstract: 

Schizosaccharomyces japonicus, a species of fission yeast first identified in Japan in 1928, is still readily isolated from natural environments such as flowers, mosses, soil, and fruit flies across Japan. Unlike laboratory strains, many wild isolates exhibit constitutive sporulation—frequent spore formation via meiosis—even under nutrient-rich conditions, particularly in nitrogen-containing medium (Seike et al., 2021).

Sexual reproduction is generally more costly than asexual reproduction, and its induction is tightly regulated by nutrient availability in yeasts. Nevertheless, its persistence in the wild suggests that it counters a selective pressure. We hypothesised that insect predation, particularly by fruit flies, may favour spore-forming variants of yeast. Spores are resistant to digestion, whereas vegetative cells are not. Symbiotic experiments with Drosophila melanogaster supported this hypothesis, showing that fly feeding behaviour serves as a selective force maintaining the trait in nature. In fission yeast, the HMG-type transcription factor Ste11 is a key regulator of sexual development because it controls the expression of numerous meiosis-related genes. We identified genetic variations in Ste11 and its upstream regulatory factors that control its expression. Results from genetic crosses revealed that certain combinations of these genetic variants led to constitutive sporulation, suggesting epistatic interactions among upstream regulators involved in the control of ste11 expression.

Our findings demonstrate that S. japonicus variants with constitutive sporulation provide a clear case of natural selection mediated by insect interactions. These strains serve as a valuable model system for studying the ecological and evolutionary impacts of insect predation on microbial life histories.