Genetic and environmental determinants of multicellular-like phenotypes in fission yeast
Multicellular fungi have repeatedly given rise to primarily unicellular yeast species. Some of these, including Schizosaccharomyces pombe, are able to revert to multicellular-like phenotypes (MLP). Our bioinformatic analysis of existing data suggested that, besides some regulatory proteins, most proteins involved in MLP formation are not functionally conserved between S. pombe and budding yeast. We developed high-throughput assays for two types of MLP in S. pombe: flocculation and surface adhesion, which correlated in minimal medium, suggesting a common mechanism. Using a library of 57 natural S. pombe isolates, we found MLP formation to widely vary across different nutrient and drug conditions. Next, in a segregant S. pombe library generated from an adhesive natural isolate and the standard laboratory strain, MLP formation correlated with expression levels of the transcription-factor gene mbx2 and several flocculins. Quantitative trait locus mapping of MLP formation located a causal frameshift mutation in the srb11 gene encoding cyclin C, a part of the Cdk8 kinase module (CKM) of the Mediator complex. Other CKM deletions also resulted in MLP formation, consistently through upregulation of mbx2, and only in minimal media. We screened a library of 3721 gene-deletion strains, uncovering additional genes involved in surface adhesion on minimal media. We identified 31 high-confidence hits, including 19 genes that have not been associated with MLPs in fission or budding yeast. Notably, deletion of srb11, unlike deletions of the 31 hits, did not compromise cell growth, which might explain its natural occurrence as a QTL for MLP formation.