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 the fission yeast Schizosaccharomyces pombe, can revert to multicellular-like phenotypes (MLPs). As MLP formation remains understudied in fission yeast compared with budding yeast, we aimed to narrow this gap. We developed high-throughput assays for two MLPs: flocculation and surface adhesion, which correlated in minimal media, suggesting a common mechanism. Using a library of 57 natural S. pombe isolates, we found that MLP formation varied widely across different nutrient and drug conditions. In a segregant S. pombe library generated by crossing an adhesive natural isolate with the standard laboratory strain, MLP formation correlated with expression levels of the transcription factor gene mbx2 and several flocculin genes. Quantitative trait locus (QTL) mapping of MLP formation located a frameshift mutation in the srb11 gene encoding cyclin C, a part of the Cdk8 kinase module (CKM) of the Mediator complex. Deletion of either srb11 or srb10 (encoding the Cdk8 kinase) resulted in MLP formation through upregulation of mbx2. Screening a library of 3,721 gene-deletion strains uncovered 31 additional genes involved in surface adhesion, including 15 genes not previously 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. Our findings provide a comprehensive genetic survey of MLP formation in fission yeast and a functional description of a causal variant that drives MLP formation in nature.