Re-routing MAP kinase signaling for penetration peg formation in predator yeasts.

Saccharomycopsis yeasts are natural organic sulfur auxotrophs due to lack of genes required for the uptake and assimilation of sulfate/sulfite. Starvation for methionine induces a shift to a predatory, mycoparasitic life strategy that is unique amongst ascomycetous yeasts. Similar to fungal plant pathogens that separated from Saccharomycopsis more than 400 million years ago, a specialized infection structure called penetration peg is used for prey cell invasion. Penetration pegs are highly enriched with chitin. Here we demonstrate that an ancient and conserved MAP kinase signaling pathway regulates penetration peg formation and successful predation in the predator yeast S. schoenii. Deletion of the MAP kinase gene SsKIL1, a homolog of the Saccharomyces cerevisiae ScKSS1/ScFUS3 and the rice blast Magnaporthe oryzae MoPMK1 genes, as well as deletion of the transcription factor SsSTE12 generate non-pathogenic mutants that fail to form penetration pegs. Comparative global transcriptome analyses using RNAseq indicate loss of the SsKil1-SsSte12-dependent predation response in the mutant strains, while a methionine starvation response is still executed. Within the promoter sequences of genes upregulated during predation we identified a cis-regulatory element similar to the ScSte12 pheromone response element. Our results indicate that, re-routing MAP-kinase signaling by re-wiring Ste12 transcriptional control towards predation specific genes contributed to the parallel evolution of this predacious behaviour in predator yeasts. Consequently, we found that SsSTE12 is dispensable for mating. Author summary: Saccharomycopsis yeasts represent a unique yeast genus of predator yeasts that attack and kill fungal prey cells. This necrotrophic mycoparasitism is initiated upon starvation, preferably for organic sulphur/methionine as predator yeasts are methionine auxotrophs. We have characterized morphological and cellular features of predation using time lapse fluorescence microscopy. This indicated prey sensing, chitin enrichment of penetration pegs used in the attack of prey cells and coordination of predation with the cell cycle resulting in cessation of daughter cell growth and a block in mitosis during predation. A predator yeast attack from the onset of penetration peg formation to the disappearance of the nuclear GFP-signal in the Saccharomyces cerevisiae prey took ~25 minutes. We identified the MAP kinase KIL1 and its putative target STE12 as key virulence genes that govern penetration peg formation. Comparative transcriptomics of mutant strains with the wild type identified specific hunger and predation responses and associated genes. Predation genes were found to harbor a conserved DNA element in their promoters. These findings can aid in deepening our understanding of necrotrophic predator-prey interactions. [ABSTRACT FROM AUTHOR]