Pheromone MAPK pathway regulates the yeast-to-hypha transition in the parasitic mushroom Naematelia sinensis in a cell fusion–independent manner

Naematelia sinensis (Jin Er), a distinctive parasitic mushroom, relies on its interaction with Stereum hirsutum for fruiting body formation. The transition from yeast to hyphae is essential for its lifecycle, facilitating both parasitism and sexual reproduction. Under axenic conditions, however, it predominantly exists in the yeast form. This study established conditions that induce yeast-to-hypha transition and explored the underlying regulatory mechanisms. Hyphal induction was successfully achieved using a medium derived from mushroom spent substrate, with lactose significantly enhancing the transition. Hyphal formation occurred in heterokaryons or co-cultured monokaryons with different B mating-type loci, underscoring the crucial role of the B mating locus. Notably, hyphal development was observed even without physical contact between monokaryons of different B mating loci, suggesting that cell fusion is not a prerequisite and a diffusible signal likely mediates the transition. Genome resequencing revealed structural variations and sequence divergence at the B locus in compatible strains. Transcriptomic analysis revealed the pheromone MAPK pathway as the regulator of the transition, alongside a unique lactose metabolic pathway that diverges from the classical Kluyveromyces model. Lactose appears to function both as a carbon source and as a possible signaling molecule driving dimorphism. These findings provide new insights into the mechanisms of fungal dimorphism and lay a foundation for future research on fruiting body formation and parasitism in N. sinensis.