Elements of a single MAP kinase cascade in Saccharomyces cerevisiae mediate two developmental programs in the same cell type: mating and invasive growth.

Diploid Saccharomyces cerevisiae strains starved for nitrogen undergo a developmental transition from a colonial form of growth to a filamentous pseudohyphal form. This dimorphism requires a polar budding pattern and elements of the MAP kinase signal transduction pathway essential for mating pheromone response in haploids. We report here that haploid strains exhibit an invasive growth behavior with many similarities to pseudohyphal development, including filament formation and agar penetration. Haploid filament formation depends on a switch from an axial to a bipolar mode of bud site selection. Filament formation is distinct from agar penetration in both haploids and diploids. We find that the same components of the MAP kinase cascade necessary for diploid pseudohyphal development (STE20, STE11, STE7, and STE12) are also required for both filament formation and agar penetration in haploids. Thus, haploid yeast cells can enter either of two developmental pathways: mating or invasive growth, both of which depend on elements of a single MAP kinase cascade. Our results provide a novel developmental model to study the dynamics of signal transduction, with implications for higher eukaryotes.