DEVELOPMENT OF PSEUDOHYPHAE BY EMBEDDED HAPLOID AND DIPLOID YEAST

Diploid strains of S. cerevisiae are known to develop pseudohyphae in response to starvation for nitrogen. We report that both haploid and d iploid yeast grow in a filamentous form when embedded in solid media. This is not a response to starvation, since yeast grown on rich media and overlaid with rich agar grow within the agar as pseu dohyphae. Whi le we find that the only element of diploidy required for formation of pseudohyphae in response to nitrogen starvation is the alpha 1/alpha 2 repressor, pseudohyphal development by embedded cells does not requi re alpha 1/alpha 2. Deletion of BUD 5 prevented the formation of pseud ohyphae by embedded cells, suggesting that these structures are the re sult of ordered filament formation rather than agar penetration. Delet ion of STE12 prevented the formation of pseudohyphae by all cell types , showing that the same signal transduction pathway is used by embedde d cells as by those responding to nitrogen starvation. Different cell types of yeast thus form filaments in response to several kinds of env ironmental stimuli.