Courtship in Saccharomyces cerevisiae: An early cell-cell interaction during mating

During conjugation in Saccharomyces cerevisiae, two cells of opposite mating type (MATa and MATα) fuse to form a diploid zygote. Conjugation requires that each cell locate an appropriate mating partner. To investigate how yeast cells select a mating partner, we developed a competition mating assay in which wild-type MATα cells have a choice of two MATa cell mating partners. We first demonstrated that sterile MATα1 cells (expressing no a- or α-specific gene products) do not compete with fertile MATa cells in the assay; hence, wild-type MATa and MATα cells can efficiently locate an appropriate mating partner. Second, we showed that a MATa strain need not be fertile to compete with a fertile MATa strain in the assay. This result defines an early step in conjugation, which we term courtship. We showed that the ability to agglutinate is not necessary in MATa cells for courtship but that production of a-pheromone and response to α-pheromone are necessary. Thus, MATa cells must not only transmit but must also receive and then respond to information for effective courtship; hence, there is a "conversation" between the courting cells. We showed that the only α-pheromone-induced response necessary in MATa cells for courtship is production of a-pheromone. In all cases tested, a strain producing a higher level of a-pheromone was more proficient in courtship than one producing a lower level. We propose that during courtship, a MATα cell selects the adjacent MATa cell producing the highest level of a-pheromone.