SACCHAROMYCES-CEREVISIAE CELLS EXECUTE A DEFAULT PATHWAY TO SELECT A MATE IN THE ABSENCE OF PHEROMONE GRADIENTS

During conjugation, haploid S. cerevisiae cells find one another by po larizing their growth toward each other along gradients of pheromone ( chemotropism), We demonstrate that yeast cells exhibit a second mating behavior: when their receptors are saturated with pheromone, wild-typ e a cells execute a default pathway and select a mate at random. These matings are less efficient than chemotropic matings, are induced by t he same dose of pheromone that induces shmoo formation, and appear to use a site near the incipient bud site for polarization. We show that the SPA2 gene is specifically required for the default pathway: spa2 D elta mutants cannot mate if pheromone concentrations are high and grad ients are absent, but can mate if gradients are present. ste2 Delta, s st2 Delta, and far1 Delta mutants are chemotropism-defective and there fore must choose a mate by using a default pathway; consistent with th is deduction, these strains require SPA2 to mate, In addition, our res ults suggest that far1 mutants are chemotropism-defective because thei r mating polarity is fixed at the incipient bud site, suggesting that the FAR1 gene is required for inhibiting the use of the incipient bud site during chemotropic mating. These observations reveal a molecular relationship between the mating and budding polarity pathways.