MATING IN THE HETEROTHALLIC HAPLOID YEAST CLAVISPORA-OPUNTIAE, WITH SPECIAL REFERENCE TO MATING-TYPE IMBALANCES IN LOCAL-POPULATIONS

Mating was studied in the haploid, heterothallic yeast Clavispora opun tiae to assess the importance of nutritional, genetic, and other facto rs that may favour mating and recombination. Local populations of this yeast generally exhibit dramatic inequalities in mating type distribu tions, suggesting that mating is rare in nature even though most isola tes mate freely in the laboratory. The absence of assimilable nitrogen is prerequisite to mating competence, presumably by causing G, arrest . Maximum mating competence is found in cells entering stationary phas e in nitrogen-limited media. Unlike the vast majority of mating yeasts , C. opuntiae does not appear to produce diffusible mating factors (se x pheromones), and mating-competent cells do not undergo sexual agglut ination. Pairwise cell contact appears to be the only signal that trig gers the sexual process in this case. In order to determine if mating type imbalances in nature are caused by reduced fertility of 'consangu ine' crosses, meiotic recombination was measured in pairs of strains t hat varied in their genetic distances as indicated by restriction mapp ing. That hypothesis was rejected, as recombination efficiency decreas ed with increasing genetic distance. We conclude that the rarity of ma ting in local populations is exacerbated by the stringent physical (pa irwise cell contact) and nutritional (nitrogen depletion) conditions t hat will allow mating to proceed. Parallels are drawn with mating patt erns observed in Clavispora lusitaniae.