Mating in the heterothallic haploid yeast Clavispora opuntiae, with special reference to mating type imbalances in local populations

Abstract

Mating was studied in the haploid, heterothallic yeast Clavispora opuntiae to assess the importance of nutritional, genetic, and other factors that may favour mating and recombination. Local populations of this yeast generally exhibit dramatic inequalities in mating type distributions, suggesting that mating is rare in nature even though most isolates mate freely in the laboratory. The absence of assimilable nitrogen is prerequisite to mating competence, presumably by causing G1 arrest. Maximum mating competence is found in cells entering stationary phase in nitrogen-limited media. Unlike the vast majority of mating yeasts, C. opuntiae does not appear to produce diffusible mating factors (sex pheromones), and mating-competent cells do not undergo sexual agglutination. Pairwise cell contact appears to be the only signal that triggers the sexual process in this case. In order to determine if mating type imbalances in nature are caused by reduced fertility of 'consanguine' crosses, meiotic recombination was measured in pairs of strains that varied in their genetic distances as indicated by restriction mapping. That hypothesis was rejected, as recombination efficiency decreased with increasing genetic distance. We conclude that the rarity of mating in local populations is exacerbated by the stringent physical (pairwise cell contact) and nutritional (nitrogen depletion) conditions that will allow mating to proceed. Parallels are drawn with mating patterns observed in Clavispora lusitaniae.

Alternative Identification Test Relying upon Sexual Reproductive Abilities of Candida lusitaniae Strains Isolated from Hospitalized Patients

“…We showed that ammonium ions are the major nitrogen source whose depletion triggers mating, a finding which is consistent with those of previous studies (12,38). Other nitrogenous compounds (such as nitrates and urea) had no effect on mating, consistent with the fact that C. lusitaniae cannot assimilate these sources.…”

“…A previous work reported the apparent lack of diffusible mating factors and of sexual agglutination in both C. opuntiae and C. lusitaniae and concluded that cell-to-cell contact was required for a mating response (12). The fact that in our experiments, mating and meiosis could occur only on a solid support is consistent with that idea.…”

“…In our experiments, none of the media generally described as supporting the sexual reproduction of C. lusitaniae (5) was as effective as YCB medium, whose use was reported for mating the cactophilic yeast C. opuntiae (12). Conjugating cells, asci, and free ascospores were generally observed within 24 to 48 h after mixing and incubating in the 18 to 28°C temperature range two sexually compatible strains.…”

“…Using a mixture of cells from the 6936 MATa and 5094 MAT␣ reference strains, we first tested the solid media previously described as supporting the sexual cycle of C. lusitaniae, i.e., Gorodkowa, sodium acetate, diluted potato dextrose agar (5), and 1% malt agar (28). None of these media was as effective in our experiments as YCB medium (12) in term of swiftness (conjugation events observed within 16 to 48 h) and quantity of ascospores released (within 24 to 72 h). In order to determine the nutritional deprivation signal that triggers the sexual cycle, we further compared the efficiencies of YCB and YNB media.…”

“…Mating is possible only between two haploid cells of opposite mating types. Meiosis gives rise to clavate, echinulate ascospores that are liberated by ascus disruption (5,12,28). Like that of many other yeasts and fungi, the sexual cycle is triggered and completed during nutrient starvation.…”