Expression and Function of Sex Pheromones and Receptors in the Homothallic AscomyceteGibberella zeae

In heterothallic ascomycete fungi, idiomorphic alleles at theMAT locus control two sex pheromone-receptor pairs that function in the recognition and chemoattraction of strains with opposite mating types. In the ascomyceteGibberella zeae , theMAT locus is rearranged such that both alleles are adjacent on the same chromosome. Strains ofG. zeae are self-fertile but can outcross facultatively. Our objective was to determine if pheromones retain a role in sexual reproduction in this homothallic fungus. Putative pheromone precursor genes (ppg1 andppg2 ) and their corresponding pheromone receptor genes (pre2 andpre1 ) were identified in the genomic sequence ofG. zeae by sequence similarity and microsynteny with other ascomycetes.ppg1 , a homolog of theSaccharomyces α-factor pheromone precursor gene, was expressed in germinating conidia and mature ascospores. Expression ofppg2 , a homolog of thea -factor pheromone precursor gene, was not detected in any cells.pre2 was expressed in all cells, butpre1 was expressed weakly and only in mature ascospores.ppg1 orpre2 deletion mutations reduced fertility in self-fertilization tests by approximately 50%. Δppg1 reduced male fertility and Δpre2 reduced female fertility in outcrossing tests. In contrast, Δppg2 and Δpre1 had no discernible effects on sexual function. Δppg1 /Δppg2 and Δpre1 /Δpre2 double mutants had the same phenotype as the Δppg1 and Δpre2 single mutants. Thus, one of the putative pheromone-receptor pairs (ppg1 /pre2 ) enhances, but is not essential for, selfing and outcrossing inG. zeae whereas no functional role was found for the other pair (ppg2 /pre1 ).