Genetic analysis of the sam mutations, which induce sexual development with no requirement for nutritional starvation in fission yeast.

The cAMP pathway and the Ras pathway are the two major pathways to sexual development in the fission yeast Schizosaccharomyces pombe. To understand the cAMP pathway or the related pathway, we analyzed mutants that display a phenotype similar to cyr1-, that is, hyper-sporulation. Nine mutants termed sam (sporulation abnormal mutant), which are highly inclined to sexual development despite the presence of nitrogen sources, were partially characterized. Cyclic AMP was detected in all nine sam mutant cells, and over-expression of the adenylyl cyclase gene (cyr1) failed to suppress the hyper-sporulation phenotype of these sam mutants, suggesting that none of the sam mutants were likely to be allelic to cyr1. Epistatic tests of sam mutants showed that they were divided into two dominant and seven recessive mutants. Dominants were able to make spores in sam/sam+ heterodiploid cells upon abundant nutrients. Both two dominant mutants bypassed the inability to make spores in ras1 deficient diploid cells, suppressed the deficiency to execute sporulation in byr2 deficient diploid cells, but failed to suppress the byr1 deficiency. Two dominant mutations seem not to occur within the byr2 gene.