Mating-type genes have been identified not only in heterothallic and pseudohomothallic, but also in homothallic members of the Sordariaceae. This chapter provides a comprehensive and up-to-date overview of the mating-type genes and their roles in the homothallic ascomycete S. macrospora. The homothallic S. macrospora is the most studied species in the genus. Even though single-spore cultures of S. macrospora are self-fertile, they may be crossed. For example, when two S. macrospora strains are paired in a petri dish, each mycelium gives rise to perithecia and homokaryotic asci. Cloning and sequencing of the entire mating-type locus of S. macrospora revealed that sequences homologous to the MATA and MATa idiomorphs of Neurospora crassa are directly linked in the mating-type locus of S. macrospora. To elucidate how reproductive modes might have evolved in different members of the Sordariaceae, phylogenetic relationships of MATA- and MATa-specific mating-type genes from heterothallic and homothallic members of the genera Sordaria and Neurospora have been examined. The phylogenetic analyses suggest that changes in the reproductive strategy may represent a single event in each genus. Transcription of cell-type-specific genes in the ascomycete yeast S. cerevisiae has been shown to rely on the interaction mating-type proteins. Thus, heterodimerization provides a way of regulating transcription factor function by joining different functional domains.
