Mating-Type Switching in Filamentous Ascomycetes

Little is known concerning the mating-type genes of filamentous ascomycetes, compared to those of the yeasts Saccharomyces cerevisiae (reviewed by HERSKOWITZ and OSHIMA 1981; KLAR, STRATHERN and HICKS 1984) and Schizosaccharomyces pombe (see BEACH 1983, EGEL 1984). The two mating-type alleles that are typically present in genetically well-studied filamentous species, such as Neurospora crassa, Sordaria brevicollis and Ascobolus immersus, appear to be extremely stable. Mutations to loss of mating-type function have been obtained in Neurospora (GRIFFITHS and DELANCE 1978; GRIFFITHS 1982). Some of these mutations are revertible, but no changes or reversals of mating specificity have been found. In N . crassa, the mating-type genes specify functions not only in the sexual phase of the life cycle but also in the vegetative phase, where strains of opposite mating type show vegetative (heterokaryon) incompatibility with one another. Efforts to separate these different functions of the Neurospora mating-type gene by recombination have failed (PITTENCER 1957; NEWMEYER, HOWE and GALEAZZI 1973), but the two have been resolved by mutation. GRIFFITHS and DELANCE (1978) obtained a single, unique mutant in which the vegetative incompatibility function was inactivated while the mating function was retained. In all other mating-type mutations which they obtained, both the vegetative and sexual functions were inactivated. Recent developments with Neurospora may have opened the way for a better understanding. The mating-type locus has been cloned molecularly (VOLLMER and YANOFSKY 1986). A biological assay has been developed for the pheromone involved in attracting trichogynes to cells of opposite mating type, and the pheromone is being characterized (BISTIS 198 1, 1983). Complementing self-sterile mutants have been obtained and have been used for genetic manipulations in the homothallic (normally self-fertile) species Neurospora africana (SANDS 1982; ARNOLD 1983). In contrast to the stability of mating type in species such as N . crassa, what appears to be a regularly occurring unidirectional reversal of mating type has been described in the filamentous ascomycetes Chromocrea spinulosa (MATHIESON 1952), Sclerotinia trgoliorum (UHM and FUJII 1983a,b), and Glomerella cingulata (WHEELER 1950).'