Functional heterothallism resulting from homokaryotic conidia and ascospores in Neurospora tetrasperma

Each ascospore of Neurospora tetrasperma normally contains haploid nuclei of both mating types, A and a, and produces a self-fertile culture. A majority of cultures from single conidia are also self-fertile, because most conidia are multinucleate and contain nuclei of both mating types. It has long been known that some single-mating type, self-sterile cultures are produced, either from exceptional homokaryotic ascospores or from the minority of conidia that are homokaryotic. The homokaryotic, self-sterile cultures are crossfertile with strains of opposite mating type, and can be fertilized by compatible nuclei from either homokaryotic or heterokaryotic cultures. The present study shows that the spontaneous proportion of self-sterile propagules from nonmutant strains can be as high as 10% for ascospores and 20% for conidia. Thus, although N. tetrasperma is predominantly inbred, the species is nevertheless capable of significant heterothallic behaviour and outcrossing. As shown by B. O. Dodge, the Eight-spore mutation E causes most of the asci to produce five to eight ascospores by affecting spindle behaviour, pairwise association of nuclei, and ascospore delimitation. Cytological events in crosses heterozygous for E have been re-examined and crosses have also been examined that involve two wild strains which superficially resemble E in producing many homokaryotic ascospores by a somewhat different mechanism. Events in the developing wild type and mutant asci are documented photographically for the first time.