SEXUAL DYSFUNCTION ASSOCIATED WITH OUTCROSSING IN NEUROSPORA-TETRASPERMA, A PSEUDOHOMOTHALLIC ASCOMYCETE

The vegetative thallus of Neurospora tetrasperma naturally exists as a self-fertile heterokaryon with nuclei of both A and a mating types. M eiosis and ascospore development are programmed to package A and a nuc lei in each spore, maintaining self-fertility in subsequent generation s. This pseudohomothallic life cycle is predominately inbreeding. Mech anisms exist, however, which give rise to single mating-type homokaryo ns that may behave as functionally heterothallic outcrossing individua ls. Ten self-fertile isolates from five sites were resolved into their A and a components. These, together with A and a laboratory wild type strains, were crossed in all combinations to assess the potential for outbreeding. Reproductive success was judged by rating perithecium an d ascus production, ascospore abortion, and viability and self-fertili ty of progeny. Disruption of the sexual cycle at various developmental stages was seen in 91% or 100 of 110 outcrosses. Few, if any, heterok aryotic self-fertile progeny were produced from most outcrosses, indic ating that sexual reproduction was relatively unsuccessful and that ou tbreeding largely interrupts the pseudohomothallic life cycle. Sexual dysfunction was apparent in crosses whether parents originated from th e same local population or from geographically distant populations. Ho wever, severity of disruption and thus reproductive isolation were som ewhat correlated with geographic distance between parents. Dysfunction appeared to be initiated by protoplasmic incompatibility between mate d haplotypes not normally combined in nature. Control crosses which re stored the original inbreeding pairs were normal. Presumably, normal i nbreeding heterokaryons pair genetically similar haplotypes which are both protoplasmically and mating-type compatible. Outbreeding could in troduce heteroallelism at protoplasmic incompatibility loci. Reproduct ive success therefore may be dependent on avoiding protoplasmic incomp atibility. These results suggest that although N. tetrasperma has mech anisms that potentially allow outbreeding, protoplasmic incompatibilit y and its resulting sexual dysfunction may actually limit outbreeding in nature.