Sexual dysfunction associated with outcrossing in Neurospora tetrasperma, a pseudohomothallic ascomycete

Abstract

The vegetative thallus of Neurospora tetrasperma naturally exists as a self-fertile heterokaryon with nuclei of both A and a mating types. Meiosis and ascospore development are programmed to package A and a nuclei in each spore, maintaining self-fertility in subsequent generations. This pseudohomothallic life cycle is predominately inbreeding. Mechanisms exist, however, which give rise to single mating-type homokaryons that may behave as functionally heterothallic outcrossing individuals. 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 and ascus production, ascospore abortion, and viability and self-fertility of progeny. Disruption of the sexual cycle at various developmental stages was seen in 91% or 100 of 110 outcrosses. Few, if any, heterokaryotic self-fertile progeny were produced from most outcrosses, indicating that sexual reproduction was relatively unsuccessful and that outbreeding largely interrupts the pseudohomothallic life cycle. Sexual dysfunction was apparent in crosses whether parents originated from the same local population or from geographically distant populations. However, severity of disruption and thus reproductive isolation were somewhat correlated with geographic distance between parents. Dysfunction appeared to be initiated by protoplasmic incompatibility between mated haplotypes not normally combined in nature. Control crosses which restored the original inbreeding pairs were normal. Presumably, normal inbreeding heterokaryons pair genetically similar haplotypes which are both protoplasmically and mating-type compatible. Outbreeding could introduce heteroallelism at protoplasmic incompatibility loci. Reproductive success therefore may be dependent on avoiding protoplasmic incompatibility. These results suggest that although N. tetrasperma has mechanisms that potentially allow outbreeding, protoplasmic incompatibility and its resulting sexual dysfunction may actually limit outbreeding in nature.

Key Words:

• inbreeding
• protoplasmic incompatibility
• reproductive isolation
• secondary homothallism