Vegetative Incompatibility in Ascomycetes: Highly Polymorphic but Selectively Neutral?

Vegetative incompatibility, the prevention of somatic fusion between individuals, occurs frequently in ascomycetes. In all species studied, a large number of Vegetative Compatibility Groups (VCGs) is found. In this study the role of mutation rate, random genetic drift and selection for the evolution of these large numbers of VCGs are investigated, both analytically and by computer simulation. We find that, irrespective of selection, the product of population size and mutation rate for new VCGs has to be larger than one to get realistic numbers of VCGs in asexual populations. If the number of VCGs is large, selection does not lead to a significant increase in the number of VCGs. Sexual reproduction, however, may enlarge the number of VCGs, due to recombination of incompatiblity genes. Here strong selective pressure does give an extra increase in VCG numbers. These results suggest that vegetative incompatibility may be a selectively neutral trait, although that does not explain it s ubiquity. Finally, it can be crucial that fungi can live in extremely large populations. An extremely large population size may offer an alternative explanation for a high degree of polymorphism. This explanation does not apply for incompatibility systems of higher organisms.