EVOLUTION OF VEGETATIVE INCOMPATIBILITY IN FILAMENTOUS ASCOMYCETES .1. DETERMINISTIC MODELS

In ascomycetes vegetative incompatibility can prevent the somatic exch ange of genetic material between conspecifics. It must occur frequentl y in natural populations, since in all species studied many vegetative compatibility groups (VCGs) are found. Using a population-genetic app roach, this paper explores two possible selective explanations for the evolution of vegetative incompatibility in asexual fungi: selection b y a nuclear parasitic gene, and selection by a harmful cytoplasmic ele ment. In a deterministic model, assuming a random spatial distribution of VCGs in an infinitely sized population, it is found that neither o f these forms of frequency-dependent selection can explain the large n umber of VCGs found in nature. The selective pressure for more VCGs di sappears once a limited number of VCGs exist, because the frequency of compatible interactions decreases when the number of VCGs increases. In comparing the two selective explanations, selection by a cytoplasmi c element seems a more plausible explanation than selection by a nucle ar gene.