The mating system of the fungus Cryphonectria parasitica: selfing and self-incompatibility

Although the genetic components of mating systems in fungi are well underst ood as laboratory phenomena, surprisingly little is known about their funct ion in nature or about their role in determining mating patterns and popula tion genetic structure. Our study of the mating system of the haploid ascom ycete fungus, Cryphonectria parasitica, resulted in the following. (1) Labo ratory crosses among 20 isolates, chosen randomly from North America and Ch ina, resolved into two incompatibility groups (occurring on both continents ), confirming that C. parasitica has a diallelic, bipolar sexual self-incom patibility system, typical of other self-incompatible Ascomycetes, in which mating is only successful between isolates of opposite mating type. (2) PC R-based markers for mating-type alleles correlated perfectly with mating-ty pe phenotypes of individual isolates. (3) Three genotypes, isolated from na tural populations in Virginia and West Virginia, were inoculated onto chest nut trees in two sites in West Virginia and were confirmed to have self-fer tilized and outcrossed in both sites. (4) Ten isolates, of a total of over 200 assayed, were confirmed to have self-fertilized in the laboratory, albe it at very low frequency. Five of these 10 isolates were ramets of a single genet, suggesting a genetic basis underlying the proclivity to self-fertil ize in the laboratory. (5) Self-fertilization could not be induced in the l aboratory with exudates (ostensibly containing pheromones) from isolates of opposite mating type. These results demonstrate that, a sexual self-incomp atibility system notwithstanding, self-fertilization occurs under both labo ratory and field conditions in C. parasitica. The disparity between observa tions of frequent selfing in nature and rare selfing in the laboratory sugg ests that the mating system is under ecological as well as genetic control.