Genetics, Vol 144, 557-567, Copyright © 1996

INVESTIGATIONS

Female Fertility and Mating Type Effects on Effective Population Size and Evolution in Filamentous Fungi

J. F. Leslie and K. K. Klein
Department of Plant Pathology, Kansas State University, Manhattan, Kansas 66506-5502

The idealized individual in many fungal species is a haploid self-sterile hermaphrodite that may be propagated by asexually produced spores or that may reproduce sexually. In field populations, polymorphism occurs for female-sterile/hermaphrodite status, and female-sterile mutants, which function only as males during sexual reproduction, may comprise >50% of the population. The effective population number may be based on the number of strains of different mating type or the relative frequency of hermaphrodites. The female-sterile mutants are at a selective disadvantage every time sexual reproduction occurs, and must have an advantage during vegetative propagation to persist at a significant frequency. When a high frequency of female-sterile strains is observed in field populations, it indicates that vegetative propagation is a significant component of the fungus' natural history. Depending on the mutation rate to female sterility and the selective advantage of the female-sterile strains during vegetative propagation, the ratio of sexual:asexual generations can range from 1:15 to 1:2300 for species in the Gibberella fujikuroi complex. The relative rarity of sexual reproduction may permit female-sterile strains to accumulate to a level such that local populations could completely lose sexuality and appear as asexual (imperfect) species.

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