THEORETICAL POPULATION-GENETICS OF MATING-TYPE LINKED HAPLO-LETHAL ALLELES

The anther-smut fungus Ustilago violacea normally produces haploid spo ridia of two mating types, and conjugation between them is thought to be a prerequisite for infection of the host plant Silene alba. How eve r, some natural populations contain high frequencies of individuals wi th mating-type bias, from which sporidia of only one mating type, usua lly A1, can be isolated. Such populations show no reduction in fungal transmission rate. The bias is most readily interpreted as caused by t he presence of deleterious recessive alleles, ''haplo-lethals,'' that are linked to mating type. Haplo-lethals may persist in a heterozygous state if, during teliospore germination, there is premature conjugati on among the immediate products of meiosis, i.e., intratetrad selfing, whereby the free-living haploid stage is bypassed. We develop a theor etical model that shows how such alleles may spread if they provide a compensatory advantage in the diploid or dikaryotic phase, for example , through increased disease transmission. There is a limited range of conditions under which such haplo-lethal alleles may be maintained in a stable polymorphism, but if intratetrad selfing is high and/or they have substantial advantage in the dikaryotic phase, haplo-lethal allel es linked to mating type can spread to fixation. The occurrence of pop ulations with a high degree of mating-type bins is therefore readily e xplained. Haplo-lethal alleles unlinked to mating type are much less l ikely to spread. In U. violacea, mating type shows first-division segr egation; under such situations, haplo-lethal alleles may also readily spread if they are linked to another centromere.