Maintenance of sex-linked deleterious alleles by selfing and group selection in metapopulations of the phytopathogenic fungus Microbotryum violaceum.

Microbotryum violaceum is a fungus that causes the sterilizing anther smut disease in many Caryophyllaceae. Its diploid teliospores are heterozygous at the mating-type locus, normally producing equal proportions of haploid sporidia of the two mating types. However, natural populations contain high frequencies of individuals producing sporidia of only one mating type. This mating-type ratio bias is caused by the presence of deleterious alleles at haploid phase ("haplo-lethals") linked to the mating-type locus. These haplo-lethals can be transmitted if there is conjugation among the products of meiosis (intratetrad selfing). Haplo-lethals still suffer from selective disadvantages, through reducing the infection probability of strains that carry them, and thus cannot persist in a panmictic population. We develop a realistic model of a metapopulation of M. violaceum on its host Silene latifolia. Simulations show that if intratetrad selfing rate is high, haplo-lethals can be maintained under a metapopulation structure because of founder effects and selection at the population level. Populations founded only by strains carrying haplo-lethals experience a lower extinction rate precisely because of their lower infection ability; they spread more slowly and sterilize fewer plants, thereby allowing their host population to grow more rapidly and therefore to be less prone to extinction.