Sex-ratio bias in populations of the phytopathogenic fungus Microbotryum violaceum from several host species

The present study explores the maintenance of a segregation distortion in the phytopathogenic fungus Microbotryum violaceum (anther smut). Some individuals of this species produce spores with completely biased sex ratio because of the presence of haplo-lethal alleles linked to the mating-type gene. To test the different hypotheses proposed to explain the maintenance of these haplo-lethal alleles (diploid advantage to counterbalance haploid deleterious effects, neutrality of haplo-lethal alleles only in populations with high interstrain competition, and complete neutrality), sex-ratio bias was investigated in natural European populations of M. violaceum from Silene latifolia and other species of Caryophyllaceae. The negative relationship observed between proportions of strains expressing a sex-ratio bias and disease prevalence was opposite the one predicted by the second hypothesis. On the contrary, the pattern of the variances, with proportions of biased strains more extreme in smaller populations, fitted perfectly with the prediction of the third hypothesis. Experimental studies are now needed to confirm the complete neutrality of the haplo-lethal alleles linked to the mating-type gene in M. violaceum.