Host-specific differentiation in the anther smut fungus Microbotryum violaceum as revealed by microsatellites

The anther smut fungus Microbotryum violaceum ( = Ustilago violacea) is a p arasite of many species of the Caryophyllaceae. Its host specificity has be en debated since early this century, when cross-inoculation experiments ind icated the existence of host-specific lineages. Recently, on the basis of s pore ultrastructure, all presumed host races were lumped within M. violaceu m. To measure gene flow among natural populations of anther smuts from diff erent host species, we used microsatellite variation at 5 loci among sample s from 8 Silene, 2 Saponaria, 2 Dianthus and 1 Gypsophila species. Most of the 326 M. violaceum samples investigated originated from the Swiss Alps an d close surroundings. Microsatellite variation revealed almost perfect isol ation among anther smut fungi from different host species. In addition, dif ferentiation was supported by the nonrandom distribution of null alleles am ong samples from different host species and host genera. Null alleles were most abundant in anther smut samples from non-Silene hosts. The resolution of genetic differentiation among anther smuts from different host species w as highest in those from Silene species. Genetic relationships among sample s as indicated by Neighbour-Joining analysis based on genetic distances are discussed with respect to host phylogeny and host ecology. One sample was identified as Ustilago gausseni because it had verrucose instead of reticul ate spores and was collected from Silene italica. Neighbour-Joining analysi s revealed that this sample was similar to the M. violaceum samples from ot her Silene host species. Therefore, our data question spore morphology as a reliable character for anther smut systematics.