A High incidence of clustered microsatellite mutations revealed by parent-offpring analysis in an African freshwater snail, Bulinus forskalii (Gastropoda, Pulmonata).

Genotyping of 11 microsatellites in 432 offspring from 28 families of the hermaphroditic, freshwater snail Bulinus forskalii detected 10 de novo mutant alleles. This gave an estimated mutation rate of 1.1 X 10(-3) per locus per gamete per generation. There was a trend towards repeat length expansion and, unlike most studies, multi-step mutations predominated, suggesting that the microsatellite mutation process does not conform to a strict stepwise mutation model. Interestingly, the ten mutant alleles appear to have arisen from only six independent germline mutation events within the microsatellite array, with seven of them residing in three mutational clusters. Our results extend observations of clustered microsatellite mutations to another taxonomic group and type of mating system, self-fertile gastropods, and provide compelling evidence of premeiotic germ,line mutations, a phenomenon that could greatly impact upon our understanding of mutation dynamics but which has received little attention.