Clustered microsatellite mutations in the pipefish Syngnathus typhle.

Clustered mutations are copies of a mutant allele that enter a population's gene pool together due to replication from a premeiotic germline mutation and distribution to multiple successful gametes of an individual. Although the phenomenon has been studied in Drosophila and noted in a few other species, the topic has received scant attention despite claims of being of major importance to population genetics theory. Here we capitalize upon the reproductive biology of male-pregnant pipefishes to document the occurrence of clustered microsatellite mutations and to estimate their rates and patterns from family data. Among a total of 3195 embryos genetically screened from 110 families, 40% of the 35 detected de novo mutant alleles resided in documented mutational clusters. Most of the microsatellite mutations appeared to involve small-integer changes in repeat copy number, and they arose in approximately equal frequency in paternal and maternal germlines. These findings extend observations on clustered mutations to another organismal group and motivate a broader critique of the mutation cluster phenomenon. They also carry implications for the evolution of microsatellites with respect to mutational models and homoplasy among alleles.