Mutation rate at swine microsatellite loci

During genotyping of 38 microsatellites for QTL (quantitative trait loci) mapping in three F2 swine populations, five mutant alleles were detected in a total of 66,436 parent-offspring transfers of microsatellite alleles, which gives an overall mutation rate of 7.52 x 10(-5) per locus per generation. No significant (P0.05) association between mutation rates and other factors (i.e., GC contents in the flanking regions, heterozygosity, and repeat number) was revealed. Detailed sequencing showed that four out of five mutant alleles were caused by insertions of one to five repeats, respectively. The other mutant allele was produced by either an insertion of three repeats or a change of 30 base pairs (a deletion of 16 CT repeats and an insertion of one CA repeat). An insertion of one base pair in the flanking region of a microsatellite was also detected. Together, these data indicate that expansions are more common than contractions among microsatellites and that the mutation processes are very complicated, do not fit with the strict stepwise mutation model and may vary from locus to locus.