Unraveling the processes of microsatellite evolution through analysis of germ line mutations in barn swallows Hirundo rustica

Direct analysis of mutation events provides important clues for understanding the processes of microsatellite evolution. A barn swallow Hirundo rustica (AAAG)(n), tetranucleotide repeat locus displayed hypervariability and hypermutability, with 44 mutations observed among 1,209 meioses, yielding an average mutation rate of 3.6%. The magnitude of mutations was compatible with a stepwise mutation model, although not exclusively involving single-step changes (7 out of 44 mutations involved 2-5 repeat units). Mutations were significantly biased toward expansions rather than contractions, a situation that, in the absence of any balancing mechanism, would lead to gradual and eventually uncontrolled growth. There was a positive relationship between repeat length and the mutation rate, at least for alleles up to about 140 repeat units. The size differences of an individual's two alleles ("allele span") did not affect the mutation rate. The sex of the mutating individual had an influence on the mutation process in several ways; expansions were more common in the male germ line than in the female germ line, the magnitude of size alteration was larger in females than in males, and the average mutation rate was nearly twice as high in males as in females. These observations demonstrate that the mutation process of microsatellites may be more complex than previously thought.