UNRAVELING THE PROCESSES OF MICROSATELLITE EVOLUTION THROUGH ANALYSISOF GERM-LINE MUTATIONS IN BARN SWALLOWS HIRUNDO-RUSTICA

Direct analysis of mutation events provides important clues for unders tanding the processes of microsatellite evolution. A barn swallow Hiru ndo rustica (AAAG)(n), tetranucleotide repeat locus displayed hypervar iability and hypermutability, with 44 mutations observed among 1,209 m eioses, yielding an average mutation rate of 3.6%. The magnitude of mu tations was compatible with a stepwise mutation model, although not ex clusively involving single-step changes (7 out of 44 mutations involve d 2-5 repeat units). Mutations were significantly biased toward expans ions rather than contractions, a situation that, in the absence of any balancing mechanism, would lead to gradual and eventually uncontrolle d 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 h ad an influence on the mutation process in several ways; expansions we re 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 fem ales. These observations demonstrate that the mutation process of micr osatellites may be more complex than previously thought.