The mutation rates of di-, tri- and tetranucleotide repeats in Drosophila melanogaster

In a recent study, we reported that the combined average mutation rate of 1 0 di-, 6 tri-, and 8 tetranucleotide repeats in Drosophila melanogaster was 6.3 X 10(-6) mutations per locus per generation, a rate substantially belo w that of microsatellite repeat units in mammals studied to date (range = 1 0(-2)-10(-5) per locus per generation). To obtain a more precise estimate o f mutation rate for dinucleotide repeat motifs alone, we assayed 39 new din ucleotide repeat microsatellite loci in the mutation accumulation lines fro m our earlier study. Our estimate of mutation rate for a total of 49 dinucl eotide repeats is 9.3 x 10(-6) per locus per generation, only slightly high er than the estimate from our earlier study. We also estimated the relative difference in microsatellite mutation rate among di-, tri-, and tetranucle otide repeats in the genome of D. melanogaster using a method based on popu lation variation, and we found that tri- and tetranucleotide repeats mutate at rates 6.4 and 8.4 times slower than that of dinucleotide repeats, respe ctively. The slower mutation rates of tri- and tetranucleotide repeats appe ar to be: associated with a relatively short repeat unit length of these re peat motifs in the genome of D. melanogaster. A positive correlation betwee n repeat unit length and allelic variation suggests that mutation rate incr eases as the repeat unit lengths of microsatellites increase.