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

In a recent study, we reported that the combined average mutation rate of10 di-, 6 tri-, and 8 tetranucleotide repeats in Drosophila melanogasterwas 6.3 x 10(-6) mutations per locus per generation, a rate substantiallybelow that of microsatellite repeat units in mammals studied to date (range= 10(-2)-10(-5) per locus per generation). To obtain a more preciseestimate of mutation rate for dinucleotide repeat motifs alone, we assayed39 new dinucleotide repeat microsatellite loci in the mutation accumulationlines from our earlier study. Our estimate of mutation rate for a total of49 dinucleotide repeats is 9.3 x 10(-6) per locus per generation, onlyslightly higher than the estimate from our earlier study. We also estimatedthe relative difference in microsatellite mutation rate among di-, tri-,and tetranucleotide repeats in the genome of D. melanogaster using a methodbased on population variation, and we found that tri- and tetranucleotiderepeats mutate at rates 6.4 and 8.4 times slower than that of dinucleotiderepeats, respectively. The slower mutation rates of tri- andtetranucleotide repeats appear to be associated with a relatively shortrepeat unit length of these repeat motifs in the genome of D. melanogaster.A positive correlation between repeat unit length and allelic variationsuggests that mutation rate increases as the repeat unit lengths ofmicrosatellites increase.