MICROSATELLITE ALLELE FREQUENCIES IN HUMANS AND CHIMPANZEES, WITH IMPLICATIONS FOR CONSTRAINTS ON ALLELE SIZE

The distributions of allele sizes at eight simple-sequence repeat (SSR ) or microsatellite loci in chimpanzees are found and compared with th e distributions previously obtained from several human populations. At several loci, the differences in average allele size between chimpanz ees and humans are sufficiently small that there might be a constraint on the evolution of average allele size. Furthermore, a model that al lows for a bias in the mutation process shows that for some loci a wea k bias can account for the observations. Several alleles at one of the loci (Mfd 59) were sequenced. Differences between alleles of differen t lengths were found to be more complex than previously assumed. An 8- base-pair deletion was present in the nonvariable region of the chimpa nzee locus. This locus contains a previously unrecognized repeated reg ion, which is imperfect in humans and perfect in chimpanzees. The appa rently greater opportunity for mutation conferred by the two perfect r epeat regions in chimpanzees is reflected in the higher variance in re peat number at Mfd 59 in chimpanzees than in humans. These data indica te that interspecific differences in allele length are not always attr ibutable to simple changes in the number of repeats.