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) ormicrosatellite loci in chimpanzees are found and compared with thedistributions previously obtained from several human populations. Atseveral loci, the differences in average allele size between chimpanzeesand humans are sufficiently small that there might be a constraint on theevolution of average allele size. Furthermore, a model that allows for abias in the mutation process shows that for some loci a weak bias canaccount for the observations. Several alleles at one of the loci (Mfd 59)were sequenced. Differences between alleles of different lengths were foundto be more complex than previously assumed. An 8-base-pair deletion waspresent in the nonvariable region of the chimpanzee locus. This locuscontains a previously unrecognized repeated region, which is imperfect inhumans and perfect in chimpanzees. The apparently greater opportunity formutation conferred by the two perfect repeat regions in chimpanzees isreflected in the higher variance in repeat number at Mfd 59 in chimpanzeesthan in humans. These data indicate that interspecific differences inallele length are not always attributable to simple changes in the numberof repeats.