HOMOPLASY FOR SIZE AT MICROSATELLITE LOCI IN HUMANS AND CHIMPANZEES

Homoplasy (convergence in the size of different alleles) at microsatel lite loci was examined by sequencing multiple alleles of two compound microsatellites and single copies of alleles of the same size at two o ther compound loci in both chimpanzees and humans. At one of the two l oci for which multiple alleles were sequenced, extensive homoplasy For size was uncovered both within and between species. At the three loci for which alleles of the same size were examined in the two species, sequencing demonstrated different internal structures. These results c onfirm theoretical predictions that a certain fraction of mutations at microsatellite loci should produce alleles that are identical in size but differ by a number of mutations. The sequence data reveal a previ ously unrecognized class of variation at microsatellites and open up t he possibility that DNA sequencing may allow the extraction of more in formation from these loci, thus increasing their power as variable mar kers for genetic mapping studies. Conversely, the data also indicate t hat the assumption that alleles of the same size are identical in sequ ence, which is implicit in several methods of analysis, is violated in some cases. Therefore, caution should be used when employing microsat ellites in phylogenetic and other studies in which the individuals bei ng examined are separated by a great number of generations from a comm on ancestor.