MICROSATELLITE EVOLUTION - A RECIPROCAL STUDY OF REPEAT LENGTHS AT HOMOLOGOUS LOCI IN CATTLE AND SHEEP

The application of microsatellites in evolutionary studies requires an understanding of the patterns governing their evolution in different species. The finding that homologous microsatellite loci are longer, i .e., containing more repeat units, in human than in other primates has been taken as evidence for directional microsatellite evolution and f or a difference in the rate of evolution between species. However, it has been argued that this finding is an inevitable consequence of bias ed selection of longer-than-average microsatellites in human, because cloning procedures are adopted to generate polymorphic and, hence, lon g markers. As a test of this hypothesis, we conducted a reciprocal com parison of the lengths of microsatellite loci in cattle and sheep usin g markers derived from the bovine genome as well as the ovine genome. In both cases, amplification products were longer in the focal species , and loci were also more polymorphic in the species from which they w ere originally cloned. The crossing pattern that we found suggests tha t interspecific length differences detected at homologous microsatelli te loci are the result of biased selection of loci associated with clo ning procedures. Hence, comparisons of microsatellite evolution betwee n species are flawed unless they are based on reciprocal analyses or o n genuinely random selection of loci with respect to repeat length.