Evolutionary history of microsatellites in the obscure group of Drosophila

The evolutionary origins of microsatellites are not well understood. Some i nvestigators have suggested that point mutations that expand repeat arrays beyond a threshold size trigger microsatellites to become variable. However , little empirical data has been brought forth on this and related issues. In this study, we examine the evolutionary history of microsatellites in si x species within the obscura group of Drosophila. tracing changes in micros atellite alleles using both PCR product size and sequence data. We found li ttle evidence supporting a general role of point mutations triggering initi al microsatellite expansion, and no consistent threshold size for expansion was observed. Flanking region length variation was extensive when alleles were sequenced in distantly related species, and some species possessed alt ogether different repeat arrays between the same primer binding sites. Our results suggest extreme caution in using microsatellite allele sizes for ph ylogenetic analyses or to infer divergences between populations.