SIZE HOMOPLASY AND MUTATIONAL PROCESSES OF INTERRUPTED MICROSATELLITES IN 2 BEE SPECIES, APIS-MELLIFERA AND BOMBUS-TERRESTRIS (APIDAE)

Similar microsatellite electromorphs (PCR products of the same size) c an arise from independent mutational events. Such alleles are not iden tical by descent. This phenomenon, termed size homoplasy, was studied by sequencing electromorphs of two microsatellite loci in which the st retch of basic repeats is interrupted by different short (1-2 bp) DNA motifs. The number and position of these interruptions were establishe d for electromorphs from closely and distantly related populations of honeybees and bumblebees. No sequence difference was found when electr omorphs came from the same subspecies or from closely related subspeci es, suggesting that they were probably identical by descent. In contra st, sequence differences were often detected in distantly related subs pecies, showing that size homoplasy frequently occurs at this level of population differentiation. Size homoplasy is increased by limits to free length variation of alleles, a phenomenon that seems to act on in terrupted microsatellites when comparing distantly related taxa, that is, honeybee subspecies from different evolutionary lineages. Electrom orph sequences suggest that, within the scope of these limits, large m utation events have occurred frequently at both interrupted loci studi ed. In good agreement with the molecular data, computations based on t he observed heterozygosity and number of electromorphs and simulation studies showed that neither locus fits the one-step stepwise mutant mo del (SMM). We speculate that interrupted microsatellites in general co uld be characterized by a higher variance in repeat number and consequ ently a lower homoplasy rate than pure ones. Hence, interrupted micros atellites should be most appropriate for investigating population diff erentiation and evolutionary relationship between relatively distant p opulations.