Size homoplasy and mutational processes of interrupted microsatellites in two bee species, Apis mellifera and Bombus terrestris (Apidae)

Similar microsatellite electromorphs (PCR products of the same size) canarise from independent mutational events. Such alleles are not identical bydescent. This phenomenon, termed size homoplasy, was studied by sequencingelectromorphs of two microsatellite loci in which the stretch of basicrepeats is interrupted by different short (1-2 bp) DNA motifs. The numberand position of these interruptions were established for electromorphs fromclosely and distantly related populations of honeybees and bumblebees. Nosequence difference was found when electromorphs came from the samesubspecies or from closely related subspecies, suggesting that they wereprobably identical by descent. In contrast, sequence differences were oftendetected in distantly related subspecies, showing that size homoplasyfrequently occurs at this level of population differentiation. Sizehomoplasy is increased by limits to free length variation of alleles, aphenomenon that seems to act on interrupted microsatellites when comparingdistantly related taxa, that is, honeybee subspecies from differentevolutionary lineages. Electromorph sequences suggest that, within thescope of these limits, large mutation events have occurred frequently atboth interrupted loci studied. In good agreement with the molecular data,computations based on the observed heterozygosity and number ofelectromorphs and simulation studies showed that neither locus fits theone-step stepwise mutant model (SMM). We speculate that interruptedmicrosatellites in general could be characterized by a higher variance inrepeat number and consequently a lower homoplasy rate than pure ones.Hence, interrupted microsatellites should be most appropriate forinvestigating population differentiation and evolutionary relationshipbetween relatively distant populations.