Abstract:
Varroa destructor is the most devastating parasite of the Western honeybee, Apis
mellifera. In the light of the arm race opposing the host and its parasite, the population
dynamics and genetic diversity of these organisms are key parameters. However, the life
cycle of V. destructor is characterized by extreme inbreeding due to full sibling mating in the
host brood cells. We here present an equation reflecting the evolution of inbreeding in such a
clonal system, and compare our predictions with empirical data based on the analysis of
seven microsatellite markers. This comparison revealed that the mites perform essentially
incestuous mating in the beginning of the brood season. However, this pattern changes with
the development of mite infestation. Despite the fact that the overall level of genetic diversity
of the mites remained low through the season, multiple inbred lineages were identified in the
mites we sampled in June. As a response to the decrease of brood availability and the
increase of the parasite population in parallel in the colonies, these lineages recombined
towards the end of the season as mites co-infest brood cells. Our results suggest that the ratio
of the number of mite per brood cell in the colony determines the genetic structure of the
populations of V. destructor. This intracolonial population dynamics has great relevance for
the selection of acaricide resistance in V. destructor. If chemical treatments occur before the
recombination phase, inbreeding will greatly enhance the fixation of resistance alleles at the
colony level.
