Abstract:
The mite Varroa destructor is one of the most dangerous parasites of the Western
honeybee (Apis mellifera) causing enormous colony losses worldwide. Various
chemical treatments for the control of the Varroa mite are currently in use, which,
however, lead to residues in bee products and often to resistance in mites. This
facilitated the exploration of alternative treatment methods and breeding for mite
resistant honeybees has been in focus for breeders in many parts of the world with
variable results.
Another approach has been applied to a honeybee population on Gotland (Sweden) that
was exposed to natural selection and survived Varroa-infestation for more than 10 years
without treatment. Eventually this population became resistant to the parasite by
suppressing the reproduction of the mite. A previous QTL mapping study had identified
a region on chromosome 7 with major loci contributing to the mite resistance. Here, a
microsatellite scan of the significant candidate QTL regions was used to investigate
potential footprints of selection in the original population by comparing the study
population on Gotland before (2000) and after selection (2007). Genetic drift had
caused an extreme loss of genetic diversity in the 2007 population for all genetic
markers tested. In addition to this overall reduction of heterozygosity, two loci on
chromosome 7 showed an even stronger and significant reduction in diversity than
expected from genetic drift alone. Within the selective sweep eleven genes are
annotated, one of them being a putative candidate to interfere with reduced mite
reproduction. A glucose-methanol-choline oxidoreductase (GMCOX18) might be
involved in changing volatiles emitted by bee larvae that might be essential to trigger
oogenesis in Varroa.
