Use of Apis mellifera drone's olfactory sensitivity towards pathological odours as a selection trait in the breeding against Varroa destructor

The Varroa destructor parasitisation has remained one of the main causes for colony losses all around the world since the introduction of the mite in Apis mellifera populations. One of the natural defence mechanisms used by the honeybee in the fight against V. destructor is hygienic behaviour, particularly a specialized form of it – Varroa-sensitive hygiene (VSH) – which includes the uncapping and removing of parasite-infested, or dead brood. When infested, capped brood changes its cuticular profile sending an olfactory signal which only a few workers in the colony can sense. Subsequently, the brood cell is opened, and the mite removed. Hygienic behaviour and olfactory sensitivity to brood-related odours are observed to be strongly influenced by genetics. Breeding efforts with the goal of enhancing the European honeybee’s resilience have been ongoing for more than three decades, but to date no breeding strategy has reached a broad-scale host-parasite balance. Due to the strong environmental dependence of resistance traits, the reproductive and genetic peculiarities of the honeybee, as well the small-scale structure of the German beekeeping community, very labour-intensive traditional breeding programmes for this species are in need of transformation. Having this in mind, this project focused on the development of a new strategy that has the potential to make breeding in honeybees significantly more efficient by utilising drones’ olfactory as a selection trait. As drones are haploid, the genetic potential for Varroa-resistance can be assessed on the gametes. This allows genetic differences to be recognised significantly better and enables faster selection success. The proboscis extension response (PER) conditioning as a non-invasive method of observing olfactory sensitivity in both workers and drones enabled the gathering of insights into VSH and its inheritance and was chosen for the selection of the most suitable individuals. In a first step, workers bred for VSH and nonselected control line workers were tested via PER conditioning for differences in their speed and perception ability when presented with highly diluted stimuli. Two pairs of odours (Pair 1: citral as Cs+: linalool as Cs-; Pair 2: Varroa-parasitised brood extract as Cs+, isopropanol as Cs-) were used as tactile stimuli for the differential conditioning. Citral – a floral odour – and the brood extract were especially chosen in order to observe whether breeding for resistance in one of the tested groups had an effect on odour sensitivity to all or only to special odours connected to VSH. The VSH-selected line exhibited a significantly higher speed of perception for the parasitised brood extract than the nonselected line. The two lines showed no differences when conditioned with the floral stimulus citral as Cs+. The results suggested an increased specific sensitivity to chemical stimuli emanating from the brood in VSH-selected workers, which could play a role in recognising and removing V. destructor. In a second step, the odour sensitivity of drones to the Varroa-parasitised-brood extract was examined through PER conditioning. Sperm from drones, sensitive/insensitive to two Varroa-parasitised-brood odours concentrations was extracted, and queens from VSH-selected and nonselected lines were inseminated accordingly, following a mating scheme. The VSH behaviour of the offspring was observed, and the genetic origin of queens and drones as well as the drones’ perception of the brood odour were considered. While drone PER conditioning did not significantly correlate with VSH results, the genetic origin of participating queens and drones played a crucial role in VSH manifestation. A tendency for maternal effects for the inheritance of VSH was observed, suggesting that the choice of father drones would have less influence on VSH than that of mothers as genes connected to VSH are inherited via mitochondrial DNA of the mother. Additive genetic effects were also observed when drones from the VSH-selected line were paired with queens from the VSH-selected line, suggesting that drone’s genes nevertheless play an important role in resistance breeding. The role of the drone should therefore not be underestimated.