Researchers at the Institut de recherche sur la biologie de l'insecte (CNRS/Université François-Rabelais de Tours) and the Laboratoire Abeilles et Environnement at INRA, in collaboration with US and Chinese colleagues, have shown that the parasitic bee mite Varroa destructor, which can mimic the chemical composition of its host's cuticle, is also capable of adapting this composition according to the bee species that it infests. Such remarkable adaptability could explain how this parasite of the Asian bee was able to colonize the European bee during the 20th century, contributing to the decline of the latter species. This work was published on June 3, 2015 in the journal Biology Letters.
Varroa destructor is an ectoparasitic mite of the honeybee, responsible for many health problems in bee colonies. In the hives, this mite penetrates the brood cells containing bee larvae and feeds on their hemolymph. It also parasitizes pupae and adult bees, contributing to the decline observed in this species and causing significant economic losses for beekeepers. The original host of this mite is the Asian bee (Apis cerana), but it has become a serious threat to the European bee (Apis mellifera), which it started to parasitize in the 1940s-50s and which is less resistant. The behavior of Asian bees (adult grooming and larval verification by worker bees) allows them to detect and eliminate the parasite. These behaviors are less frequent in European honeybees and their colonies can be wiped out by the parasite in two to three years if no chemical treatment is applied.
Bee cuticles are made up of about fifty hydrocarbon compounds, which are used for chemical communication, among other purposes. Bees are able to recognize the composition of a cuticle and thereby identify the species or age of an individual. This also allows them to detect the presence of parasites, which have a different cuticle. Previous studies have shown that Varroa destructor mites can mimic the cuticular hydrocarbons of their host and thus escape the hygienic behavior of bees. In this new study, the researchers transferred mites from an Asian bee colony to European bee larvae and vice versa, and looked at how the parasites' ability to mimic the composition of the cuticle of a new host depends on the host bee species they originate from.
Their results show that the parasites are able to mimic both hosts, even when transferred artificially. Indeed, the proportions of cuticular hydrocarbons of the mites changed after transfer in order to mimic the cuticle of their new host. The chemical mimicry is therefore maintained and its remarkable adaptability could explain how this parasite of the Asian bee was able to colonize the domestic European honeybee.
Analysis of the cuticles also revealed that the mites from Asian honeybee colonies were better at mimicking than mites from European bees. Thus, it would seem that the long coevolution between Varroa destructor and Apis cerana has allowed the mites to be more effective in their chemical mimicry and the Asian bees to develop more suitable behavioral strategies in their fight against this parasite. In contrast, the relatively recent transfer of the mite to Apis mellifera explains why the European bee finds it difficult to detect the parasite. This host-parasite system provides a good example of an "arms race" between a parasite and its host in the course of their common evolution.
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