Genetic basis of the honey bee Apis mellifera resistance to Varroa destructor
Evolutionary Cell and Developmental Biology (Zoology)
Final Report Abstract
The western honey bee Apis mellifera experiences major colony losses caused by Varroa destructor still after six decades after its introduction. Varroa mite's distribution was originally limited to Asia and it lived in its original host the Asian honey bee A. cerana colonies causing little to no damage. Varroa came in contact with the western honey bee due to its introduction in Asia and eventually some mite individuals switched host. After the eventual spread of Varroa destructor in Europe different western honey bee populations were screened for their resistance to Varroa. A set of resistant western honey bee populations were collected in locations like Gotland in Sweden and Toulouse in France among few others and they were left without Varroa treatment. These populations experienced initial collapse and subsequent recovery due to emerging resistance. The resistant western honey bee populations likely possess key traits necessary for viable treatment free apiculture to exist in Europe and North America. Given that in the original host of Varroa the resistance is based on Varroa being able to reproduce only in the drone brood we speculated that the brood is the essential place to look for emerging resistance. Therefore, we suggested to study the original host worker and drone brood right at the beginning of pupation and compare it to the western honey bee worker and drone brood to find clues to resistance mechanisms. One of the western honey bee resistance traits reported was drone brood resistance to Varroa. Drones are haploid in the honey bees containing only one set of chromosomes and thus making genetic studies much simpler than in diploid organism as there is not dominance and epistatic interactions become more exposed. We then suggested making a cross between resistant and susceptible western honey bees and analyse the resulting drone offspring by using quantitative trait locus (QTL) analysis. When we analysed the drone resistance to Varroa in different western honey bee populations in Gotland, Sweden and Toulouse, France, it is likely that ecdysone signalling system at the beginning of pupation may represent a key mechanism and selection tool for resistance to Varroa in the western honey bee. The resistance was based on different loci in the genomes of different resistant populations, but we could find candidate genes for resistance that were associated with the ecdysone signalling system in both resistant populations. In future studies it might be important to focus in candidate genes from our two QTL analyses. The candidate genes sequence variation may be associated with resistance or RNA interference (RNAi) could be used to see how they influence Varroa reproduction. When we compared the Asian honey bee worker and drone brood to the western honey bee worker and drone brood, there was a clear difference in the Asian honey bee worker development. We concluded that the worker and drone development at the beginning of pupation is so differentiated in the Asian honey bee that Varroa does not receive signals necessary to initiate reproductive cascade in the Asian honey bee worker brood as it had to specialise to reproduce in the drone brood. For future studies it might be fruitful to study the western honey bee populations that have shorter worker pupation time to see if this correlates with resistance to Varroa. These projects have contributed significantly to understanding the basis of resistance to Varroa in its original host the Asian honey bee and in the resistant western honey bee populations. The findings form a solid basis for many future research projects that study immediate effects of emerging resistance to Varroa mite parasites or long-term coevolutionary Varroa resistance patterns in the honey bees.
Publications
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Identifying the genetic basis of Apis mellifera drone resistance to Varroa. EurBee 2016, Cluj Napoca, Romania
Conlon BH, Oertelt E, Moritz RFA & Routtu J
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Host-parasite evolution between honeybees and Varroa. ESEB 2017, Groningen, the Netherlands
Conlon BH, Frey E, Rosenkranz P, Locke B, Moritz RFA, Routtu J
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Reproduction of Varroa in the original host the Asian honey bee. ESEB 2017, Groningen, the Netherlands
Routtu J, Conlon BH, Devaraju S, Brockmann A, Moritz RFA
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The role of epistatic interactions underpinning resistance to parasitic Varroa mites in haploid honey bee (Apis mellifera) drones. Journal of Evolutionary Biology (2018) 31: 801-809
Conlon BH, Frey E, Rosenkranz P, Locke B, Moritz RFA & Routtu J
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Transcriptomics of early Varroa infection in the Asian honey bee. IUSSI 2018, Guaruja, Brazil
Routtu J, Conlon BH, Devaraju S, Brockmann A, Moritz RFA
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A gene for resistance to the Varroa mite (Acari) in honey bee (Apis mellifera) pupae. Molecular Ecology (2019) 28: 2958-2966
Conlon BH, Aurori A, Giurgiu A-I, Kefuss J, Dezmirean DS, Moritz RFA & Routtu J
