Temporal changes in the viromes of Swedish Varroa-resistant and Varroa-susceptible honeybee populations

Abstract

The parasitic mite, Varroa destructor, in combination with the viruses it vectors, is the main cause for global colony losses of the European honeybee, Apis mellifera. However, an isolated honeybee population established in 1999 on the Island of Gotland, Sweden has naturally acquired resistance to the mite, and has survived without mite control treatment for more than 18 years. A recent study has shown that this mite resistant (MR) population also appears to be resistant to Black queen cell virus (BQCV) and Sacbrood virus (SBV) and tolerant to Deformed wing virus (DWV), relative to nearby mite susceptible (MS) honeybee populations. In this study, RNA sequencing was employed to corroborate these previous findings and identify other viral factors that may play a role in the enhanced survival of this mite resistant honeybee population. Two additional honeybee-infecting viruses, Apis rhabdovirus-1 (ARV-1) and Lake Sinai virus (LSV), were identified and near-complete genomes of these two viruses were obtained. Phylogenetic analyses of the assembled virus sequences revealed consistent separation between the MR and MS honeybee populations, although it is unclear whether this is due to pre-existing differences between the viruses in the two populations when they were established, and isolated, or due to virus genetic adaptation towards reduced virulence in the MR population, to promote colony survival. Reverse transcription quantitative polymerase chain reaction(RT-qPCR) analyses show higher ARV and LSV titres in MS colonies compared to MR colonies, gradually increasing from summer to autumn 2009, and reaching maximum titres in the following spring 2010. While the DWV and BQCV titres in MR colonies increased between autumn 2009 and spring 2010, the SBV practically disappeared entirely by spring 2010. Possible explanations for the apparent virus tolerance or resistance in the Gotland mite-resistant honeybee population are discussed.

Diversity and Global Distribution of Viruses of the Western Honey Bee, Apis mellifera

“…For instance, naturally mite-resistant colonies show a significant reduction of the mite population size relative to non-resistant colonies. Consequently, they display much lower DWV titers and different virus infection dynamics [51,146]. To minimize colony losses, the beekeeping industry has developed several strategies and tools to control mite populations infesting mite-susceptible honey bee stocks.…”

Bee Viruses: Routes of Infection in Hymenoptera

“…More conclusive evidence is needed to elucidate the precise role of mites in the transmission of this virus (de Miranda et al, 2015). Similarly, there is no conclusive evidence that SBV is directly transmitted by V. destructor, although it is frequently associated with varroarelated damage (Dubois et al, 2020), host-virus molecular interactions (Di Prisco et al, 2016;Ryabov et al, 2016;Remnant et al, 2019) and tolerance (Thaduri et al, 2018) as well as in V. destructor behavior (Giuffre et al, 2019). For BQCV, there is no evidence that it replicates in or is transmitted by V. destructor.…”

Natural selection, selective breeding, and the evolution of resistance of honeybees (Apis mellifera) against Varroa

“…In addition to lower virulence of the parasite, increased tolerance/resistance of the host is also favoured in a system with predominantly vertical transmission. Evidence for the role of virus tolerance in the Gotland population comes from [130][131][132] and [133]. Likewise, A. m. scutellata seems resistant or tolerant to DWV [134].…”