Parasite and virus dynamics in the honeybee Apis mellifera unicolor on a tropical island recently invaded by Varroa destructor.

In La Réunion, the established honeybee subspecies Apis mellifera unicolor, an endemic subspecies of African lineage, is facing considerable challenges. Since the introduction of the Varroa destructor mite in 2017 high colony losses have been recorded. We investigated the dynamics of V. destructor and two viruses, the Deformed Wing Virus (DWV), known to be transmitted by the mite, and the Chronic Bee Paralysis Virus (CBPV), in A. m. unicolor. Colonies from two apiaries located at 300 and 900 m a.s.l were monitored twice for one year without any acaricide treatment. The brood area, V. destructor infestation rates, DWV and CBPV prevalence and load were recorded monthly. A. m. unicolor maintained brood rearing throughout the year. Varroa destructor infestation resulted in high colony mortality (up to 85 %) and high phoretic mite rates (up to 52 mites per hundred bees). The establishment of DWV in colonies occurred after that of V. destructor and the mite infestation rate had a significant effect on the virus prevalence and load. CBPV appeared only transiently throughout the surveys. The data showed that, in tropical colonies with permanent brood rearing, V. destructor and DWV can reach high levels, but are still subject to seasonal variations that appear to be influenced by environmental conditions. This suggests that beekeeping practices could be adapted by favouring sites and periods for transhumance or acaricide treatment.

Caught in the act: the invasion of a viral vector changes viral prevalence and titre in native honeybees and bumblebees.

Novel transmission routes change pathogen landscapes and may facilitate disease emergence. The varroa mite is a virus vector that switched to western honeybees at the beginning of the last century, leading to hive mortality, particularly in combination with RNA viruses. A recent invasion of varroa on the French island of Ushant introduced vector-mediated transmission to one of the last varroa-naive native honeybee populations and caused rapid changes in the honeybee viral community. These changes were characterized by a drastic increase in deformed wing virus type B prevalence and titre in honeybees, as well as knock-on effects in bumblebees, particularly in the year following the invasion. Slow bee paralysis virus also appeared in honeybees and bumblebees, with a 1 year delay, while black queen cell virus declined in honeybees. This study highlights the rapid and far-reaching effects of vector-borne transmission that can extend beyond the directly affected host species, and that the direction of the effect depends on the pathogen's virulence.

Effects of natural treatments on the varroa mite infestation levels and overall health of honey bee (Apis mellifera) colonies.

The survival of the honey bee (Apis mellifera), which has a crucial role in pollination and ecosystem maintenance, is threatened by many pathogens, including parasites, bacteria, fungi and viruses. The ectoparasite Varroa destructor is considered the major cause of the worldwide decline in honey bee colony health. Although several synthetic acaricides are available to control Varroa infestations, resistant mites and side effects on bees have been documented. The development of natural alternatives for mite control is therefore encouraged. The study aims at exploring the effects of cinnamon and oregano essential oils (EOs) and of a mixed fruit cocktail juice on mite infestation levels and bee colony health. A multi-method study including hive inspection, mite count, molecular detection of fungal, bacterial and viral pathogens, analysis of defensin-1, hymenoptaecin and vitellogenin immune gene expression, colony density and honey production data, was conducted in a 20-hive experimental apiary. The colonies were divided into five groups: four treatment groups and one control group. The treatment groups were fed on a sugar syrup supplemented with cinnamon EO, oregano EO, a 1:1 mixture of both EOs, or a juice cocktail. An unsupplemented syrup was, instead, used to feed the control group. While V. destructor affected all the colonies throughout the study, no differences in mite infestation levels, population density and honey yield were observed between treatment and control groups. An overexpression of vitellogenin was instead found in all EO-treated groups, even though a significant difference was only found in the group treated with the 1:1 EO mixture. Viral (DWV, CBPV and BQCV), fungal (Nosema ceranae) and bacterial (Melissococcus plutonius) pathogens from both symptomatic and asymptomatic colonies were detected.

Eggs sampling as an effective tool for identifying the incidence of viruses in honey bees involved in artificial queen rearing.

The Carniolan honey bee (Apis mellifera carnica) plays an essential role in crop pollination, environment diversity, and the production of honey bee products. However, the health of individual honey bees and their colonies is under pressure due to multiple stressors, including viruses as a significant threat to bees. Monitoring various virus infections could be a crucial selection tool during queen rearing. In the present study, samples from all developmental stages (eggs, larvae, pupae, and queens) were screened for the incidence of seven viruses during queen rearing in Slovenia. The screening of a total of 108 samples from five queen breeders was performed by the RT-qPCR assays. The results showed that the highest incidence was observed for black queen cell virus (BQCV), Lake Sinai virus 3 (LSV3), deformed wing virus B (DWV-B), and sacbrood virus (SBV). The highest viral load was detected in queens (6.07 log10 copies/queen) and larvae (5.50 log10 copies/larva) for BQCV, followed by SBV in larvae (5.47 log10 copies/larva). When comparing all the honey bee developmental stages, the eggs exhibited general screening for virus incidence and load in queen mother colonies. The results suggest that analyzing eggs is a good indicator of resilience to virus infection during queen development.

Two Novel Geminiviruses Identified in Bees (Apis mellifera and Nomia sp.).

Members of the Geminviridae family are circular single-stranded DNA plant-infecting viruses, some of which impact global food production. Geminiviruses are vectored by sap-feeding insects such as leafhoppers, treehoppers, aphids, and whiteflies. Additionally, geminivirus sequences have also been identified in other insects such as dragonflies, mosquitoes, and stingless bees. As part of a viral metagenomics study on honeybees and solitary bees (Nomia sp.), two geminivirus genomes were identified. These represent a novel citlodavirus (from honeybees collected from Westmoreland, Jamaica) and a mastrevirus-like genome (from a solitary bee collected from Tempe, Arizona, USA). The novel honeybee-derived citlodavirus genome shares ~61 to 69% genome-wide nucleotide pairwise identity with other citlodavirus genome sequences and is most closely related to the passion fruit chlorotic mottle virus identified in Brazil. Whereas the novel solitary bee-derived mastrevirus-like genome shares ~55 to 61% genome-wide nucleotide identity with other mastreviruses and is most closely related to tobacco yellow dwarf virus identified in Australia, based on pairwise identity scores of the full genome, replication-associated protein, and capsid protein sequences. Previously, two geminiviruses in the Begomovirus genus were identified in samples of stingless bee (Trigona spp.) samples. Here, we identify viruses that represent two new species of geminiviruses from a honeybee and a solitary bee, which continues to demonstrate that plant pollinators can be utilized for the identification of plant-infecting DNA viruses in ecosystems.

Host Barriers Limit Viral Spread in a Spillover Host: A Study of Deformed Wing Virus in the Bumblebee Bombus terrestris.

The transmission of pathogens from reservoir to recipient host species, termed pathogen spillover, can profoundly impact plant, animal, and public health. However, why some pathogens lead to disease emergence in a novel species while others fail to establish or do not elicit disease is often poorly understood. There is strong evidence that deformed wing virus (DWV), an (+)ssRNA virus, spills over from its reservoir host, the honeybee Apis mellifera, into the bumblebee Bombus terrestris. However, the low impact of DWV on B. terrestris in laboratory experiments suggests host barriers to virus spread in this recipient host. To investigate potential host barriers, we followed the spread of DWV genotype B (DWV-B) through a host's body using RT-PCR after experimental transmission to bumblebees in comparison to honeybees. Inoculation was per os, mimicking food-borne transmission, or by injection into the bee's haemocoel, mimicking vector-based transmission. In honeybees, DWV-B was present in both honeybee faeces and haemolymph within 3 days of inoculation per os or by injection. In contrast, DWV-B was not detected in B. terrestris haemolymph after inoculation per os, suggesting a gut barrier that hinders DWV-B's spread through the body of a B. terrestris. DWV-B was, however, detected in B. terrestris faeces after injection and feeding, albeit at a lower abundance than that observed for A. mellifera, suggesting that B. terrestris sheds less DWV-B than A. mellifera in faeces when infected. Barriers to viral spread in B. terrestris following oral infection may limit DWV's impact on this spillover host and reduce its contribution to the community epidemiology of DWV.

Varroa destructor parasitism and Deformed wing virus infection in honey bees are linked to peroxisome-induced pathways.

The ectoparasitic mite Varroa destructor transmits and triggers viral infections that have deleterious effects on honey bee colonies worldwide. We performed a manipulative experiment in which worker bees collected at emergence were exposed to Varroa for 72 h, and their proteomes were compared with those of untreated control bees. Label-free quantitative proteomics identified 77 differentially expressed A. mellifera proteins (DEPs). In addition, viral proteins were identified by orthogonal analysis, and most importantly, Deformed wing virus (DWV) was found at high levels/intensity in Varroa-exposed bees. Pathway enrichment analysis suggested that the main pathways affected included peroxisomal metabolism, cyto-/exoskeleton reorganization, and cuticular proteins. Detailed examination of individual DEPs revealed that additional changes in DEPs were associated with peroxisomal function. In addition, the proteome data support the importance of TGF-ß signaling in Varroa-DWV interaction and the involvement of the mTORC1 and Hippo pathways. These results suggest that the effect of DWV on bees associated with Varroa feeding results in aberrant autophagy. In particular, autophagy is selectively modulated by peroxisomes, to which the observed proteome changes strongly corresponded. This study complements previous research with different study designs and suggests the importance of the peroxisome, which plays a key role in viral infections.

Virus replication in the honey bee parasite, Varroa destructor.

IMPORTANCE: The parasitic mite Varroa destructor is a significant driver of worldwide colony losses of our most important commercial pollinator, the Western honey bee Apis mellifera. Declines in honey bee health are frequently attributed to the viruses that mites vector to honey bees, yet whether mites passively transmit viruses as a mechanical vector or actively participate in viral amplification and facilitate replication of honey bee viruses is debated. Our work investigating the antiviral RNA interference response in V. destructor demonstrates that key viruses associated with honey bee declines actively replicate in mites, indicating that they are biological vectors, and the host range of bee-associated viruses extends to their parasites, which could impact virus evolution, pathogenicity, and spread.

Exploring viral infections in honey bee colonies: insights from a metagenomic study in southern Brazil.

The decline in honey bee colonies in different parts of the world in recent years is due to different reasons, such as agricultural practices, climate changes, the use of chemical insecticides, and pests and diseases. Viral infections are one of the main causes leading to honey bee population declines, which have a major economic impact due to honey production and pollination. To investigate the presence of viruses in bees in southern Brazil, we used a metagenomic approach to sequence adults' samples of concentrated extracts from Apis mellifera collected in fifteen apiaries of six municipalities in the Rio Grande do Sul state, Brazil, between 2016 and 2017. High-throughput sequencing (HTS) of these samples resulted in the identification of eight previously known viruses (Apis rhabdovirus 1 (ARV-1), Acute bee paralysis virus (ABPV), Aphid lethal paralysis virus (ALPV), Black queen cell virus (BQCV), Bee Macula-like virus (BeeMLV), Deformed wing virus (DWV), Lake Sinai Virus NE (LSV), and Varroa destructor virus 3 (VDV-3)) and a thogotovirus isolate. This thogotovirus shares high amino acid identities in five of the six segments with Varroa orthomyxovirus 1, VOV-1 (98.36 to 99.34% identity). In contrast, segment 4, which codes for the main glycoprotein (GP), has no identity with VOV-1, as observed for the other segments, but shares an amino acid identity of 34-38% with other glycoproteins of viruses from the Orthomyxoviridae family. In addition, the putative thogotovirus GP also shows amino acid identities ranging from 33 to 41% with the major glycoprotein (GP64) of insect viruses of the Baculoviridae family. To our knowledge, this is the second report of a thogotovirus found in bees and given this information, this thogotovirus isolate was tentatively named Apis thogotovirus 1 (ATHOV-1). The detection of multiple viruses in bees is important to better understand the complex interactions between viruses and their hosts. By understanding these interactions, better strategies for managing viral infections in bees and protecting their populations can be developed.

Lake Sinai virus is a diverse, globally distributed but not emerging multi-strain honeybee virus.

Domesticated honeybees and wild bees are some of the most important beneficial insects for human and environmental health, but infectious diseases pose a serious risk to these pollinators, particularly following the emergence of the ectoparasitic mite Varroa destructor as a viral vector. The acquisition of this novel viral vector from the Asian honeybee Apis ceranae has fundamentally changed viral epidemiology in its new host, the western honeybee A. mellifera. While the recently discovered Lake Sinai Viruses (LSV) have been associated with weak honeybee colonies, they have not been associated with vector-borne transmission. By combining a large-scale multi-year survey of LSV in Chinese A. mellifera and A. cerana honeybee colonies with globally available LSV-sequence data, we investigate the global epidemiology of this virus. We find that globally distributed LSV is a highly diverse multi-strain virus, which is predominantly associated with the western honeybee A. mellifera. In contrast to the vector-borne deformed wing virus, LSV is not an emerging disease. Instead, demographic reconstruction and strong global and local population structure indicates that it is a highly variable multi-strain virus in a stable association with its main host, the western honeybee. Prevalence patterns in China suggest a potential role for migratory beekeeping in the spread of this pathogen, demonstrating the potential for disease transmission with the man-made transport of beneficial insects.

Virus Prevalence in Egg Samples Collected from Naturally Selected and Traditionally Managed Honey Bee Colonies across Europe.

Monitoring virus infections can be an important selection tool in honey bee breeding. A recent study pointed towards an association between the virus-free status of eggs and an increased virus resistance to deformed wing virus (DWV) at the colony level. In this study, eggs from both naturally surviving and traditionally managed colonies from across Europe were screened for the prevalence of different viruses. Screenings were performed using the phenotyping protocol of the 'suppressed in ovo virus infection' trait but with qPCR instead of end-point PCR and a primer set that covers all DWV genotypes. Of the 213 screened samples, 109 were infected with DWV, 54 were infected with black queen cell virus (BQCV), 3 were infected with the sacbrood virus, and 2 were infected with the acute bee paralyses virus. It was demonstrated that incidences of the vertical transmission of DWV were more frequent in naturally surviving than in traditionally managed colonies, although the virus loads in the eggs remained the same. When comparing virus infections with queen age, older queens showed significantly lower infection loads of DWV in both traditionally managed and naturally surviving colonies, as well as reduced DWV infection frequencies in traditionally managed colonies. We determined that the detection frequencies of DWV and BQCV in honey bee eggs were lower in samples obtained in the spring than in those collected in the summer, indicating that vertical transmission may be lower in spring. Together, these patterns in vertical transmission show that honey bee queens have the potential to reduce the degree of vertical transmission over time.

Context-Dependent Viral Transgenerational Immune Priming in Honey Bees (Hymenoptera: Apidae).

Transgenerational immune priming is the process of increased resistance to infection in offspring due to parental pathogen exposure. Honey bees (Apis mellifera L. (Hymenoptera: Apidae)) are hosts to multiple pathogens, and this complex immune function could help protect against overwhelming infection. Honey bees have demonstrated transgenerational immune priming for the bacterial pathogen Paenibacillus larvae; however, evidence for viral transgenerational immune priming is lacking across insects in general. Here we test for the presence of transgenerational immune priming in honey bees with Deformed wing virus (DWV) by injecting pupae from DWV-exposed queens and measuring virus titer and immune gene expression. Our data suggest that there is evidence for viral transgenerational immune priming in honey bees, but it is highly context-dependent based on route of maternal exposure and potentially host genetics or epigenetic factors.

Impact of Honey Bee Migratory Management on Pathogen Loads and Immune Gene Expression is Affected by Complex Interactions With Environment, Worker Life History, and Season.

The effects of honey bee management, such as intensive migratory beekeeping, are part of the ongoing debate concerning causes of colony health problems. Even though comparisons of disease and pathogen loads among differently managed colonies indicate some effects, the direct impact of migratory practices on honey bee pathogens is poorly understood. To test long- and short-term impacts of managed migration on pathogen loads and immunity, experimental honey bee colonies were maintained with or without migratory movement. Individuals that experienced migration as juveniles (e.g., larval and pupal development), as adults, or both were compared to control colonies that remained stationary and therefore did not experience migratory relocation. Samples at different ages and life-history stages (hive bees or foragers), taken at the beginning and end of the active season, were analyzed for pathogen loads and physiological markers of health. Bees exposed to migratory management during adulthood had increased levels of the AKI virus complex (Acute bee paralysis, Kashmir bee, and Israeli acute bee paralysis viruses) and decreased levels of antiviral gene expression (dicer-like). However, those in stationary management as adults had elevated gut parasites (i.e. trypanosomes). Effects of environment during juvenile development were more complex and interacted with life-history stage and season. Age at collection, life-history stage, and season all influenced numerous factors from viral load to immune gene expression. Although the factors that we examined are not independent, the results illuminate potential factors in both migratory and nonmigratory beekeeping that are likely to contribute to colony stress, and also indicate potential mitigation measures.

Honey bees and climate explain viral prevalence in wild bee communities on a continental scale.

Viruses are omnipresent, yet the knowledge on drivers of viral prevalence in wild host populations is often limited. Biotic factors, such as sympatric managed host species, as well as abiotic factors, such as climatic variables, are likely to impact viral prevalence. Managed and wild bees, which harbor several multi-host viruses with a mostly fecal-oral between-species transmission route, provide an excellent system with which to test for the impact of biotic and abiotic factors on viral prevalence in wild host populations. Here we show on a continental scale that the prevalence of three broad host viruses: the AKI-complex (Acute bee paralysis virus, Kashmir bee virus and Israeli acute paralysis virus), Deformed wing virus, and Slow bee paralysis virus in wild bee populations (bumble bees and solitary bees) is positively related to viral prevalence of sympatric honey bees as well as being impacted by climatic variables. The former highlights the need for good beekeeping practices, including Varroa destructor management to reduce honey bee viral infection and hive placement. Furthermore, we found that viral prevalence in wild bees is at its lowest at the extreme ends of both temperature and precipitation ranges. Under predicted climate change, the frequency of extremes in precipitation and temperature will continue to increase and may hence impact viral prevalence in wild bee communities.

Virome Analysis Reveals Diverse and Divergent RNA Viruses in Wild Insect Pollinators in Beijing, China.

Insect pollinators provide major pollination services for wild plants and crops. Honeybee viruses can cause serious damage to honeybee colonies. However, viruses of other wild pollinating insects have yet to be fully explored. In the present study, we used RNA sequencing to investigate the viral diversity of 50 species of wild pollinating insects. A total of 3 pathogenic honeybee viruses, 8 previously reported viruses, and 26 novel viruses were identified in sequenced samples. Among these, 7 novel viruses were shown to be closely related to honeybee pathogenic viruses, and 4 were determined to have potential pathogenicity for their hosts. The viruses detected in wild insect pollinators were mainly from the order Picornavirales and the families Orthomyxoviridae, Sinhaliviridae, Rhabdoviridae, and Flaviviridae. Our study expanded the species range of known insect pollinator viruses, contributing to future efforts to protect economic honeybees and wild pollinating insects.

Phenomic analysis of the honey bee pathogen-web and its dynamics on colony productivity, health and social immunity behaviors.

Many pathogens and parasites have evolved to overwhelm and suppress their host's immune system. Nevertheless, the interactive effects of these agents on colony productivity and wintering success have been relatively unexplored, particularly in large-scale phenomic studies. As a defense mechanism, honey bees have evolved remarkable social behaviors to defend against pathogen and parasite challenges, which reduce the impact of disease and improve colony health. To investigate the complex role of pathogens, parasites and social immunity behaviors in relation to colony productivity and outcomes, we extensively studied colonies at several locations across Canada for two years. In 2016 and 2017, colonies founded with 1-year-old queens of diverse genetic origin were evaluated, which represented a generalized subset of the Canadian bee population. During each experimental year (May through April), we collected phenotypic data and sampled colonies for pathogen analysis in a standardized manner. Measures included: colony size and productivity (colony weight, cluster size, honey production, and sealed brood population), social immunity traits (hygienic behavior, instantaneous mite population growth rate, and grooming behavior), as well as quantification of gut parasites (Nosema spp., and Lotmaria passim), viruses (DWV-A, DWV-B, BQCV and SBV) and external parasites (Varroa destructor). Our goal was to examine: 1) correlations between pathogens and colony phenotypes; 2) the dynamics of pathogens and parasites on colony phenotypes and productivity traits; and 3) the effects of social immunity behaviors on colony pathogen load. Our results show that colonies expressing high levels of some social immunity behaviors were associated with low levels of pathogens/parasites, including viruses, Nosema spp., and V. destructor. In addition, we determined that elevated viral and Nosema spp. levels were associated with low levels of colony productivity, and that five out of six pathogenic factors measured were negatively associated with colony size and weight in both fall and spring periods. Finally, this study also provides information about the incidence and abundance of pathogens, colony phenotypes, and further disentangles their inter-correlation, so as to better understand drivers of honey bee colony health and productivity.

First detection of Lake Sinai virus in honeybees (Apis mellifera) and wild arthropods in Japan.

Lake Sinai virus (LSV), an RNA virus, is suspected to be associated with poor health in honeybees (Apis mellifera). We examined LSV in 26 specimens of healthy honeybees and 44 specimens of wild arthropods in the Gifu Prefecture, Japan. LSV was found more frequently in honeybee specimens (11/26, 42.3%) than in wild arthropod specimens (1/44, 2.3%) (P<0.01). Phylogenetic and nucleotide sequence analysis revealed two lineages: LSV3 in honeybees, and LSV4 in both honeybees and wild arthropods. To our knowledge, this is the first report of LSV prevalence in honeybees and wild arthropods in Japan.

Development of a Kit for Rapid Immunochromatographic Detection of Sacbrood Virus Infecting Apis cerana (AcSBV) Based on Polyclonal and Monoclonal Antibodies Raised against Recombinant VP1 and VP2 Expressed in Escherichia coli.

A Korean isolate of the sacbrood virus infecting Apis cerana (AcSBV-Kor) is the most destructive honeybee virus, causing serious economic damage losses in Korean apiculture. To address this, here, we attempted to develop an assay for the rapid detection of AcSBV-Kor based on immunochromatographic detection of constituent viral proteins. Genes encoding VP1 and VP2 proteins of AcSBV-Kor were cloned into an expression vector (pET-28a) and expressed in Escherichia coli BL21(DE3). During purification, recombinant VP1 (rVP1) and VP2 (rVP2) proteins were found in the insoluble fraction, with a molecular size of 26.7 and 24.9 kDa, respectively. BALB/c mice immunized with the purified rVP1 and rVP2 produced polyclonal antibodies (pAbs) such as pAb-rVP1 and pAb-rVP2. Western blot analysis showed that pAb-rVP1 strongly reacted with the homologous rVP1 but weakly reacted with heterologous rVP2. However, pAb-rVP2 strongly reacted not only with the homologous rVP2 but also with the heterologous rVP1. Spleen cells of the immunized mice fused with SP2/0-Ag14 myeloma cells produced monoclonal antibodies (mAbs) such as mAb-rVP1-1 and mAb-rVP2-13. Western blot analysis indicated that pAb-rVP1, pAb-rVP2, mAb-rVP1-1, and mAb-rVP2-13 reacted with AcSBV-infected honeybees and larvae as well as the corresponding recombinant proteins. These antibodies were then used in the development of a rapid immunochromatography (IC) strip assay kit with colloidal gold coupled to pAb-rVP1 and pAb-rVP2 at the conjugate pad and mAb-rVP1-1 and mAb-rVP2-13 at the test line. One antibody pair, pAb-rVP1/mAb-VP1-1, showed positive reactivity as low as 1.38 × 103 copies, while the other pair, pAb-rVP2/mAb-VP2-13, showed positive reactivity as low as 1.38 × 104 copies. Therefore, the antibody pair pAb-rVP1/mAb-VP1-1 was selected as a final candidate for validation. To validate the detection of AcSBV, the IC strip tests were conducted with 50 positive and 50 negative samples and compared with real-time PCR tests. The results confirm that the developed IC assay is a sufficiently sensitive and specific detection method for user-friendly and rapid detection of AcSBV.

Global similarity, and some key differences, in the metagenomes of Swedish varroa-surviving and varroa-susceptible honeybees.

There is increasing evidence that honeybees (Apis mellifera L.) can adapt naturally to survive Varroa destructor, the primary cause of colony mortality world-wide. Most of the adaptive traits of naturally varroa-surviving honeybees concern varroa reproduction. Here we investigate whether factors in the honeybee metagenome also contribute to this survival. The quantitative and qualitative composition of the bacterial and viral metagenome fluctuated greatly during the active season, but with little overall difference between varroa-surviving and varroa-susceptible colonies. The main exceptions were Bartonella apis and sacbrood virus, particularly during early spring and autumn. Bombella apis was also strongly associated with early and late season, though equally for all colonies. All three affect colony protein management and metabolism. Lake Sinai virus was more abundant in varroa-surviving colonies during the summer. Lake Sinai virus and deformed wing virus also showed a tendency towards seasonal genetic change, but without any distinction between varroa-surviving and varroa-susceptible colonies. Whether the changes in these taxa contribute to survival or reflect demographic differences between the colonies (or both) remains unclear.

Simulated vector transmission differentially influences dynamics of two viral variants of deformed wing virus in honey bees (Apis mellifera).

Understanding how vectors alter the interactions between viruses and their hosts is a fundamental question in virology and disease ecology. In honey bees, transmission of deformed wing virus (DWV) by parasitic Varroa mites has been associated with elevated disease and host mortality, and Varroa transmission has been hypothesized to lead to increased viral titres or select for more virulent variants. Here, we mimicked Varroa transmission by serially passaging a mixed population of two DWV variants, A and B, by injection through in vitro reared honey bee pupae and tracking these viral populations through five passages. The DWV-A and DWV-B variant proportions shifted dynamically through passaging, with DWV-B outcompeting DWV-A after one passage, but levels of both variants becoming equivalent by Passage 5. Sequencing analysis revealed a dominant, recombinant DWV-B strain (DWV-A derived 5' IRES region with the rest of the genome DWV-B), with low nucleotide diversity that decreased through passaging. DWV-A populations had higher nucleotide diversity compared to DWV-B, but this also decreased through passaging. Selection signatures were found across functional regions of the DWV-A and DWV-B genomes, including amino acid mutations in the putative capsid protein region. Simulated vector transmission differentially impacted two closely related viral variants which could influence viral interactions with the host, demonstrating surprising plasticity in vector-host-viral dynamics.