Olav Rueppell’s research while affiliated with University of Alberta and other places

Gut microbiomes play a significant role in the health, development, and behavior of numerous species, including honey bees ( Apis mellifera ). Worker honey bees exhibit varying degrees of hygienic behavior, which involves the removal of unhealthy brood to mitigate disease within their colony. However, the potential relationship between hygienic behavior and the honey bee gut microbiome has not been previously investigated. In this study, we compared gut microbiota in honey bees engaged in hygienic behavior (hygiene performers) versus those not exhibiting this behavior (non-hygiene performers) using 16S rRNA gene amplicon sequencing. Proteobacteria, Firmicutes, and Actinobacteria were identified as the predominant phyla. Notably, three bacterial species ( Apilactobacillus kunkeei , Bartonella apis, and Frischella perrara ) were found to be more abundant in hygiene performer bees compared to non-hygiene performer bees. Additionally, hygiene performer bees showed a higher diversity of amplicon sequence variants, with Apibacter mensalis being exclusively present in hygiene performer bees and absent in non-hygiene performer bees. These findings reveal an association between gut microbiota composition and hygienic behavior in honey bees, which may provide a foundation for future research exploring probiotic development and other strategies to enhance honey bee health.

Honey bees (Apis mellifera) confront a multitude of challenges to their health throughout their lifespan and have naturally evolved protective mechanisms to defend against biological stressors. Transgenerational immune priming (TGIP) is one such defense mechanism that confers protection against bacterial infections from parents to offspring. However, it is unclear whether TGIP in honey bees also protects against viral infections, which may offer a promising pathway to decrease the honey bees’ susceptibility to viral infections. We studied our hypothesis that honey bees can prime their offspring against Israeli acute paralysis virus (IAPV). We tested the prediction that the offspring of queens exposed to thermally inactivated IAPV would exhibit higher survival of an acute IAPV infection than the offspring of sham-treated queens. Based on pilot studies that compared the effects of different inoculation methods, we topically inoculated experimental queens with heat-inactivated IAPV and compared survival of an infection with active IAPV between their offspring and offspring of sham-treated control queens. IAPV infection significantly decreased offspring survival but maternal exposure to the inactive virus did not affect this outcome. Our results fail to support the notion that maternal exposure confers the same level of protection against virus infections as observed against bacterial infections, at least in this specific instant, underscoring the intricate nature of the honey bees’ transgenerational immune response. Further development of effective strategies against viral threats to improve honey bee health is needed.

The ectoparasitic mite Varroa destructor remains a great threat for the beekeeping industry, for example contributing to excessive winter colony loss in Canada. For decades, beekeepers have sequentially used the registered synthetic varroacides tau-fluvalinate, coumaphos, amitraz, and flumethrin, leading to the risk of resistance evolution in the mites. In addition to the widespread resistance to coumaphos and pyrethroids, a decline in amitraz efficacy has recently been reported in numerous beekeeping regions in Canada. The goals of this study were to assess the evolution of resistance to amitraz in Canadian mite populations and to evaluate the presence and incidence of mutations previously associated with resistance to amitraz and pyrethroids in V. destructor. Our bioassay results confirmed the presence of amitraz-resistant mites in the population of Alberta. These phenotypic results were complemented by targeted genotyping of the octopamine receptor gene Octβ2R which revealed the presence of the mutation Y215H in 90% of tested apiaries with local allele frequencies ranging from 5 to 95%. The phenotypic resistance showed a significant correlation with the presence of this mutation across apiaries. In parallel, the L925I and L925M mutations in the voltage-gated sodium channel were identified in 100% of the tested apiaries with frequencies ranging from 33 to 97%, suggesting that resistance to pyrethroids remains widespread. These results support the notion that the practice of relying on a single treatment for a prolonged period can increase rates of resistance to current varroacides. Our findings suggest the need for large-scale resistance monitoring via genotyping to provide timely information to beekeepers and regulators. This will enable them to make an effective management plan, including rotation of available treatments to suppress or at least delay the evolution of resistance in V. destructor populations.

Social insects, particularly honey bees, have exceptionally high genomic frequencies of genetic recombination. This phenomenon and underlying mechanisms are poorly understood. To characterise the patterns of crossovers and gene conversion in the honey bee genome, a recombination map of 187 honey bee brothers was generated by whole‐genome resequencing. Recombination events were heterogeneously distributed without many true hotspots. The tract lengths between phase shifts were bimodally distributed, indicating distinct crossover and gene conversion events. While crossovers predominantly occurred in G/C‐rich regions and seemed to cause G/C enrichment, the gene conversions were found predominantly in A/T‐rich regions. The nucleotide composition of sequences involved in gene conversions that were associated with or distant from crossovers corresponded to the differences between crossovers and gene conversions. These combined results suggest two types of DNA double‐strand break repair during honey bee meiosis: non‐canonical homologous recombination, leading to gene conversion and A/T enrichment of the genome, and the canonical homologous recombination based on completed double Holliday Junctions, which can result in gene conversion or crossover and is associated with G/C bias. This G/C bias may be selected for to balance the A/T‐rich base composition of eusocial hymenopteran genomes. The lack of evidence for a preference of the canonical homologous recombination for double‐strand break repair suggests that the high genomic recombination rate of honey bees is mainly the consequence of a high rate of double‐strand breaks, which could in turn result from the life history of honey bees and their A/T‐rich genome.

Social insects have the highest rates of meiotic recombination among Metazoa, but there is considerable variation within the Hymenoptera. We synthesize the literature to investigate several hypotheses for these elevated recombination rates. We reexamine the long-standing Red Queen hypothesis, considering how social aspects of immunity could lead to increases in recombination. We examine the possibility of positive feedback between gene duplication and recombination rate in the context of caste specialization. We introduce a novel hypothesis that recombination rate may be driven up by direct selection on recombination activity in response to increases in lifespan. Finally, we find that the role of population size in recombination rate evolution remains opaque, despite the long-standing popularity of this hypothesis. Moreover, our review emphasizes how the varied life histories of social insect species provide an effective framework for advancing a broader understanding of adaptively driven variation in recombination rates.

The life cycle of Varroa destructor, the ectoparasitic mite of honey bees (Apis mellifera), includes a dispersal phase, in which mites attach to adult bees for transport and feeding, and a reproductive phase, in which mites invade worker and drone brood cells just prior to pupation to reproduce while their bee hosts complete development. In this study, we wanted to determine whether increased nurse bee visitations of adjacent drone and worker brood cells would increase the likelihood of Varroa mites invading those cells. We also explored whether temporarily restricting the nurses’ access to sections of worker brood for 2 or 4 h would subsequently cause higher nurse visitations, and thus, higher Varroa cell invasions. Temporarily precluding larvae from being fed by nurses subsequently led to higher Varroa infestation of those sections in some colonies, but this pattern was not consistent across colonies. Therefore, removing highly infested sections of capped worker brood could be further explored as a potential mechanical/cultural method for mite control. Our results provide more information on how nurse visitations affect the patterns of larval cell invasion by Varroa. Given that the mite’s successful reproduction depends on the nurses’ ability to visit and feed developing brood, more studies are needed to understand the patterns of Varroa mite invasion of drone and worker cells to better combat this pervasive honey bee parasite.

The size of animal groups has profound effects on individual and collective behavior, particularly in social insect colonies. Larger colonies are predicted to be more complex with more specialization among members. However, the empirical support of this theoretical expectation is limited. Hygienic behavior of honey bees is a complex cooperative behavior of workers detecting, uncapping, and removing unhealthy brood. It is an important defense against brood diseases, including the ectoparasitic mite Varroa destructor. We support the prediction that hygienic behavior increases with group size using a simulation model. To also test this prediction empirically, we performed five experiments, to compare the hygienic performance of small and large honey bee groups at four different scales, roughly representing four orders of magnitude. Hygienic performance qualitatively increased across the different scales, but different methodologies limit quantitative comparisons across experiments. Within experiments, group size was also positively related to hygienic behavior. The strongest effects of group size were measured in groups that were smaller than what honey bees adopt under natural conditions. The group-size effect on hygienic performance decreased with increasing scale and at the full colony scale, group size was unrelated to hygienic assay scores. Therefore, colony size is unlikely to confound the hygienic evaluation of colonies in apicultural practice although we demonstrate clear effects of group size on hygienic behavior. Direct observations of individual behavior that were performed in two small scale experiments did not support our prediction of increased individual specialization in larger groups. Thus, our study supports the notion of performance benefits of larger groups in the context of social immunity, although the mechanisms of how group size enhances hygienic behavior remain to be investigated further. Significance Statement Social insects owe their ecological success partly to their efficient division of labor and behavioral specialization of colony members. Empirical support for the theoretical argument that group performance increases with group size is insufficient. Hygienic behavior is an important defense against brood pests and diseases that threaten honey bee health. Yet, it has not been investigated with respect to group size. Here, we analyze a simulation model, demonstrating theoretically that group size is predicted to increase hygienic behavior. We then provide experimental support for this prediction across a range of group sizes, though results of some experiments are equivocal and sample sizes are limited, constraining our empirical conclusions. In small groups, we find support for the theoretical idea that hygienic performance increases with group size, but our study also indicates that this effect is not very important under apiculturally relevant group size conditions. We find no support for higher individual specialization in larger groups. Furthermore, our study indicates that beekeepers can disregard deviations in colony size when assessing their stock for hygienic behavior.

Resistance to and avoidance of stress slow aging and confer increased longevity in numerous organisms. Honey bees and other superorganismal social insects have two main advantages over solitary species to avoid or resist stress: individuals can directly help each other by resource or information transfer, and they can cooperatively control their environment. These benefits have been recognised in the context of pathogen and parasite stress as the concept of social immunity, which has been extensively studied. However, we argue that social immunity is only a special case of a general concept that we define here as social stress protection to include group‐level defences against all biotic and abiotic stressors. We reason that social stress protection may have allowed the evolution of reduced individual‐level defences and individual life‐history optimization, including the exceptional aging plasticity of many social insects. We describe major categories of stress and how a colonial lifestyle may protect social insects, particularly against temporary peaks of extreme stress. We use the honey bee ( Apis mellifera L.) to illustrate how patterns of life expectancy may be explained by social stress protection and how modern beekeeping practices can disrupt social stress protection. We conclude that the broad concept of social stress protection requires rigorous empirical testing because it may have implications for our general understanding of social evolution and specifically for improving honey bee health.

Ectoparasitic mites of the genera Varroa and Tropilaelaps have evolved to exclusively exploit honey bees as food sources during alternating dispersal and reproductive life history stages. Here we show that the primary food source utilized by Varroa destructor depends on the host life history stage. While feeding on adult bees, dispersing V. destructor feed on the abdominal membranes to access to the fat body as reported previously. However, when V. destructor feed on honey bee pupae during their reproductive stage, they primarily consume hemolymph, indicated by wound analysis, preferential transfer of biostains, and a proteomic comparison between parasite and host tissues. Biostaining and proteomic results were paralleled by corresponding findings in Tropilaelaps mercedesae, a mite that only feeds on brood and has a strongly reduced dispersal stage. Metabolomic profiling of V. destructor corroborates differences between the diet of the dispersing adults and reproductive foundresses. The proteome and metabolome differences between reproductive and dispersing V. destructor suggest that the hemolymph diet coincides with amino acid metabolism and protein synthesis in the foundresses while the metabolism of non-reproductive adults is tuned to lipid metabolism. Thus, we demonstrate within-host dietary specialization of ectoparasitic mites that coincides with life history of hosts and parasites.