Immune pathways and defence mechanisms in honey bees Apis mellifera

Immunosuppression response to the neonicotinoid insecticide thiacloprid in females and males of the red mason bee Osmia bicornis L

Solitary bees are frequently exposed to pesticides, which are considered as one of the main stress factors that may lead to population declines. A strong immune defence is vital for the fitness of bees. However, the immune system can be weakened by environmental factors that may render bees more vulnerable to parasites and pathogens. Here we demonstrate for the first time that field-realistic concentrations of the commonly used neonicotinoid insecticide thiacloprid can severely affect the immunocompetence of Osmia bicornis. In detail, males exposed to thiacloprid solutions of 200 and 555 µg/kg showed a reduction in hemocyte density. Moreover, functional aspects of the immune defence - the antimicrobial activity of the hemolymph - were impaired in males. In females, however, only a concentration of 555 µg/kg elicited similar immunosuppressive effects. Although males are smaller than females, they consumed more food solution. This leads to a 2.77 times higher exposure in males, probably explaining the different concentration thresholds observed between the sexes. In contrast to honeybees, dietary exposure to thiacloprid did not affect melanisation or wound healing in O. bicornis. Our results demonstrate that neonicotinoid insecticides can negatively affect the immunocompetence of O. bicornis, possibly leading to an impaired disease resistance capacity.

Transcriptomic analysis to elucidate the response of honeybees (Hymenoptera: Apidae) to amitraz treatment

Amitraz is an acaricide that is widely used in apiculture. Several studies have reported that in honeybees (Apis mellifera Linnaeus; Hymenoptera: Apidae), amitraz affects learning, memory, behavior, immunity, and various other physiological processes. Despite this, few studies have explored the molecular mechanisms underlying the action of amitraz on honeybees. Here, we investigated the transcriptome of honeybees after exposure to 9.4 mg/L amitraz for 10 d, a subchronic dose. Overall, 279 differentially expressed genes (DEGs) were identified (237 upregulated, 42 downregulated). Several, including Pla2, LOC725381, LOC413324, LOC724386, LOC100577456, LOC551785, and P4504c3, were validated by quantitative PCR. According to gene ontology, DEGs were mainly involved in metabolism, biosynthesis, and translation. Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that amitraz treatment affected the relaxin signaling pathway, platelet activation, and protein digestion and absorption.

Phylogenomics Identifies an Ancestral Burst of Gene Duplications Predating the Diversification of Aphidomorpha

Aphids (Aphidoidea) are a diverse group of hemipteran insects that feed on plant phloem sap. A common finding in studies of aphid genomes is the presence of a large number of duplicated genes. However, when these duplications occurred remains unclear, partly due to the high relatedness of sequenced species. To better understand the origin of aphid duplications we sequenced and assembled the genome of Cinara cedri, an early branching lineage (Lachninae) of the Aphididae family. We performed a phylogenomic comparison of this genome with 20 other sequenced genomes, including the available genomes of five other aphids, along with the transcriptomes of two species belonging to Adelgidae (a closely related clade to the aphids) and Coccoidea. We found that gene duplication has been pervasive throughout the evolution of aphids, including many parallel waves of recent, species-specific duplications. Most notably, we identified a consistent set of very ancestral duplications, originating from a large-scale gene duplication predating the diversification of Aphidomorpha (comprising aphids, phylloxerids, and adelgids). Genes duplicated in this ancestral wave are enriched in functions related to traits shared by Aphidomorpha, such as association with endosymbionts, and adaptation to plant defenses and phloem-sap-based diet. The ancestral nature of this duplication wave (106-227 Ma) and the lack of sufficiently conserved synteny make it difficult to conclude whether it originated from a whole-genome duplication event or, alternatively, from a burst of large-scale segmental duplications. Genome sequencing of other aphid species belonging to different Aphidomorpha and related lineages may clarify these findings.

The Heat Shock Response in the Western Honey Bee (Apis mellifera) is Antiviral

Honey bees (Apismellifera) are an agriculturally important pollinator species that live in easily managed social groups (i.e., colonies). Unfortunately, annual losses of honey bee colonies in many parts of the world have reached unsustainable levels. Multiple abiotic and biotic stressors, including viruses, are associated with individual honey bee and colony mortality. Honey bees have evolved several antiviral defense mechanisms including conserved immune pathways (e.g., Toll, Imd, JAK/STAT) and dsRNA-triggered responses including RNA interference and a non-sequence specific dsRNA-mediated response. In addition, transcriptome analyses of virus-infected honey bees implicate an antiviral role of stress response pathways, including the heat shock response. Herein, we demonstrate that the heat shock response is antiviral in honey bees. Specifically, heat-shocked honey bees (i.e., 42 °C for 4 h) had reduced levels of the model virus, Sindbis-GFP, compared with bees maintained at a constant temperature. Virus-infection and/or heat shock resulted in differential expression of six heat shock protein encoding genes and three immune genes, many of which are positively correlated. The heat shock protein encoding and immune gene transcriptional responses observed in virus-infected bees were not completely recapitulated by administration of double stranded RNA (dsRNA), a virus-associated molecular pattern, indicating that additional virus-host interactions are involved in triggering antiviral stress response pathways.

Kinome Analysis of Honeybee (Apis mellifera L.) Dark-Eyed Pupae Identifies Biomarkers and Mechanisms of Tolerance to Varroa Mite Infestation

The mite Varroa destructor is a serious threat to honeybee populations. Selective breeding for Varroa mite tolerance could be accelerated by biomarkers within individual bees that could be applied to evaluate a colony phenotype. Previously, we demonstrated differences in kinase-mediated signaling between bees from colonies of extreme phenotypes of mite susceptibility. We expand these findings by defining a panel of 19 phosphorylation events that differ significantly between individual pupae from multiple colonies with distinct Varroa mite tolerant phenotypes. The predictive capacity of these biomarkers was evaluated by analyzing uninfested pupae from eight colonies representing a spectrum of mite tolerance. The pool of biomarkers effectively discriminated individual pupae on the basis of colony susceptibility to mite infestation. Kinome analysis of uninfested pupae from mite tolerant colonies highlighted an increased innate immune response capacity. The implication that differences in innate immunity contribute to mite susceptibility is supported by the observation that induction of innate immune signaling responses to infestation is compromised in pupae of the susceptible colonies. Collectively, biomarkers within individual pupae that are predictive of the susceptibility of colonies to mite infestation could provide a molecular tool for selective breeding of tolerant colonies.

Immune signaling pathways in the endoparasitoid, Pteromalus puparum

Parasitoids serve as effective biocontrol agents for agricultural pests. However, they face constant challenges from host immune defense and numerous pathogens and must develop potent immune defense against these threats. Despite the recent advances in innate immunity, little is known about the immunological mechanisms of parasitoids. Here, we identified and characterized potential immune-related genes of the endoparasitoid, Pteromalus puparum, which act in regulating populations of some members of the Pieridae. We identified 216 immune-related genes based on interrogating the P. puparum genome and transcriptome databases. We categorized the cognate gene products into recognition molecules, signal moieties and effector proteins operating in four pathways, Toll, IMD, JAK/STAT, and JNK. Comparative analyses of immune-related genes from seven insect species indicate that recognition molecules and effector proteins are more expanded and diversified than signaling genes in these signal pathways. There are common 1:1 orthologs between the endoparasitoid P. puparum and its relative, the ectoparasitoid Nasonia vitripennis. The developmental expression profiles of immune genes randomly selected from the transcriptome analysis were verified by a quantitative polymerase chain reaction. Our work provides comprehensive analyses of P. puparum immune genes, some of which may be exploited in advancing parasitoid-based biocontrol technologies.

Flumethrin at sublethal concentrations induces stresses in adult honey bees (Apis mellifera L.)

Flumethrin is a typical pyrethroid varroacide widely used for mite control in beekeeping worldwide. Currently, information on the toxicological characteristics of flumethrin on bees at sublethal concentrations is still lacking. To fill this gap in information, we performed a 48-h acute oral and 14-day chronic toxicity testing of flumethrin in newly emerged adult honey bees under laboratory conditions. Results showed that flumethrin had high acute toxicity to honey bees with a 48-h LD₅₀ of 0.47 µg/bee (95% CI, 0.39 ∼ 0.57 µg/bee), which is higher than that of many other commercial pyrethroid insecticides, but lower than that of tau-fluvalinate. After 14 days of chronic exposure to flumethrin at 0.01, 0.10, and 1.0 mg/L, significant antioxidant response, detoxification, immune reaction, and apoptosis were observed in the midguts. These findings indicated that flumethrin had potential risks to bees, and it can disturb the homeostasis of bees at sublethal concentrations under longer exposure conditions. Flumethrin is highly lipophilic and easy to accumulate in beeswax; thus, careless practices might pose risks to colony development in commercial beekeeping and native populations. This laboratory study can serve as an early warning, and further studies are required to understand the real residual level of flumethrin in bees and the risks of flumethrin in field condition.

Exposure of honey bee larvae to thiamethoxam and its interaction with Nosema ceranae infection in adult honey bees

During their lifetime honey bees (Apis mellifera) rarely experience optimal conditions. Sometimes, a simultaneous action of multiple stressors, natural and chemical, results in even greater effect than of any stressor alone. Therefore, integrative investigations of different factors affecting honey bees have to be carried out. In this study, adult honey bees exposed to thiamethoxam in larval and/or adult stage and infected with Nosema ceranae were examined. Newly emerged bees from colonies, non-treated or treated with thiamethoxam, were organized in six groups and kept in cages. Thiamethoxam treated bees were further exposed to either thiamethoxam or Nosema (groups TT and TN), or simultaneously to both (group TTN). Newly emerged bees from non-treated colonies were exposed to Nosema (group CN). From both, treated and non-treated colonies two groups were organized and further fed only with sugar solution (groups C and TC). Here, we present the expression profile of 19 genes in adult worker honey bees comprising those involved in immune, detoxification, development and apoptosis response. Results showed that gene expression patterns changed with time and depended on the treatment. In group TC at the time of emergence the majority of tested genes were downregulated, among which nine were significantly altered. The same gene pattern was observed on day six, where the only significantly upregulated gene was defensin-1. On day nine most of analyzed genes in all experimental groups showed upregulation compared to control group, where upregulation of antimicrobial peptide genes abaecin, defensin-1 and defensin-2 was significant in groups TT and TTN. On day 15 we observed a similar pattern of expression in groups TC and TT exposed to thiamethoxam only, where most of the detoxification genes were downregulated. Additionally RNA loads of Nosema and honey bee viruses were recorded. We detected a synergistic interaction of thiamethoxam and Nosema, reflected in lowest honey bee survival.

Consequences of a short time exposure to a sublethal dose of Flupyradifurone (Sivanto) pesticide early in life on survival and immunity in the honeybee (Apis mellifera)

Dramatic losses of pollinating insects have become of global concern, as they threaten not only key ecosystem services but also human food production. Recent research provided evidence that interactions between ecological stressors are drivers of declining pollinator health and responsible for observed population collapses. We used the honeybee Apis mellifera and conducted a series of experiments to test for long-term effects of a single short exposure to the agricultural pesticide flupyradifurone to a second environmental stressor later in life. To do this, we exposed individuals during their larval development or early adulthood to sublethal dosages of flupyradifurone (0.025 μg for larvae and 0.645 μg for imagos), either pure or as part of an agricultural formulation (Sivanto). We afterwards exposed bees to a second ecological stressor infecting individuals with 10,000 spores of the fungal gut parasite Nosema ceranae. We found that pesticide exposures significantly reduced survival of bees and altered the expression of several immune and detoxification genes. The ability of bees to respond to these latter effects differed significantly between colonies, offering opportunities to breed bees with elevated levels of pesticide tolerance in the future. We conclude that short episodes of sublethal pesticide exposures during development are sufficient to trigger effects later in life and could therefore contribute to the widespread declines in bee health.

De novo transcriptome analysis and identification of genes associated with immunity, detoxification and energy metabolism from the fat body of the tephritid gall fly, Procecidochares utilis

The fat body, a multifunctional organ analogous to the liver and fat tissue of vertebrates, plays an important role in insect life cycles. The fat body is involved in protein storage, energy metabolism, elimination of xenobiotics, and production of immunity regulator-like proteins. However, the molecular mechanism of the fat body's physiological functions in the tephritid stem gall-forming fly, Procecidochares utilis, are still unknown. In this study, we performed transcriptome analysis of the fat body of P. utilis using Illumina sequencing technology. In total, 3.71 G of clean reads were obtained and assembled into 30,559 unigenes, with an average length of 539 bp. Among those unigenes, 21,439 (70.16%) were annotated based on sequence similarity to proteins in NCBI's non-redundant protein sequence database (Nr). Sequences were also compared to NCBI's non-redundant nucleotide sequence database (Nt), a manually curated and reviewed protein sequence database (SwissProt), and KEGG and gene ontology annotations were applied to better understand the functions of these unigenes. A comparative analysis was performed to identify unigenes related to detoxification, immunity and energy metabolism. Many unigenes involved in detoxification were identified, including 50 unigenes of putative cytochrome P450s (P450s), 18 of glutathione S-transferases (GSTs), 35 of carboxylesterases (CarEs) and 26 of ATP-binding cassette (ABC) transporters. Many unigenes related to immunity were identified, including 17 putative serpin genes, five peptidoglycan recognition proteins (PGRPs) and four lysozyme genes. In addition, unigenes potentially involved in energy metabolism, including 18 lipase genes, five fatty acid synthase (FAS) genes and six elongases of very long chain fatty acid (ELOVL) genes, were identified. This transcriptome improves our genetic understanding of P. utilis and the identification of a numerous transcripts in the fat body of P. utilis offer a series of valuable molecular resources for future studies on the functions of these genes.

Evaluation of reference genes for gene expression analysis by real-time quantitative PCR (qPCR) in three stingless bee species (Hymenoptera: Apidae: Meliponini)

Stingless bees are generalist pollinators distributed through the pantropical region. There is growing evidence that their wild populations are experiencing substantial decline in response to habitat degradation and pesticides. Policies for conservation of endangered species will benefit from studies focusing on genetic and molecular aspects of their development and behavior. The most common method for looking at gene expression is real-time quantitative polymerase chain reaction preceded by reverse transcription (RT-qPCR) of the mRNA of interest. This method requires the identification of reliable reference genes to correctly estimate fluctuations in transcript levels. To contribute to molecular studies on stingless bees, we used Frieseomelitta varia, Melipona quadrifasciata, and Scaptotrigona bipunctata species to test the expression stability of eight reference genes (act, ef1-α, gapdh, rpl32, rps5, rps18, tbp, and tbp-af) in RT-qPCR procedures in five physiological and experimental conditions (development, sex, tissues, bacteria injection, and pesticide exposure). In general, the rpl32, rps5 and rps18 ribosomal protein genes and tpb-af gene showed the highest stability, thus being identified as suitable reference genes for the three stingless bee species and defined conditions. Our results also emphasized the need to evaluate the stability of candidate genes for any designed experimental condition and stingless bee species.

Systematic identification of circular RNAs and corresponding regulatory networks unveil their potential roles in the midguts of eastern honeybee workers

Currently, knowledge of circular RNAs (circRNAs) in insects including honeybee is extremely limited. Here, differential expression profiles and regulatory networks of circRNAs in the midguts of Apis cerana cerana workers were comprehensively investigated using transcriptome sequencing and bioinformatics. In total, 9589 circRNAs (201-800 nt in length) were identified from 8-day-old and 11-day-old workers' midguts (Ac1 and Ac2); among them, 5916 (61.70%) A. cerana cerana circRNAs showed conservation with our previously indentified circRNAs in Apis mellifera ligucstica workers' midguts (Xiong et al., Acta Entomologica Sinica 61:1363-1375, 2018). Five circRNAs were confirmed by RT-PCR and Sanger sequencing. Interestingly, novel_circ_003723, novel_circ_002714, novel_circ_002451, and novel_circ_001980 were highly expressed in both Ac1 and Ac2. In addition, the source genes of circRNAs were involved in 34 GO terms including organelle and cellular process and 141 pathways such as endocytosis and Wnt signaling pathway. Moreover, 55 DEcircRNAs including 34 upregulated and 21 downregulated circRNAs were identified in Ac2 compared with Ac1. circRNA-miRNA regulatory networks indicated that 1060 circRNAs can target 74 miRNAs; additionally, the DEcircRNA-miRNA-mRNA networks suggested that 13 downregulated circRNAs can bind to eight miRNAs and 29 miRNA-targeted mRNAs, while 16 upregulated circRNAs can link to 9 miRNAs and 29 miRNA-targeted mRNAs. These results indicated that DEcircRNAs as ceRNAs may play a comprehensive role in the growth, development, and metabolism of the worker's midgut via regulating source genes and interacting with miRNAs. Notably, eight DEcircRNAs targeting miR-6001-y were likely to be key participants in the midgut development. Our findings not only offer a valuable resource for further studies on A. cerana cerana circRNA and novel insights into understanding the molecular mechanisms underlying the midgut development of eastern honeybee but also provide putative circRNA candidates for functional research in the near future and novel biomarkers for identification of eastern honeybee species including A. cerana cerana and honeybee diseases such as chalkbrood and microsporidiosis.

Towards Precision Nutrition: A Novel Concept Linking Phytochemicals, Immune Response and Honey Bee Health

The high annual losses of managed honey bees (Apis mellifera) has attracted intensive attention, and scientists have dedicated much effort trying to identify the stresses affecting bees. There are, however, no simple answers; rather, research suggests multifactorial effects. Several works have been reported highlighting the relationship between bees' immunosuppression and the effects of malnutrition, parasites, pathogens, agrochemical and beekeeping pesticides exposure, forage dearth and cold stress. Here we analyze a possible connection between immunity-related signaling pathways that could be involved in the response to the stress resulted from Varroa-virus association and cold stress during winter. The analysis was made understanding the honey bee as a superorganism, where individuals are integrated and interacting within the colony, going from social to individual immune responses. We propose the term "Precision Nutrition" as a way to think and study bees' nutrition in the search for key molecules which would be able to strengthen colonies' responses to any or all of those stresses combined.

Defining Pollinator Health: A Holistic Approach Based on Ecological, Genetic, and Physiological Factors

Evidence for global bee population declines has catalyzed a rapidly evolving area of research that aims to identify the causal factors and to effectively assess the status of pollinator populations. The term pollinator health emerged through efforts to understand causes of bee decline and colony losses, but it lacks a formal definition. In this review, we propose a definition for pollinator health and synthesize the available literature on the application of standardized biomarkers to assess health at the individual, colony, and population levels. We focus on biomarkers in honey bees, a model species, but extrapolate the potential application of these approaches to monitor the health status of wild bee populations. Biomarker-guided health measures can inform beekeeper management decisions, wild bee conservation efforts, and environmental policies. We conclude by addressing challenges to pollinator health from a One Health perspective that emphasizes the interplay between environmental quality and human, animal, and bee health.

Biopesticide spinosad induces transcriptional alterations in genes associated with energy production in honey bees (Apis mellifera) at sublethal concentrations

Bees experience substantial colony losses, which are often associated with pesticides. Besides synthetic insecticides biological compounds such as spinosad are used in agriculture and organic farming against insect pests. However, potential adverse effect at sublethal concentrations to pollinators are poorly known. Here we aim to determine potential adverse outcome pathways of spinosad and to identify molecular effects by investigating transcriptional alterations in the brain of honey bees. We experimentally exposed bees to three sublethal concentrations of 0.05, 0.5 and 5 ng spinosad/bee, and assessed transcriptional alterations of target genes. Additionally, we evaluated whether spinosad-induced transcriptional alterations were influenced by the time of the year. In April, alterations were most pronounced after 24 h exposure, while in June alterations occurred mostly after 48 h. In July, expressional alterations were often lower but the pattern was more similar to that in June than that in April. Down-regulation of genes encoding acetylcholine receptors, enzymes involved in oxidative phosphorylation (cox5a, ndufb7 and cox17), cytochrome P450 dependent monooxygenases (cyp9q1, cyp9q2 and cyp9q3) and insulin-like peptide-1 were among the most significant transcriptional alterations. This suggests adverse effects of spinosad to energy production and metabolism and thus negative consequences on foraging. Together, our study indicates that spinosad causes adverse effects at environmentally realistic concentrations, which may pose a risk to bee populations.

Identification of novel antimicrobial peptides from rice planthopper, Nilaparvata lugens

In this study, two novel antibacterial peptide genes, termed lugensin A and B were identified and characterized from a rice sap-sucking hemipteran insect pest, the brown planthopper, Nilaparvata lugens. Lugensin gene expression was significantly induced by Gram-negative and Gram-positive bacterial stains under the regulation of a signal receptor, the long peptidoglycan recognition protein (PGRP-LC) in the IMD pathway. Knockdown of PGRP-LC by RNAi eliminated bacterium induced Lugensin gene expression. Lugensins had the apparent antibacterial activities against Escherichia coli K12, Bacillus subtilis and the rice bacterial brown stripe pathogen Acidovorax avenae subsp. avenae (Aaa) strain RS-1. Lugensins inhibited bacterial proliferation by disrupting the integrity of the bacterial membranes. Scanning electron microscopy revealed abnormal membrane morphology of the recombinant Lugensin-treated bacteria. Lugensins induced complete cell disruption of E. coli K12 and B. subtilis strains while formed the holes on the cell surface of Aaa RS-1 strain. Immunofluorescence showed that Lugensins localized in the cell membrane of E. coli K12 while accumulated in the cytosol of B. subtilis. Differently, Lugensins remained in both the cell membrane and the cytosol of Aaa RS-1 strain, suggesting different action modes of Lugensins to different microbes. This is the first report of the novel antibacterial peptides found in the rice sap-sucking hemipteran insect species.

Deleterious Effects of Neonicotinoid Pesticides on Drosophila melanogaster Immune Pathways

Sublethal exposure to certain pesticides (e.g., neonicotinoid insecticides) is suspected to contribute to honey bee (Apis mellifera) population decline in North America. Neonicotinoids are known to interfere with immune pathways in the gut of insects, but the underlying mechanisms remain elusive. We used a Drosophila melanogaster model to understand how imidacloprid (a common neonicotinoid) interferes with two innate immune pathways—Duox and Imd. We found that imidacloprid dysregulates these pathways to reduce hydrogen peroxide production, ultimately leading to a dysbiotic shift in the gut microbiota. Intriguingly, we found that presupplementation with probiotic bacteria could mitigate the harmful effects of imidacloprid. Thus, these observations uncover a novel mechanism of pesticide-induced immunosuppression that exploits the interconnectedness of two important insect immune pathways.

Neonicotinoid insecticides are common agrochemicals that are used to kill pest insects and improve crop yield. However, sublethal exposure can exert unintentional toxicity to honey bees and other beneficial pollinators by dysregulating innate immunity. Generation of hydrogen peroxide (H₂O₂) by the dual oxidase (Duox) pathway is a critical component of the innate immune response, which functions to impede infection and maintain homeostatic regulation of the gut microbiota. Despite the importance of this pathway in gut immunity, the consequences of neonicotinoid exposure on Duox signaling have yet to be studied. Here, we use a Drosophila melanogaster model to investigate the hypothesis that imidacloprid (a common neonicotinoid) can affect the Duox pathway. The results demonstrated that exposure to sublethal imidacloprid reduced H₂O₂ production by inhibiting transcription of the Duox gene. Furthermore, the reduction in Duox expression was found to be a result of imidacloprid interacting with the midgut portion of the immune deficiency pathway. This impairment led to a loss of microbial regulation, as exemplified by a compositional shift and increased total abundance of Lactobacillus and Acetobacter spp. (dominant microbiota members) found in the gut. In addition, we demonstrated that certain probiotic lactobacilli could ameliorate Duox pathway impairment caused by imidacloprid, but this effect was not directly dependent on the Duox pathway itself. This study is the first to demonstrate the deleterious effects that neonicotinoids can have on Duox-mediated generation of H₂O₂ and highlights a novel coordination between two important innate immune pathways present in insects.

Comparative susceptibility and immune responses of Asian and European honey bees to the American foulbrood pathogen, Paenibacillus larvae

American foulbrood (AFB) disease is caused by Paenibacillus larvae. Currently, this pathogen is widespread in the European honey bee-Apis mellifera. However, little is known about infectivity and pathogenicity of P. larvae in the Asiatic cavity-nesting honey bees, Apis cerana. Moreover, comparative knowledge of P. larvae infectivity and pathogenicity between both honey bee species is scarce. In this study, we examined susceptibility, larval mortality, survival rate and expression of genes encoding antimicrobial peptides (AMPs) including defensin, apidaecin, abaecin, and hymenoptaecin in A. mellifera and A. cerana when infected with P. larvae. Our results showed similar effects of P. larvae on the survival rate and patterns of AMP gene expression in both honey bee species when bee larvae are infected with spores at the median lethal concentration (LC₅₀ ) for A. mellifera. All AMPs of infected bee larvae showed significant upregulation compared with noninfected bee larvae in both honey bee species. However, larvae of A. cerana were more susceptible than A. mellifera when the same larval ages and spore concentration of P. larvae were used. It also appears that A. cerana showed higher levels of AMP expression than A. mellifera. This research provides the first evidence of survival rate, LC₅₀ and immune response profiles of Asian honey bees, A. cerana, when infected by P. larvae in comparison with the European honey bee, A. mellifera.

RNA-seq reveals disruption of gene regulation when honey bees are caged and deprived of hive conditions

In this study, we present phenotypic and genetic data characterizing the impact of imidacloprid and caging stress on honey bee Apis mellifera physiological responses and regulation of 45 genes using targeted-RNA seq. The term 'caging stress' characterizes the effects of depriving honey bees of all hive aspects and conditions. Two cohorts of 1 day old sister bees were subjected to different conditions. One cohort was caged and fed different imidacloprid-tainted sugar solutions and the second was marked and introduced back to its natal hive. Physiological bee parameters and diet behavior were monitored daily for caged bees over several weeks. Bee samples from both cohorts were sampled weekly for RNA sequencing and oxidative stress analyses. Imidacloprid induced significant protein damage and post-ingestive aversion responses in caged bees, leading to lower tainted syrup consumption and higher water intake compared with the controls. No differentially expressed genes were observed among caged bees in regards to imidacloprid treatment. However, significant upregulation in antioxidant genes was recorded in caged bees as compared with hive bees, with overwhelming downregulation in all gene categories in caged bees at week 4. We identified two sets of genes that were constantly regulated in caged bees, including Rsod with unknown function in insects that could potentially characterize caging stress in honey bees.

Effects of a Resident Yeast from the Honeybee Gut on Immunity, Microbiota, and Nosema Disease

The western honeybee (Apis mellifera) has a core bacterial microbiota that is well described and important for health. Honeybees also host a yeast community that is poorly understood with respect to host nutrition and immunity, and also the symbiotic bacterial microbiota. In this work, we present two studies focusing on the consequences of dysbiosis when honeybees were control-fed a yeast that was isolated from a honeybee midgut, Wickerhamomyces anomalus. Yeast augmentation for bees with developed microbiota appeared immunomodulatory (lowered immunity and hormone-related gene expression) and affected the microbial community, while yeast augmentation for newly emerged bees without an established bacterial background did not lead to decreased immunity— and hormone—related gene expression. In newly emerged bees that had a naturally occurring baseline level of W. anomalus, we observed that the addition of N. ceranae led to a decrease in yeast levels. Overall, we show that yeasts can affect the microbiome, immunity, and physiology.

Spodoptera frugiperda transcriptional response to infestation by Steinernema carpocapsae

Steinernema carpocapsae is an entomopathogenic nematode (EPN) used in biological control of agricultural pest insects. It enters the hemocoel of its host via the intestinal tract and releases its symbiotic bacterium Xenorhabdus nematophila. In order to improve our knowledge about the physiological responses of its different hosts, we examined the transcriptional responses to EPN infestation of the fat body, the hemocytes and the midgut in the lepidopteran pest Spodoptera frugiperda. The tissues poorly respond to the infestation at an early time post-infestation of 8 h with only 5 genes differentially expressed in the fat body of the caterpillars. Strong transcriptional responses are observed at a later time point of 15 h post-infestation in all three tissues. Few genes are differentially expressed in the midgut but tissue-specific panels of induced metalloprotease inhibitors, immune receptors and antimicrobial peptides together with several uncharacterized genes are up-regulated in the fat body and the hemocytes. Among the most up-regulated genes, we identified new potential immune effectors, unique to Lepidoptera, which show homology with bacterial genes of unknown function. Altogether, these results pave the way for further functional studies of the responsive genes' involvement in the interaction with the EPN.

Native and introduced Argentine ant populations are characterised by distinct transcriptomic signatures associated with behaviour and immunity

Biological invasions can be influenced by trait variation in the invader, such as behavioural traits and ecological factors, such as variation in pathogen pressure. High-throughput nucleotide sequencing has increased our capacity to investigate the genomic basis of the functional changes associated with biological invasions. Here, we used RNA-sequencing in Argentina and California, Australia and New Zealand to investigate if native and introduced Argentine ant populations were characterised by distinct transcriptomic signatures. We focused our analysis on viral pressure and immunity, as well as genes associated with biogenic amines known to modulate key behaviour in social insects. Using a combination of differential expression analysis, gene co-expression network analysis and candidate gene approach, we show that native and introduced populations have distinct transcriptomic signatures. Genes associated with biogenic amines were overall up-regulated in the native range compared to introduced populations. Although we found no significant variation in overall viral loads amongst regions for viruses known to infect Argentine ants, viral diversity was lower in most of the introduced range which was interestingly associated with down-regulation of the RNAi immune pathway, primarily directed against viruses. Altogether, our data show that Argentine ant populations exhibit range-specific transcriptomic signatures, perhaps reflecting regional adaptations that may contribute to the ecological success of introduced populations.

The Year of the Honey Bee (Apis mellifera L.) with Respect to Its Physiology and Immunity: A Search for Biochemical Markers of Longevity

It has been known for many years that in temperate climates the European honey bee, Apis mellifera, exists in the form of two distinct populations within the year, short-living summer bees and long-living winter bees. However, there is only limited knowledge about the basic biochemical markers of winter and summer populations as yet. Nevertheless, the distinction between these two kinds of bees is becoming increasingly important as it can help beekeepers to estimate proportion of long-living bees in hives and therefore in part predict success of overwintering. To identify markers of winter generations, we employed the continuous long-term monitoring of a single honey bee colony for almost two years, which included measurements of physiological and immunological parameters. The results showed that the total concentration of proteins, the level of vitellogenin, and the antibacterial activity of haemolymph are the best three of all followed parameters that are related to honey bee longevity and can therefore be used as its markers.

Honeybee-Specific Lactic Acid Bacterium Supplements Have No Effect on American Foulbrood-Infected Honeybee Colonies

Paenibacillus larvae, the causative agent of American foulbrood (AFB), is the primary bacterial pathogen affecting honeybees and beekeeping. The main methods for controlling AFB are incineration of diseased colonies or prophylactic antibiotic treatment (e.g., with tylosin), neither of which is fully satisfactory. The search for superior means for controlling AFB has led to an increased interest in the natural relationships between the honeybee-pathogenic and mutualistic microorganisms and, in particular, the antagonistic effects of honeybee-specific lactic acid bacteria (hbs-LAB) against P. larvae These effects have been demonstrated only on individual larvae in controlled laboratory bioassays. Here we investigated whether supplemental administration of hbs-LAB had a similar beneficial effect on P. larvae infection at colony level. We compared experimentally AFB-infected colonies treated with hbs-LAB supplements to untreated and tylosin-treated colonies and recorded AFB symptoms, bacterial spore levels, and two measures of colony health. To account for the complexity of a bee colony, we focused on (Bayesian) probabilities and magnitudes of effect sizes. Tylosin reduced AFB disease symptoms but also had a negative effect on colony strength. The tylosin treatment did not, however, affect P. larvae spore levels and might therefore "mask" the potential for disease. hbs-LAB tended to reduce brood size in the short term but was unlikely to affect AFB symptoms or spores. These results do not contradict demonstrated antagonistic effects of hbs-LAB against P. larvae at the individual bee level but rather suggest that supplementary administration of hbs-LAB may not be the most effective way to harness these beneficial effects at the colony level.IMPORTANCE The previously demonstrated antagonistic effects of honeybee-derived bacterial microbiota on the infectivity and pathogenicity of P. larvae in laboratory bioassays have identified a possible new approach to AFB control. However, honeybee colonies are complex superorganisms where social immune defenses play a major role in resistance against disease at the colony level. Few studies have investigated the effect of beneficial microorganisms on bee diseases at the colony level. Effects observed at the individual bee level do not necessarily translate into similar effects at the colony level. This study partially fills this gap by showing that, unlike at the individual level, hbs-LAB supplements did not affect AFB symptoms at the colony level. The inference is that the mechanisms regulating the honeybee microbial dynamics within a colony are too strong to manipulate positively through supplemental feeding of live hbs-LAB and that new potential remedies identified through laboratory research have to be tested thoroughly in situ, in colonies.

Integrative Genomics Reveals the Genetics and Evolution of the Honey Bee's Social Immune System

Social organisms combat pathogens through individual innate immune responses or through social immunity—behaviors among individuals that limit pathogen transmission within groups. Although we have a relatively detailed understanding of the genetics and evolution of the innate immune system of animals, we know little about social immunity. Addressing this knowledge gap is crucial for understanding how life-history traits influence immunity, and identifying if trade-offs exist between innate and social immunity. Hygienic behavior in the Western honey bee, Apis mellifera, provides an excellent model for investigating the genetics and evolution of social immunity in animals. This heritable, colony-level behavior is performed by nurse bees when they detect and remove infected or dead brood from the colony. We sequenced 125 haploid genomes from two artificially selected highly hygienic populations and a baseline unselected population. Genomic contrasts allowed us to identify a minimum of 73 genes tentatively associated with hygienic behavior. Many genes were within previously discovered QTLs associated with hygienic behavior and were predictive of hygienic behavior within the unselected population. These genes were often involved in neuronal development and sensory perception in solitary insects. We found that genes associated with hygienic behavior have evidence of positive selection within honey bees (Apis), supporting the hypothesis that social immunity contributes to fitness. Our results indicate that genes influencing developmental neurobiology and behavior in solitary insects may have been co-opted to give rise to a novel and adaptive social immune phenotype in honey bees.

Transcriptomic investigation of immune responses of the Apis cerana cerana larval gut infected by Ascosphaera apis

Chalkbrood is the most common fungal disease in honeybees. The objective of this study was to reveal immune responses in the Apis cerana cerana larval gut following Ascosphaera apis invasion. Combining a previously assembled transcriptome of A. c. cerana larval gut and the high-throughput sequencing data obtained in this study, 6152 differentially expressed genes (DEGs) were clustered into eight profiles. Trend analysis showed three significant up-regulated profiles (p ≤ 0.05) and three down-regulated profiles. Gene Ontology (GO) term analysis suggested that DEGs within significant up-regulated and down-regulated clusters were enriched in 46 and 38 functional groups, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis indicated a majority of DEGs were involved in ribosome structure or function, carbon metabolism, biosynthesis of amino acids, and oxidative phosphorylation. In addition, 142 and 14 DEGs were annotated in the cellular immune- and humoral immune-related pathways, respectively. Further investigation indicated that DEGs up-regulated in cellular immune and humoral immune pathways outnumbered those that were down-regulated. Moreover, immune responses of A. c. cerana and Apis mellifera ligustica larvae were compared and studied to decipher resistance of eastern honeybee larvae to A. apis. These results demonstrated that a large number of genes involved in immunity-related pathways were activated by A. apis. Our findings provided valuable information for elucidating the molecular mechanisms underlying immune responses of A. c. cerana larvae to A. apis infection and pathogen-host interactions during chalkbrood infection.

Ingestion of killed bacteria activates antimicrobial peptide genes in Drosophila melanogaster and protects flies from septic infection

Drosophila melanogaster possesses a sophisticated and effective immune system composed of humoral and cellular immune responses, and production of antimicrobial peptides (AMPs) is an important defense mechanism. Expression of AMPs is regulated by the Toll and IMD (immune deficiency) pathways. Production of AMPs can be systemic in the fat body or a local event in the midgut and epithelium. So far, most studies focus on systemic septic infection in adult flies and little is known about AMP gene activation after ingestion of killed bacteria. In this study, we investigated activation of AMP genes in the wild-type w¹¹¹⁸, MyD88 and Imd mutant flies after ingestion of heat-killed Escherichia coli and Staphylococcus aureus. We showed that ingestion of E. coli activated most AMP genes, including drosomycin and diptericin, in the first to third instar larvae and pupae, while ingestion of S. aureus induced only some AMP genes in some larval stages or in pupae. In adult flies, ingestion of killed bacteria activated AMP genes differently in males and females. Interestingly, ingestion of killed E. coli and S. aureus in females conferred resistance to septic infection by both live pathogenic Enterococcus faecalis and Pseudomonas aeruginosa, and ingestion of E. coli in males conferred resistance to P. aeruginosa infection. Our results indicated that E. coli and S. aureus can activate both the Toll and IMD pathways, and systemic and local immune responses work together to provide Drosophila more effective protection against infection.

Trait-Based Modeling of Multihost Pathogen Transmission: Plant-Pollinator Networks

Epidemiological models for multihost pathogen systems often classify individuals taxonomically and use species-specific parameter values, but in species-rich communities that approach may require intractably many parameters. Trait-based epidemiological models offer a potential solution but have not accounted for within-species trait variation or between-species trait overlap. Here we propose and study trait-based models with host and vector communities represented as trait distributions without regard to species identity. To illustrate this approach, we develop susceptible-infectious-susceptible models for disease spread in plant-pollinator networks with continuous trait distributions. We model trait-dependent contact rates in two common scenarios: nested networks and specialized plant-pollinator interactions based on trait matching. We find that disease spread in plant-pollinator networks is impacted the most by selective pollinators, universally attractive flowers, and cospecialized plant-pollinator pairs. When extreme pollinator traits are rare, pollinators with common traits are most important for disease spread, whereas when extreme flower traits are rare, flowers with uncommon traits impact disease spread the most. Greater nestedness and specialization both typically promote disease persistence. Given recent pollinator declines caused in part by pathogens, we discuss how trait-based models could inform conservation strategies for wild and managed pollinators. Furthermore, while we have applied our model to pollinators and pathogens, its framework is general and can be transferred to any kind of species interactions in any community.

Comparative analysis of C-type lectin domain proteins in the ghost moth, Thitarodes xiaojinensis (Lepidoptera: Hepialidae)

Insects have a large family of C-type lectins involved in cell adhesion, pathogen recognition and activation of immune responses. In this study, 32 transcripts encoding C-type lectin domain proteins (CTLDPs) were identified from the Thitarodes xiaojinensis transcriptome. According to their domain structures, six CTLDPs with one carbohydrate-recognition domain (CRD) were classified into the CTL-S subfamily. The other 23 CTLDPs with two CRDs were grouped into the immulectin (IML) subfamily. The remaining three with extra regulatory domains were sorted into the CTL-X subfamily. Phylogenetic analysis showed that CTL-S and CTL-X members from different insects could form orthologous groups. In contrast, no T. xiaojinensis IML orthologues were found in other insects. Remarkable lineage-specific expansion in this subfamily was observed reflecting that these CTLDPs, as important receptors, have evolved diversified members in response to a variety of microbes. Prediction of binding ligands revealed that T. xiaojinensis, a cold-adapted species, conserved the ability of CRDs to combine with Ca²⁺ to keep its receptors from freezing. Comparative analysis of induction of CTLDP genes after different immune challenges indicated that IMLs might play critical roles in immune defenses. This study examined T. xiaojinensis CTLDPs and provides a basis for further studies of their characteristics.

In-hive variation of the gut microbial composition of honey bee larvae and pupae from the same oviposition time

BACKGROUND

Knowledge of microbiota composition, persistence, and transmission as well as the overall function of the bacterial community is important and may be linked to honey bee health. This study aimed to investigate the inter-individual variation in the gut microbiota in honey bee larvae and pupae.

RESULTS

Individual larvae differed in the composition of major bacterial groups. In the majority of 5th instar bees, Firmicutes showed predominance (70%); however, after larval defecation and during pupation, the abundance decreased to 40%, in favour of Gammaproteobacteria. The 5th instar larvae hosted significantly more (P < 0.001) Firmicutes than black pupae. Power calculations revealed that 11 and 18 replicate-individuals, respectively, were required for the detection of significant differences (P < 0.05) in the Bacteroidetes and Firmicutes abundance between stages, while higher numbers of replicates were required for Actinobacteria (478 replicates) and Gammaproteobacteria (111 replicates).

CONCLUSIONS

Although sample processing and extraction protocols may have had a significant influence, sampling is very important for studying the bee microbiome, and the importance of the number of individuals pooled in samples used for microbiome studies should not be underestimated.

A Transmissible RNA Pathway in Honey Bees

Systemic RNAi, initiated by double-stranded RNA (dsRNA) ingestion, has been reported in diverse invertebrates, including honey bees, demonstrating environmental RNA uptake that undermines homologous gene expression. However, the question why any organism would take up RNA from the environment has remained largely unanswered. Here, we report on horizontal RNA flow among honey bees mediated by secretion and ingestion of worker and royal jelly diets. We demonstrate that transmission of jelly-secreted dsRNA to larvae is biologically active and triggers gene knockdown that lasts into adulthood. Worker and royal jellies harbor differential naturally occurring RNA populations. Jelly RNAs corresponded to honey bee protein-coding genes, transposable elements, and non-coding RNA, as well as bacteria, fungi, and viruses. These results reveal an inherent property of honey bees to share RNA among individuals and generations. Our findings suggest a transmissible RNA pathway, playing a role in social immunity and signaling between members of the hive.

The Mechanisms of Social Immunity Against Fungal Infections in Eusocial Insects

Entomopathogenic fungus as well as their toxins is a natural threat surrounding social insect colonies. To defend against them, social insects have evolved a series of unique disease defenses at the colony level, which consists of behavioral and physiological adaptations. These colony-level defenses can reduce the infection and poisoning risk and improve the survival of societal members, and is known as social immunity. In this review, we discuss how social immunity enables the insect colony to avoid, resist and tolerate fungal pathogens. To understand the molecular basis of social immunity, we highlight several genetic elements and biochemical factors that drive the colony-level defense, which needs further verification. We discuss the chemosensory genes in regulating social behaviors, the antifungal secretions such as some insect venoms in external defense and the immune priming in internal defense. To conclude, we show the possible driving force of the fungal toxins for the evolution of social immunity. Throughout the review, we propose several questions involved in social immunity extended from some phenomena that have been reported. We hope our review about social 'host-fungal pathogen' interactions will help us further understand the mechanism of social immunity in eusocial insects.

Immune related genes as markers for monitoring health status of honey bee colonies

Background

Honey bee population decline threatens the beekeeping sector, agriculture and global biodiversity. Early detection of colony mortality may facilitate rapid interventions to contain and prevent mortality spread. Among others, deformed wing virus (DWV) is capable of inducing colony losses, especially when combined with Varroa destructor mite. Since the bee immune system plays a crucial role in ensuring that bees are able to face these pathogens, we explored whether expression of immune genes could serve as biomarkers of colony health.

Results

Herein, we describe a preliminary immunological marker composed of two immune genes (relish and defensin), which provide insight on honey bee antiviral defense mechanism. Of the tested genes, relish expression correlated with the presence of DWV-Varroa complex, while decreased defensin expression correlated with poor resistance to this complex.

Conclusions

The monitoring of these genes may help us to better understand the complex physiology of honey bees’s immune system and to develop new approaches for managing the health impacts of DWV infection and varroa infestation in the field.

Analysis of the Gynaephora qinghaiensis pupae immune transcriptome in response to parasitization by Thektogaster sp

As a pest on the Qinghai-Tibet Plateau, Gynaephora qinghaiensis causes severe damage to grassland vegetation and its pupae are also natural hosts of Thektogaster sp. To successfully parasitize, endoparasitoids generally introduce or secrete multiple parasitic factors into the host body during the spawning stage to suppress the host immune response. To study the parasitic effects of Thektogaster sp. on G. qinghaiensis, a transcriptome analysis of immune-related genes in parasitized and nonparasitized G. qinghaiensis pupae was performed. A total of 371,260,704 clean reads were assembled into 118,144 unigenes with an average length of 884.33 base pairs. Of these, 23,660 unigenes were annotated in at least one database and 94,484 unigenes were not annotated in any databases. These findings indicated that the majority of the genetic resources (79.97% of all unigenes) in Gynaephora should be further explored. Parasitization significantly affected the transcriptional profile of G. qinghaiensis pupae. The present study identified 12,322 differentially expressed genes and 57 immune-related genes were identified in parasitized G. qinghaiensis pupae. Most immune-related genes were downregulated, potentially resulting from the inhibitory effect of Thektogaster sp. on G. qinghaiensis pupae after parasitization. Overall, the transcriptome analysis sheds valuable light on the molecular mechanisms of G. qinghaiensis parasitization by Thektogaster sp. and promotes the development of novel biocontrol strategies for Gynaephora based on immune defense.

Functional crosstalk across IMD and Toll pathways: insight into the evolution of incomplete immune cascades

In insects, antimicrobial humoral immunity is governed by two distinct gene cascades, IMD pathway mainly targeting Gram-negative bacteria and Toll pathway preferentially targeting Gram-positive bacteria, which are widely conserved among diverse metazoans. However, recent genomic studies uncovered that IMD pathway is exceptionally absent in some hemipteran lineages like aphids and assassin bugs. How the apparently incomplete immune pathways have evolved with functionality is of interest. Here we report the discovery that, in the hemipteran stinkbug Plautia stali, both IMD and Toll pathways are present but their functional differentiation is blurred. Injection of Gram-negative bacteria and Gram-positive bacteria upregulated effector genes of both pathways. Notably, RNAi experiments unveiled significant functional permeation and crosstalk between IMD and Toll pathways: RNAi of IMD pathway genes suppressed upregulation of effector molecules of both pathways, where the suppression was more remarkable for IMD effectors; and RNAi of Toll pathway genes reduced upregulation of effector molecules of both pathways, where the suppression was more conspicuous for Toll effectors. These results suggest the possibility that, in hemipterans and other arthropods, IMD and Toll pathways are intertwined to target wider and overlapping arrays of microbes, which might have predisposed and facilitated the evolution of incomplete immune pathways.

Comparative genomics and transcriptomics of host-pathogen interactions in insects: evolutionary insights and future directions

Classical evolutionary studies of protein-coding genes have established that genes in the canonical immune system are often among the most rapidly evolving within and between species. As more genomes and transcriptomes across insects are sequenced, it is becoming clear that duplications and losses of immune genes are also a likely consequence of host-pathogen interactions. Furthermore, particular species respond to diverse pathogenic challenges with a wide range of challenge-specific responses that are still poorly understood. Transcriptional studies, using RNA-seq to characterize the infection-regulated transcriptome of diverse insects, are crucial for additional progress in understanding the ecology and evolution of the full complexity of the host response.

Dyeing but not dying: Colourful dyes as a non-lethal method of food labelling for in vitro-reared honey bee (Apis mellifera) larvae

Several environmental factors (e.g. food source, pesticides, toxins, parasites and pathogens) influence development and maturation of honey bees (Apis mellifera). Therefore, controlled experimental conditions are mandatory when studying the impact of environmental factors: particularly food quality and nutrient consumption. In vitro larval rearing is a standard approach for monitoring food intake of larvae and the labelling of food is necessary to quantify intake in controlled feeding experiments. Here, we tested the suitability of two food dyes, Allura Red and Brilliant Blue, in an experimental set up using in vitro reared honey bee larvae and freshly hatched adult workers. Absorbance of both dyes was measured, in food and dye-fed larvae, to determine the optimal dye concentrations for accurate detection and quantification. By quantifying relative dye concentrations in dye mixtures, relative concentrations of mixed dyes can be estimated independent of the total food consumed by the larvae. Survival assays were conducted to test the impact of both dyes on larval and worker bee survival. Worker bees showed no increase in adult mortality, when fed with dyed honey. Larval survival was not significantly different until the late pupal stage. The physiological impact of dye feeding was tested by measuring larval immune response. No changes in innate immune gene expression were detectable for larvae fed with dyed and non-dyed food. In conclusion, we established a non-invasive food labelling protocol for food intake quantification in in vitro reared honey bee larvae, using non-toxic, inexpensive, and easy to apply food dyes.

Israeli Acute Paralysis Virus: Honey Bee Queen⁻Worker Interaction and Potential Virus Transmission Pathways

Queen loss or failure is an important cause of honey bee colony loss. A functional queen is essential to a colony, and the queen is predicted to be well protected by worker bees and other mechanisms of social immunity. Nevertheless, several honey bee pathogens (including viruses) can infect queens. Here, we report a series of experiments to test how virus infection influences queen⁻worker interactions and the consequences for virus transmission. We used Israeli acute paralysis virus (IAPV) as an experimental pathogen because it is relevant to bee health but is not omnipresent. Queens were observed spending 50% of their time with healthy workers, 32% with infected workers, and 18% without interaction. However, the overall bias toward healthy workers was not statistically significant, and there was considerable individual to individual variability. We found that physical contact between infected workers and queens leads to high queen infection in some cases, suggesting that IAPV infections also spread through close bodily contact. Across experiments, queens exhibited lower IAPV titers than surrounding workers. Thus, our results indicate that honey bee queens are better protected by individual and social immunity, but this protection is insufficient to prevent IAPV infections completely.

Pollen reverses decreased lifespan, altered nutritional metabolism, and suppressed immunity in honey bees (Apis mellifera) treated with antibiotics

Nutrition is involved in regulating multiple aspects of honeybee biology such as caste, immunity, lifespan, growth and behavioral development. Deformed wing virus (DWV) is a major pathogenic factor which threatens honeybee populations, and its replication is regulated by nutrition status and immune responses of honeybees. The alimentary canal of the honeybee is home to a diverse microbial community that provides essential nutrients and serves to bolster immune responses. However, to what extent gut bacteria affect honeybee nutrition metabolism and immunity with respect to DWV has not been investigated fully. In this study, newly emerged worker bees were subjected to four diets that contained 1) pollen, 2) pollen and antibiotics, 3) neither pollen nor antibiotics, 4) antibiotics alone. The expression level of two nutrition genes target of rapamycin (tor) and insulin like peptide (ilp1), one nutritional marker gene vitellogenin (vg), five major royal jelly proteins genes (mrjp1-5), one antimicrobial peptide regulating gene relish (rel), and DWV virus titer and its replication intermediate, negative RNA strand, were determined by qRT-PCR from the honeybees after 7 days post antibiotic treatment. Additionally, honeybee head weight and survival rate were measured. We observed that antibiotics decreased the expression of tor and rel, increased DWV titer and its replication activity. Expression of ilp1, five mrjps, vg, and honeybee head weight were also reduced compared to bees on a pollen diet. Antibiotics also caused a significant drop in survivorship, which could be rescued by addition of pollen to diets. Of importance, pollen could partially rescue the loss of vg and mrjp2 while also increasing head weight of antibiotic-treated bees. Our results illuminate the roles of bacteria in honeybee nutrition, metabolism, and immunity; which confer the capability of inhibiting virus replication, extending honeybee lifespan, and improving overall health.

Disease Transmission and Networks

While epidemiological models have traditionally assumed that diseases spread by the mass action principle, actual contact networks within social groups do not meet this assumption. Theoretical models have shown that disease dynamics could vary considerably under different types of contact networks, but these models face challenges in terms of their evaluation due to the difficulty of collecting empirical data. The honeybee colony with its elaborate social organization and large repertoire of diseases provides an ideal setting to explore how the structure of the contact network contributes to the transmission of a disease.

Transcriptional and physiological effects of the pyrethroid deltamethrin and the organophosphate dimethoate in the brain of honey bees (Apis mellifera)

The pyrethroid deltamethrin and the organophosphate insecticide dimethoate are widely used in agriculture and in urban areas. Both plant protection products (PPPs) unintendedly result in adverse effects in pollinators. Currently, the sublethal effects of both compounds are poorly known, particularly on the molecular and biochemical level. Here we analysed effects of deltamethrin and dimethoate at environmental and sublethal concentrations in honey bee workers by focusing on transcriptional changes of target genes in the brain. In addition, expression of vitellogenin protein and activity of acetylcholinesterase were assessed upon dimethoate exposure to assess physiological effects. Deltamethrin resulted in induction of the cyp9q2 transcript at 0.53 ng/bee, while dimethoate led to induction of vitellogenin on the mRNA and protein level at 2 ng/bee. Transcripts of additional cytochrome P450-dependent monooxygenases (cyps) and genes related to immune system regulation were not differentially expressed upon PPP exposure. Dimethoate but not deltamethrin led to a strong and concentration-related inhibition of the acetylcholinesterase at 2 and 20 ng/bee. Our data demonstrate that deltamethrin and dimethoate exhibit transcriptional effects at environmental concentrations in the brain of honey bees. Dimethoate also strongly affected physiological traits, which may translate to adverse effects in forager bees.

10-HDA, A Major Fatty Acid of Royal Jelly, Exhibits pH Dependent Growth-Inhibitory Activity Against Different Strains of Paenibacillus larvae

Paenibacillus larvae (P. larvae) is a bacterial pathogen causing American foulbrood (AFB), the most serious disease of honeybee larvae. The food of young larvae could play an important role in the resistance of larvae against AFB. It contains antibacterial substances produced by honeybees that may inhibit the propagation of the pathogen in larval midguts. In this study, we identified and investigated the antibacterial effects of one of these substances, trans-10-hydroxy-2-decenoic acid (10-HDA), against P. larvae strains including all Enterobacterial Repetitive Intergenic Consensus (ERIC) genotypes. Its inhibitory activities were studied by determining the minimum inhibitory concentrations (MICs). It was found that 10-HDA efficacy increases substantially with decreasing pH; up to 12-fold differences in efficacy were observed between pH = 5.5 and pH = 7.2. P. larvae strains showed different susceptibility to 10-HDA; up to 2.97-fold differences existed among various strains with environmentally important ERIC I and ERIC II genotypes. Germinating spores of the pathogen were generally more susceptible to 10-HDA than vegetative cells. Our findings suggest that 10-HDA could play significant role in conferring antipathogenic activity to larval food in the midguts of young larvae and contribute to the resistance of individual larvae to P. larvae.

Apis mellifera (Insecta: Hymenoptera) in the target of neonicotinoids: A one-way ticket? Bioinsecticides can be an alternative

The recent decline of Apis mellifera populations around the world has been subject of intense research due to ecological and economic damages resulting from the loss of pollination services. The intensive use of insecticides from the neonicotinoids group is among the possible causal factors of this decline, including also sub-lethal effects. However, the use of synthetic insecticides has been increased on a global scale in the recent decades. In order to evaluate an alternative to the use of neonicotinoids, this work investigated the effects of a bioinsecticide and its major compound on A. mellifera (Apidae: Hymenoptera), one of the main pollinators of crop plants. For this, bees were exposed, by contact and ingestion, to the essential oil of Cymbopogon martinii (Poaceae: Poales), to geraniol (major compound) and the insecticide imidacloprid to evaluate the toxicity and behavioral effects as well as the locomotion changes and immune responses of bees treated with these compounds. In general, toxicity was greater through ingestion and the insecticide imidacloprid was more toxic to A. mellifera compared to the essential oil and its major compound. The individual and collective behaviors (i.e. trophallaxis, grooming, avoidance) as well as the immune responses of bees were not significantly affected by bioinsecticides. However, the locomotion response and flight orientation of the bees were significantly altered by insecticide when administered by ingestion. Our results highlight the potential of C. martinii essential oil and its major compound as a possible alternative to mitigate the harmful effects of neonicotinoids on bees.

Social environment affects the transcriptomic response to bacteria in ant queens

Social insects have evolved enormous capacities to collectively build nests and defend their colonies against both predators and pathogens. The latter is achieved by a combination of individual immune responses and sophisticated collective behavioral and organizational disease defenses, that is, social immunity. We investigated how the presence or absence of these social defense lines affects individual-level immunity in ant queens after bacterial infection. To this end, we injected queens of the ant Linepithema humile with a mix of gram+ and gram- bacteria or a control solution, reared them either with workers or alone and analyzed their gene expression patterns at 2, 4, 8, and 12 hr post-injection, using RNA-seq. This allowed us to test for the effect of bacterial infection, social context, as well as the interaction between the two over the course of infection and raising of an immune response. We found that social isolation per se affected queen gene expression for metabolism genes, but not for immune genes. When infected, queens reared with and without workers up-regulated similar numbers of innate immune genes revealing activation of Toll and Imd signaling pathways and melanization. Interestingly, however, they mostly regulated different genes along the pathways and showed a different pattern of overall gene up-regulation or down-regulation. Hence, we can conclude that the absence of workers does not compromise the onset of an individual immune response by the queens, but that the social environment impacts the route of the individual innate immune responses.

Nosemosis control in European honey bees, Apis mellifera, by silencing the gene encoding Nosema ceranae polar tube protein 3

RNA interference (RNAi) is a post-transcriptional gene silencing mechanism triggered by double-stranded RNA (dsRNA) that is homologous in sequence to the silenced gene and is conserved in a wide range of eukaryotic organisms. The RNAi mechanism has provided unique opportunities for combating honey bee diseases caused by various parasites and pathogens. Nosema ceranae is a microsporidian parasite of European honey bees, Apis mellifera, and has been associated with honey bee colony losses in some regions of the world. Here we explored the possibility of silencing the expression of a N. ceranae putative virulence factor encoding polar tube protein 3 (ptp3) which is involved in host cell invasion as a therapeutic strategy for controlling Nosema parasites in honey bees. Our studies showed that the oral ingestion of a dsRNA corresponding to the sequences of N. ceranae ptp3 could effectively suppress the expression of the ptp3 gene in N. ceranae-infected bees and reduce Nosema load. In addition to the knockdown of ptp3 gene expression, ingestion of ptp3-dsRNA also led to improved innate immunity in bees infected with N. ceranae along with an improvement in physiological performance and lifespan compared with untreated control bees. These results strongly suggest that RNAi-based therapeutics hold real promise for the effective treatment of honey bee diseases in the future, and warrant further investigation.