Multi-host viruses in Argentine ants and honey bees: Increased viral disease in honey bees is associated with Argentine ants

Emerging infectious diseases threaten public health, livestock economies, and wildlife. Human-mediated species introductions can alter host and pathogen communities that shape the dynamics of infectious diseases. Several RNA viruses that have been linked to population declines in wild pollinators and losses of managed honey bees have been detected in multiple other species and are suspected to circulate within insect communities. Yet, we lack an understanding of how disease dynamics are affected by the introduction of novel species. These introduced species include invasive ants, which can disturb honey bees and become a pest in apiaries. The Argentine ant (Linepithema humile) is a globally successful invader that has been observed to attack bees and multiple bee-associated viruses have been detected in this ant species. Here, I studied interactions between Argentine ants and European honey bees (Apis mellifera) and how these interactions affect viral dynamics in beehives. I first tested a range of pollinators and associated insects for RNA viruses that are pathogenic to honey bees. Bee-associated viruses showed evidence for active viral replication in several pollinator species but also in species that cohabit in beehives such as ants, spiders, and cockroaches. Using phylogenetic analyses, I found that viral transmission within communities was shaped by geographic origin rather than being restricted by species barriers. Next, I used a longitudinal field study to test whether Argentine ant presence affected pathogen infections and survival in beehives. Argentine ants tested positive for three bee-associated viruses even before beehives were moved into ant-infested sites. Increased levels of deformed wing virus in beehives in autumn were associated with ant presence, although hive mortality was not affected by ants over the duration of this experiment. I used RNA sequencing on a subset of honey bee samples collected during autumn to study the RNA virome and identify transcriptomic responses associated with ant presence. Twelve RNA viruses were found in beehives, among those, three plant-associated viruses and an unclassified RNA virus that had not previously been observed in honey bees. Deformed wing virus showed the highest viral titres in most hives, but was only marginally affected by ant presence. Sacbrood virus and tomato ringspot virus levels were increased in hives with ants, however, both viruses are not known to infect Argentine ants and the plant-associated tomato ringspot virus seems unlikely to affect bee health. Lastly, I tested the feasibility of controlling Argentine ants in apiaries using a novel pest control strategy. RNA interference is a conserved cellular gene regulation mechanism that could be used to silence specific genes in ants. Using double-stranded RNA (dsRNA) to silence two immune-related genes in Argentine ants was expected to increase pathogen susceptibility, which could then lead to higher pathogen levels that reduce ant numbers. My results indicated that no consistent immune silencing could be achieved in the field. Immune gene expression changes were observed, but pathogen titres were not affected, and ant numbers stayed high. Argentine ant control using a conventional insecticide significantly increased bee survival, whereas many hives in the dsRNA and control group abandoned their hives due to ant attacks. Although population control was not successful using the two Argentine ant-specific dsRNAs, insights into ant immunity and ant-bee interactions could improve the development of novel control strategies. Bee-associated viruses have repeatedly been detected in ant species, yet, this is one of the first studies to investigate whether ants affect viral dynamics in honey bees. I showed that invasive Argentine ants are associated with increases in viral pathogens in honey bees. The mechanisms by which ants affect bee disease are unknown, although there is some evidence for ants transmitting viruses or causing stress responses in bees that affect immunity. The findings of this thesis highlight the risk of invasive ant species disrupting pollination services. New and environmentally-friendly methods to control invasive species are urgently needed to improve bee health and limit the spread of invasive ants, such as Argentine ants. The high prevalence of bee-associated viruses and viral diversity in ants suggests that pathogens that are suitable for population control might be present in ant populations, although risks of spillovers into other species need to be carefully considered.</p

Genetic strain diversity of multi-host RNA viruses that infect a wide range of pollinators and associates is shaped by geographic origins

Emerging viruses have caused concerns about pollinator population declines, as multi-host RNA viruses may pose a health threat to pollinators and associated arthropods. In order to understand the ecology and impact these viruses have, we studied their host range and determined to what extent host and spatial variation affect strain diversity. Firstly, we used RT-PCR to screen pollinators and associates, including honey bees (Apis mellifera) and invasive Argentine ants (Linepithema humile), for virus presence and replication. We tested for the black queen cell virus (BQCV), deformed wing virus (DWV), and Kashmir bee virus (KBV) that were initially detected in bees, and the two recently discovered Linepithema humile bunya-like virus 1 (LhuBLV1) and Moku virus (MKV). DWV, KBV, and MKV were detected and replicated in a wide range of hosts and commonly co-infected hymenopterans. Secondly, we placed KBV and DWV in a global phylogeny with sequences from various countries and hosts to determine the association of geographic origin and host with shared ancestry. Both phylogenies showed strong geographic rather than host-specific clustering, suggesting frequent inter-species virus transmission. Transmission routes between hosts are largely unknown. Nonetheless, avoiding the introduction of non-native species and diseased pollinators appears important to limit spill overs and disease emergence

Viruses and their effects in ants (Hymenoptera: Formicidae)

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Vespula data overview

Vespula vulgaris nests used in the nest analysis and mitochondrial DNA analysis. Sample name: internal name used to identify samle; Queen cells and worker cells are estimated from queen and worker comb area.;Latitude and longitude give the sampling location in New Zealand or Belgium; "DNA from?" indicates which DNA extraction method was used

Fitness and microbial networks of the common wasp, Vespula vulgaris (Hymenoptera: Vespidae), in its native and introduced ranges

1. Variation in microbial communities between populations is increasingly hypothesised to affect animal fitness and performance, including for invasive species. Pathogenic species may be lost during the introduction process, enhancing invader fitness and abundance. 2. This study assessed fitness, immune gene expression, and microbial network complexity of invasive common wasps, Vespula vulgaris. Microbial networks were assayed using 16S and 18S sequencing and gene expression arrays in the native (Belgium) and introduced range (New Zealand). The immune gene expression of the wasp Down syndrome cell adhesion molecule (Dscam) gene homologue was examined. Dscam expression can be induced by viruses, Gram‐positive and Gram‐negative bacteria, and parasites. 3. Individual nest fitness was higher in the native range of Belgium than in the introduced New Zealand range. Microbial communities of wasps in the introduced range were more diverse with more complex networks, although some microorganisms were range‐specific. Microbial networks in the introduced range showed higher clustering coefficients, number of connected paths, network centralisation, number of neighbours and network density. 4. Larvae, workers, virgin and foundress queens had higher expression of Dscam in the New Zealand samples. These immune gene expression patterns were associated with higher pathogen pressure and lower relative fitness. 5. Epidemiological theory predicts that a high density of pathogen and microbial hosts should result in a high rate of disease infection, prevalence, and highly connected microbial networks. The results of this study support these predictions. Wasps displayed lower relative fitness and more highly connected microbial networks in New Zealand than in Belgium.status: publishe

16S and 18S FASTA files

16S and 18S amplicon sequencing files for microbial network analysis. Files are in FASTA format. One 18S and one 16S file is provided for each of the 18 samples in the study. Codes in the file name identify the species (VV), the region (NZ for New Zealand and EU for Belgium) and the life stage (Q for queen, L for larva and W for worker)

Data from: Fitness and microbial networks of the common wasp, Vespula vulgaris (Hymenoptera: Vespidae), in its native and introduced range

1. Variation in microbial communities between populations is increasingly hypothesized to affect animal fitness and performance, including for invasive species. Pathogenic species may be lost during the introduction process, enhancing invader fitness and abundance. 2. We assessed fitness, immune gene expression, and microbial network complexity of invasive common wasps, Vespula vulgaris. Microbial networks were assayed using 16S and 18S sequencing and gene expression arrays in the native (Belgium) and introduced range (New Zealand). We examined the immune gene expression of the wasp Down syndrome cell adhesion molecule (Dscam) gene homologue. Dscam expression can be induced by viruses, Gram-positive and Gram-negative bacteria, and parasites. 3. Individual nest fitness was higher in the native range of Belgium than in the introduced, New Zealand range. Microbial communities of wasps in the introduced range were more diverse with more complex networks, although some microorganisms were range specific. Microbial networks in the introduced range showed higher clustering coefficients, number of connected paths, network centralization, number of neighbours and network density. 4. Larvae, workers, virgin and foundress queens had higher expression of Dscam in the New Zealand samples. These immune gene expression patterns were associated with higher pathogen pressure and lower relative fitness in New Zealand compared to Belgium. 5. Epidemiological theory predicts that a high density of pathogen and microbial hosts should result in a high rate of disease infection, prevalence, and highly connected microbial networks. Our results support these predictions. Wasps displayed lower relative fitness and more highly connected microbial networks in New Zealand compared to Belgium

Data from: Fitness in invasive social wasps: the role of variation in viral load, immune response and paternity in predicting nest size and reproductive output

Within any one habitat, the relative fitness of organisms in a population can vary substantially. Social insects like the common wasp are among the most successful invasive animals, but show enormous variation in nest size and other fitness-related traits. Some of this variation may be caused by pathogens such as viruses that can have serious consequences in social insects, which range from reduced productivity to colony death. Both individual immune responses and colony-level traits such as genetic diversity are likely to influence effects of pathogen infections on colony fitness. Here we investigate how infections with Kashmir Bee Virus (KBV), immune response and intracolony genetic diversity (due to queen polyandry) affect nest size in the invasive common wasp, Vespula vulgaris. We show that KBV is highly prevalent in wasps and expression of antiviral immune genes is significantly increased with higher viral loads across individuals. Patriline membership within a nest did not influence KBV susceptibility or immune response. A permutational MANCOVA revealed that polyandry, viral load, and expression of the immune gene Dicer were significant predictors of variation in nest size. High intracolony genetic diversity due to polyandry has previously been hypothesized to improve colony-level resistance to parasites and pathogens. Consistent with this hypothesis, we observed genetically diverse colonies to be significantly larger and to produce more queens, although this effect was not driven by the pathogen we investigated. Invasive wasps clearly suffer from pathogens and expend resources, as indicated here by elevated immune gene expression, toward reducing pathogen-impact on colony fitness

Honey bees and social wasps reach convergent architectural solutions to nest-building problems

The hexagonal cells built by honey bees and social wasps are an example of adaptive architecture; hexagons minimize material use, while maximizing storage space and structural stability. Hexagon building evolved independently in the bees and wasps, but in some species of both groups, the hexagonal cells are size dimorphic—small worker cells and large reproductive cells—which forces the builders to join differently sized hexagons together. This inherent tiling problem creates a unique opportunity to investigate how similar architectural challenges are solved across independent evolutionary origins. We investigated how 5 honey bee and 5 wasp species solved this problem by extracting per-cell metrics from 22,745 cells. Here, we show that all species used the same building techniques: intermediate-sized cells and pairs of non-hexagonal cells, which increase in frequency with increasing size dimorphism. We then derive a simple geometric model that explains and predicts the observed pairing of non-hexagonal cells and their rate of occurrence. Our results show that despite different building materials, comb configurations, and 179 million years of independent evolution, honey bees and social wasps have converged on the same solutions for the same architectural problems, thereby revealing fundamental building properties and evolutionary convergence in construction behavior.publishe

The association between mitochondrial genetic variation and reduced colony fitness in an invasive wasp

Despite the mitochondrion's long-recognized role in energy production, mitochondrial DNA (mtDNA) variation commonly found in natural populations was assumed to be effectively neutral. However, variation in mtDNA has now been increasingly linked to phenotypic variation in life history traits and fitness. We examined whether the relative fitness in native and invasive common wasp (Vespula vulgaris) populations in Belgium and New Zealand (NZ), respectively, can be linked to mtDNA variation. Social wasp colonies in NZ were smaller with comparatively fewer queen cells, indicating a reduced relative fitness in the invaded range. Interestingly, queen cells in this population were significantly larger leading to larger queen offspring. By sequencing 1,872 bp of the mitochondrial genome, we determined mitochondrial haplotypes and detected reduced genetic diversity in NZ. Three common haplotypes in NZ frequently produced many queens, whereas the four rare haplotypes produced significantly fewer or no queens. The entire mitochondrial genome for each of these haplotypes was sequenced to identify polymorphisms associated with fitness reduction. We found 16 variable sites; however, no nonsynonymous mutation that was clearly causing impaired mitochondrial function was detected. We discuss how detected variants may alter secondary structures, gene expression or mito-nuclear interactions, or could be associated with nuclear-encoded variation. Whatever the ultimate mechanism, we show reduced fitness and mtDNA variation in an invasive wasp population as well as specific mtDNA variants associated with fitness variation within this population. Ours is one of only a few studies that confirm fitness impacts of mtDNA variation in wild nonmodel populations.status: publishe