Reproduction of parasitic mites Varroa destructor in original and new honeybee hosts

The ectoparasitic mite, Varroa destructor, shifted host from the eastern honeybee, Apis cerana, to the western honeybee, Apis mellifera. Whereas the original host survives infestations by this parasite, they are lethal to colonies of its new host. Here, we investigated a population of A. cerana naturally infested by the V. destructor Korea haplotype that gave rise to the globally invasive mite lineage. Our aim was to better characterize traits that allow for the survival of the original host to infestations by this particular mite haplotype. A known major trait of resistance is the lack of mite reproduction on worker brood in A. cerana. We show that this trait is neither due to a lack of host attractiveness nor of reproduction initiation by the parasite. However, successful mite reproduction was prevented by abnormal host development. Adult A. cerana workers recognized this state and removed hosts and parasites, which greatly affected the fitness of the parasite. These results confirm and complete previous observations of brood susceptibility to infestation in other honeybee host populations, provide new insights into the coevolution between hosts and parasites in this system, and may contribute to mitigating the large‐scale colony losses of A. mellifera due to V. destructor.     

Reproductive interference between honeybee species in artificial sympatry

Reproductive isolation between closely related species is often incomplete. The Western honeybee, Apis mellifera, and the Eastern hive bee, Apis cerana, have been allopatric for millions of years, but are nonetheless similar in morphology and behaviour. During the last century, the two species were brought into contact anthropogenically, providing potential opportunities for interspecific matings. Hybrids between A. mellifera and A. cerana are inviable, so natural interspecific matings are of concern because they may reduce the viability of A. cerana and A. mellifera populations – two of the world's most important pollinators. We examined the mating behaviour of A. mellifera and A. cerana queens and drones from Caoba Basin, China and Cairns, Australia. Drone mating flight times overlap in both areas. Analysis of the spermathecal contents of queens with species‐specific genetic markers indicated that in Caoba Basin, 14% of A. mellifera queens mated with at least one A. cerana male, but we detected no A. cerana queens that had mated with A. mellifera males. Similarly, in Cairns, no A. cerana queens carried A. mellifera sperm, but one‐third of A. mellifera queens had mated with at least one A. cerana male. No hybrid embryos were detected in eggs laid by interspecifically mated A. mellifera queens in either location. However, A. mellifera queens artificially inseminated with A. cerana sperm produced inviable hybrid eggs or unfertilized drones. This suggests that reproductive interference will impact the viability of honeybee populations wherever A. cerana and A. mellifera are in contact.     

Varroa treatment with bromfenvinphos markedly suppresses honeybee biochemical defence levels

We examined the influence of bromfenvinphos, a commonly used acaricide, on activities of many metabolic enzymes affecting the biochemical defences/physiology of the western honeybee, Apis mellifera L. (Hymenoptera: Apidae), as well as on some metabolic compound concentrations, percentage of global DNA methylation, and Nosema spp. infection levels. Bromfenvinphos‐treated workers had decreased haemolymph volumes and higher protein concentrations on their cuticle but lower protein concentrations in the haemolymph. They had higher global DNA methylation levels independent of the age‐related variants. Bromfenvinphos decreased the activities of antioxidant enzymes (SOD, GPx, CAT, GST), acidic, neutral, and alkaline protease inhibitors and enzymatic physiological markers (AST, ALT, ALP), and concentrations of urea, uric acid, creatinine, cholesterol, glucose, Mg²⁺, and Ca²⁺ in worker haemolymph, depending on the age of the bees. Protease activities were higher only in the haemolymph of young bromfenvinphos‐treated bees in comparison with untreated bees. This compound decreased the activities of alkaline proteases and neutral protease inhibitors on the cuticle. Unexpectedly, in the treated bees, the activities of acidic and neutral proteases, and acidic and alkaline protease inhibitors, were higher in the young bees and lower in the older workers in comparison to the untreated group. The bromfenvinphos‐treated workers were more heavily infested with Nosema spp. Thus, bromfenvinphos not only supressed many levels of biochemical defences, and therefore stress‐resistance‐related biochemical pathways but also visibly increased the Nosema spp. infection levels.     

Effect of host brood type on the number of offspring laid by the honeybee parasite Varroa jacobsoni

The number of offspring laid by individual mites, varies depending on the type (drone or worker) of honeybee brood cell invaded. The number of offspring laid by individual mites increases when artificially transferred from worker to drone brood and vice versa when moved in the opposite direction.     

Human‐facilitated metapopulation dynamics in an emerging pest species,Cimex lectularius

The number and demographic history of colonists can have dramatic consequences for the way in which genetic diversity is distributed and maintained in a metapopulation. The bed bug (Cimex lectularius) is a re‐emerging pest species whose close association with humans has led to frequent local extinction and colonization, that is, to metapopulation dynamics. Pest control limits the lifespan of subpopulations, causing frequent local extinctions, and human‐facilitated dispersal allows the colonization of empty patches. Founder events often result in drastic reductions in diversity and an increased influence of genetic drift. Coupled with restricted migration, this can lead to rapid population differentiation. We therefore predicted strong population structuring. Here, using 21 newly characterized microsatellite markers and approximate Bayesian computation (ABC), we investigate simplified versions of two classical models of metapopulation dynamics, in a coalescent framework, to estimate the number and genetic composition of founders in the common bed bug. We found very limited diversity within infestations but high degrees of structuring across the city of London, with extreme levels of genetic differentiation between infestations (F_ST = 0.59). ABC results suggest a common origin of all founders of a given subpopulation and that the numbers of colonists were low, implying that even a single mated female is enough to found a new infestation successfully. These patterns of colonization are close to the predictions of the propagule pool model, where all founders originate from the same parental infestation. These results show that aspects of metapopulation dynamics can be captured in simple models and provide insights that are valuable for the future targeted control of bed bug infestations.     

Host–parasite genotypic interactions in the honey bee: the dynamics of diversity

Parasites are thought to be a major driving force shaping genetic variation in their host, and are suggested to be a significant reason for the maintenance of sexual reproduction. A leading hypothesis for the occurrence of multiple mating (polyandry) in social insects is that the genetic diversity generated within‐colonies through this behavior promotes disease resistance. This benefit is likely to be particularly significant when colonies are exposed to multiple species and strains of parasites, but host–parasite genotypic interactions in social insects are little known. We investigated this using honey bees, which are naturally polyandrous and consequently produce genetically diverse colonies containing multiple genotypes (patrilines), and which are also known to host multiple strains of various parasite species. We found that host genotypes differed significantly in their resistance to different strains of the obligate fungal parasite that causes chalkbrood disease, while genotypic variation in resistance to the facultative fungal parasite that causes stonebrood disease was less pronounced. Our results show that genetic variation in disease resistance depends in part on the parasite genotype, as well as species, with the latter most likely relating to differences in parasite life history and host–parasite coevolution. Our results suggest that the selection pressure from genetically diverse parasites might be an important driving force in the evolution of polyandry, a mechanism that generates significant genetic diversity in social insects.     

Effect of mating frequency and brood cell infestation rate on the reproductive success of the honeybee parasite Varroa jacobsoni

Abstract.

• 1 The reproductve biology of Varroa jacobsoni, whose females infest honeybee brood, was studied in natural and transparent artificial brood cells. These investigations were made under the headings of maturation behaviour and fertilization, and the influence of infestation rate of brood cells on the number of mated females produced per infesting Varroa.
• 2 Mating of Varroa daughters, observed in the transparent brood cells with time-lapse video, occurs just after ecdysis and as soon as they arrive on the faecal accumulation prepared by the mother. Such females are remated for as long as no other freshly moulted daughter arrives on the faecal accumulation.
• 3 The number of spermatozoa stocked in the spermatheca increases with remating, a strong indication for sperm mixing in this species when brood cells contain more than one Varroa foundress.
• 4 The number of daughters per infesting mother decreases at higher rates of infestation per cell, but the proportion of such daughters with a mate rises sharply due to the higher probability of finding a male within multi-infested cells. The number of mated daughters per mother is maximal in cells with two foundress Varroa females.
• 5 The frequency distributions of infesting mites in drone cells are aggregated, and approximate to negative binomial distributions.
• 6 We postulate from the above that the observed non-random infestation by Varroa in drone brood augments the mite's mean reproductive success through the production of a higher number of mated daughters than the corresponding Poisson distributions would.

     

Honey bee colonies from different races show variation in defenses against the varroa mite in a ‘common garden’

Honey bee [Apis mellifera L. (Hymenoptera: Apidae)] genetic diversity may be the key to responding to novel health challenges faced by this important pollinator. In this study, we first compared colonies of four honey bee races, A. m. anatoliaca, A. m. carnica, A. m. caucasica, and A. m. syriaca from Turkey, with respect to honey storage, bee population size, and defenses against varroa. The mite Varroa destructor Anderson & Trueman (Acari: Varroidae) is an important pest of honey bee colonies. There are genetic correlates with two main defenses of bees against this parasite: hygienic behavior, or removing infested brood, and grooming, which involves shaking and swiping off mites and biting them. In the second part of this study, we examined the relationship of these two types of defenses, hygiene and grooming, and their correlation with infestation rates in 32 genetically diverse colonies in a ‘common garden’ apiary. Mite biting was found to be negatively correlated with mite infestation levels.     

Cheaters sometimes prosper: targeted worker reproduction in honeybee (Apis mellifera) colonies during swarming

Kin selection theory predicts that honeybee (Apis mellifera) workers should largely refrain from producing their own offspring, as the workers collectively have higher inclusive fitness if they rear the sons of their mother, the queen. Studies that have quantified levels of ovary activation and reproduction among workers have largely supported this prediction. We sampled pre‐emergent male pupae and adult workers from seven colonies at regular intervals throughout the reproductive part of the season. We show that the overall contribution of workers to male (drone) production is 4.2%, nearly 40 times higher than is generally reported, and is highest during reproductive swarming, when an average of 6.2% of the males genotyped are worker‐produced. Similarly, workers in our samples were 100 times more likely to have active ovaries than previously assumed. Worker reproduction is seasonally influenced and peaks when colonies are rearing new queens. Not all worker subfamilies contribute equally to reproduction. Instead, certain subfamilies are massively over‐represented in drone brood. By laying eggs within the period in which many colonies produce virgin queens, these rare worker subfamilies increase their direct fitness via their well‐timed sons.     

Geographically widespread honeybee‐gut symbiont subgroups show locally distinct antibiotic‐resistant patterns

How long‐term antibiotic treatment affects host bacterial associations is still largely unknown. The honeybee‐gut microbiota has a simple composition, so we used this gut community to investigate how long‐term antibiotic treatment affects host‐associated microbiota. We investigated the phylogenetic relatedness, genomic content (GC percentage, genome size, number of genes and CRISPR) and antibiotic‐resistant genes (ARG) for strains from two abundant members of the honeybee core gut microbiota (Gilliamella apicola and Snodgrassella alvi). Domesticated honeybees are subjected to geographically different management policies, so we used two research apiaries, representing different antibiotic treatment regimens in their apiculture: low antibiotic usage (Norway) and high antibiotic usage (Arizona, USA). We applied whole‐genome shotgun sequencing on 48 G. apicola and 22 S. alvi. We identified three predominating subgroups of G. apicola in honeybees from both Norway and Arizona. For G. apicola, genetic content substantially varied between subgroups and distance similarity calculations showed similarity discrepancy between subgroups. Functional differences between subgroups, such as pectin‐degrading enzymes (G. apicola), were also identified. In addition, we identified horizontal gene transfer (HGT) of transposon (Tn10)‐associated tetracycline resistance (Tet B) across the G. apicola subgroups in the Arizonan honeybees, using interspace polymorphisms in the Tet B determinant. Our results support that honeybee‐gut symbiont subgroups can resist long‐term antibiotic treatment and maintain functionality through acquisition of geographically distinct antibiotic‐resistant genes by HGT.     

A mid‐Pleistocene rainforest corridor enabled synchronous invasions of the Atlantic Forest by Amazonian anole lizards

Shifts in the geographic distribution of habitats over time can promote dispersal and vicariance, thereby influencing large‐scale biogeographic patterns and ecological processes. An example is that of transient corridors of suitable habitat across disjunct but ecologically similar regions, which have been associated with climate change over time. Such connections likely played a role in the assembly of tropical communities, especially within the highly diverse Amazonian and Atlantic rainforests of South America. Although these forests are presently separated by open and dry ecosystems, paleoclimatic and phylogenetic evidence suggest that they have been transiently connected in the past. However, little is known about the timing, magnitude and the distribution of former forest connections. We employ sequence data at multiple loci from three codistributed arboreal lizards (Anolis punctatus, Anolis ortonii and Polychrus marmoratus) to infer the phylogenetic relationships among Amazonian and Atlantic Forest populations and to test alternative historical demographic scenarios of colonization and vicariance using coalescent simulations and approximate Bayesian computation (ABC). Data from the better‐sampled Anolis species support colonization of the Atlantic Forest from eastern Amazonia. Hierarchical ABC indicates that the three species colonized the Atlantic Forest synchronously during the mid‐Pleistocene. We find support of population bottlenecks associated with founder events in the two Anolis, but not in P. marmoratus, consistently with their distinct ecological tolerances. Our findings support that climatic fluctuations provided key opportunities for dispersal and forest colonization in eastern South America through the cessation of environmental barriers. Evidence of species‐specific histories strengthens assertions that biological attributes play a role in responses to shared environmental change.     

Social immunity in honeybees—Density dependence,diet, and body mass trade‐offs

Group living is favorable to pathogen spread due to the increased risk of disease transmission among individuals. Similar to individual immune defenses, social immunity, that is antiparasite defenses mounted for the benefit of individuals other than the actor, is predicted to be altered in social groups. The eusocial honey bee (Apis mellifera) secretes glucose oxidase (GOX), an antiseptic enzyme, throughout its colony, thereby providing immune protection to other individuals in the hive. We conducted a laboratory experiment to investigate the effects of group density on social immunity, specifically GOX activity, body mass and feeding behavior in caged honey bees. Individual honeybees caged in a low group density displayed increased GOX activity relative to those kept at a high group density. In addition, we provided evidence for a trade‐off between GOX activity and body mass: Individuals caged in the low group density had a lower body mass, despite consuming more food overall. Our results provide the first experimental evidence that group density affects a social immune response in a eusocial insect. Moreover, we showed that the previously reported trade‐off between immunity and body mass extends to social immunity. GOX production appears to be costly for individuals, and potentially the colony, given that low body mass is correlated with small foraging ranges in bees. At high group densities, individuals can invest less in social immunity than at low densities, while presumably gaining shared protection from infection. Thus, there is evidence that trade‐offs at the individual level (GOX vs. body mass) can affect colony‐level fitness.     

Post‐fragmentation population structure in a cooperative breeding Afrotropical cloud forest bird: emergence of a source‐sink population network

The impact of demographic parameters on the genetic population structure and viability of organisms is a long‐standing issue in the study of fragmented populations. Demographic and genetic tools are now readily available to estimate census and effective population sizes and migration and gene flow rates with increasing precision. Here we analysed the demography and genetic population structure over a recent 15‐year time span in five remnant populations of Cabanis's greenbul (Phyllastrephus cabanisi), a cooperative breeding bird in a severely fragmented cloud forest habitat. Contrary to our expectation, genetic admixture and effective population sizes slightly increased, rather than decreased between our two sampling periods. In spite of small effective population sizes in tiny forest remnants, none of the populations showed evidence of a recent population bottleneck. Approximate Bayesian modelling, however, suggested that differentiation of the populations coincided at least partially with an episode of habitat fragmentation. The ratio of meta‐N_e to meta‐N_c was relatively low for birds, which is expected for cooperative breeding species, while N_e/N_c ratios strongly varied among local populations. While the overall trend of increasing population sizes and genetic admixture may suggest that Cabanis's greenbuls increasingly cope with fragmentation, the time period over which these trends were documented is rather short relative to the average longevity of tropical species. Furthermore, the critically low N_c in the small forest remnants keep the species prone to demographic and environmental stochasticity, and it remains open if, and to what extent, its cooperative breeding behaviour helps to buffer such effects.     

SNPs selected by information content outperform randomly selected microsatellite loci for delineating genetic identification and introgression in the endangered dark European honeybee (Apis mellifera mellifera)

The honeybee (Apis mellifera) has been threatened by multiple factors including pests and pathogens, pesticides and loss of locally adapted gene complexes due to replacement and introgression. In western Europe, the genetic integrity of the native A. m. mellifera (M‐lineage) is endangered due to trading and intensive queen breeding with commercial subspecies of eastern European ancestry (C‐lineage). Effective conservation actions require reliable molecular tools to identify pure‐bred A. m. mellifera colonies. Microsatellites have been preferred for identification of A. m. mellifera stocks across conservation centres. However, owing to high throughput, easy transferability between laboratories and low genotyping error, SNPs promise to become popular. Here, we compared the resolving power of a widely utilized microsatellite set to detect structure and introgression with that of different sets that combine a variable number of SNPs selected for their information content and genomic proximity to the microsatellite loci. Contrary to every SNP data set, microsatellites did not discriminate between the two lineages in the PCA space. Mean introgression proportions were identical across the two marker types, although at the individual level, microsatellites' performance was relatively poor at the upper range of Q‐values, a result reflected by their lower precision. Our results suggest that SNPs are more accurate and powerful than microsatellites for identification of A. m. mellifera colonies, especially when they are selected by information content.     

ABC inference of multi‐population divergence with admixture from unphased population genomic data

Rapidly developing sequencing technologies and declining costs have made it possible to collect genome‐scale data from population‐level samples in nonmodel systems. Inferential tools for historical demography given these data sets are, at present, underdeveloped. In particular, approximate Bayesian computation (ABC) has yet to be widely embraced by researchers generating these data. Here, we demonstrate the promise of ABC for analysis of the large data sets that are now attainable from nonmodel taxa through current genomic sequencing technologies. We develop and test an ABC framework for model selection and parameter estimation, given histories of three‐population divergence with admixture. We then explore different sampling regimes to illustrate how sampling more loci, longer loci or more individuals affects the quality of model selection and parameter estimation in this ABC framework. Our results show that inferences improved substantially with increases in the number and/or length of sequenced loci, while less benefit was gained by sampling large numbers of individuals. Optimal sampling strategies given our inferential models included at least 2000 loci, each approximately 2 kb in length, sampled from five diploid individuals per population, although specific strategies are model and question dependent. We tested our ABC approach through simulation‐based cross‐validations and illustrate its application using previously analysed data from the oak gall wasp, Biorhiza pallida.     

Susceptibility of small honey bee colonies to invasion by the small hive beetle,Aethina tumida (Coleoptera,Nitidulidae)

Weak and small honey bee colonies are supposed to be more susceptible to infestations by the small hive beetle [Aethina tumida, small hive beetle (SHB)]. To test this, we established 24 nucleus colonies [12 with and 12 without previous SHB removal (= screening)]. Four weeks later, we compared beetle numbers and the occurrence of SHB reproduction to the corresponding full‐sized colonies. Full‐sized colonies with no screening were infested with significantly more SHBs than all other groups (mean ± standard deviation = 46.9 ± 26.7). Regardless of this, none of the full‐sized colonies showed damage or evidence of SHB reproduction. In contrast, five nucleus colonies collapsed and SHB larvae were found in an additional seven colonies. Our study demonstrates that SHB infestation levels which are harmless to full‐sized colonies may have a negative impact on small nucleus colonies.