Infection by the castrating parasitic nematode Sphaerularia bombi changes gene expression in Bombus terrestris bumblebee queens.

Parasitism can result in dramatic changes in host phenotype, which are themselves underpinned by genes and their expression. Understanding how hosts respond at the molecular level to parasites can therefore reveal the molecular architecture of an altered host phenotype. The entomoparasitic nematode Sphaerularia bombi is a parasite of bumblebee (Bombus) hosts where it induces complex behavioural changes and host castration. To examine this interaction at the molecular level, we performed genome-wide transcriptional profiling using RNA-Sequencing (RNA-Seq) of S. bombi-infected Bombus terrestris queens at two critical time-points: during and just after overwintering diapause. We found that infection by S. bombi affects the transcription of genes underlying host biological processes associated with energy usage, translation, and circadian rhythm. We also found that the parasite affects the expression of immune genes, including members of the Toll signalling pathway providing evidence for a novel interaction between the parasite and the host immune response. Taken together, our results identify host biological processes and genes affected by an entomoparasitic nematode providing the first steps towards a molecular understanding of this ecologically important host-parasite interaction.

Disease associations between honeybees and bumblebees as a threat to wild pollinators.

Emerging infectious diseases (EIDs) pose a risk to human welfare, both directly and indirectly, by affecting managed livestock and wildlife that provide valuable resources and ecosystem services, such as the pollination of crops. Honeybees (Apis mellifera), the prevailing managed insect crop pollinator, suffer from a range of emerging and exotic high-impact pathogens, and population maintenance requires active management by beekeepers to control them. Wild pollinators such as bumblebees (Bombus spp.) are in global decline, one cause of which may be pathogen spillover from managed pollinators like honeybees or commercial colonies of bumblebees. Here we use a combination of infection experiments and landscape-scale field data to show that honeybee EIDs are indeed widespread infectious agents within the pollinator assemblage. The prevalence of deformed wing virus (DWV) and the exotic parasite Nosema ceranae in honeybees and bumblebees is linked; as honeybees have higher DWV prevalence, and sympatric bumblebees and honeybees are infected by the same DWV strains, Apis is the likely source of at least one major EID in wild pollinators. Lessons learned from vertebrates highlight the need for increased pathogen control in managed bee species to maintain wild pollinators, as declines in native pollinators may be caused by interspecies pathogen transmission originating from managed pollinators.

Microsporidia: An Emerging Threat to Bumblebees?

Microsporidia may cause emerging infectious diseases (EIDs) in bumblebees. Two drivers - commercial bumblebees and managed honey bees - have been identified as possible sources of pathogen spillover. In addition, declines in bumblebee populations may have led to lower genetic diversity and subsequent higher susceptibility to infection, enabling microsporidia to increase in prevalence. There is strong evidence for relatively recent increases in the prevalence of Nosema bombi in North America. However, the lack of definitive data on spillover by microsporidia, in North America or elsewhere, makes it difficult to identify the causes of such increases. Phylogenomic studies are urgently needed to identify the global population structure of microsporidia in bumblebees, and thus identify the source of current and future epidemics.

Microsporidia - Emergent Pathogens in the Global Food Chain.

Intensification of food production has the potential to drive increased disease prevalence in food plants and animals. Microsporidia are diversely distributed, opportunistic, and density-dependent parasites infecting hosts from almost all known animal taxa. They are frequent in highly managed aquatic and terrestrial hosts, many of which are vulnerable to epizootics, and all of which are crucial for the stability of the animal-human food chain. Mass rearing and changes in global climate may exacerbate disease and more efficient transmission of parasites in stressed or immune-deficient hosts. Further, human microsporidiosis appears to be adventitious and primarily associated with an increasing community of immune-deficient individuals. Taken together, strong evidence exists for an increasing prevalence of microsporidiosis in animals and humans, and for sharing of pathogens across hosts and biomes.

Horizontal transmission success of Nosema bombi to its adult bumble bee hosts: effects of dosage, spore source and host age.

Parasite transmission dynamics are fundamental to explaining the evolutionary epidemiology of disease because transmission and virulence are tightly linked. Horizontal transmission of microsporidian parasites, e.g. Nosema bombi, may be influenced by numerous factors, including inoculation dose, host susceptibility and host population heterogeneity. Despite previous studies of N. bombi and its bumble bee hosts, neither the epidemiology nor impact of the parasite are as yet understood. Here we investigate the influence N. bombi spore dosage (1000 to 500,000 spores), spore source (Bombus terrestris and B. lucorum isolates) and host age (2- and 10-day-old bees) have on disease establishment and the presence of patent infections in adult bumble bees. Two-day-old bees were twice as susceptible as their 10-day-old sisters, and a 5-fold increase in dosage from 100,000 to 500,000 spores resulted in a 20-fold increase in the prevalence of patent infections. While intraspecific inoculations were 3 times more likely to result in non-patent infections there was no such effect on the development of patent infections. These results suggest that host-age and dose are likely to play a role in N. bombi's evolutionary epidemiology. The relatively low levels of horizontal transmission success are suggestive of low virulence in this system.

The impact of host starvation on parasite development and population dynamics in an intestinal trypanosome parasite of bumble bees.

Host nutrition plays an important role in determining the development and success of parasitic infections. While studies of vertebrate hosts are accumulating, little is known about how host nutrition affects parasites of invertebrate hosts. Crithidia bombi is a gut trypanosome parasite of the bumble bee, Bombus terrestris and here we use it as a model system to determine the impact of host nutrition on the population dynamics and development of micro-parasites in invertebrates. Pollen-starved bees supported significantly smaller populations of the parasite. In pollen-fed bees the parasite showed a temporal pattern in development, with promastigote transmission stages appearing at the start of the infection and gradually being replaced by choanomastigote and amastigote forms. In pollen-starved bees this developmental process was disrupted, and there was no pattern in the appearance of these three forms. We discuss the implications of these results for parasite transmission, and speculate about the mechanisms behind these changes.

Activation of host constitutive immune defence by an intestinal trypanosome parasite of bumble bees.

Many parasites, including important species that affect humans and livestock, must survive the harsh environment of insect guts to complete their life-cycle. Hence, understanding how insects protect themselves against such parasites has immediate practical implications. Previously, such protection has been thought to consist mainly of mechanical structures and the action of lectins. However, recently it has become apparent that gut infections may interact with the host immune system in more complex ways. Here, using bumble bees, Bombus terrestris and their non-invasive gut trypanosome, Crithidia bombi, as a model system we investigated the effects of parasitic infection, host resources and the duration of infections on the host immune system. We found that infection doubled standing levels of immune defence in the haemolymph (the constitutive pro-phenoloxidase system), which is used as a first, general defence against parasites. However, physical separation of the parasite from the haemolymph suggests the presence of a messenger system between the gut and the genes that control the pro-phenoloxidase system. Surprisingly, we found no direct effect of host resource-stress or duration of the infection on the immune system. Our results suggest a novel and tactical response of insects to gut infections, demonstrating the complexity of such host-parasite systems.

Queen-controlled sex ratios and worker reproduction in the bumble bee Bombus hypnorum, as revealed by microsatellites.

Social insect colonies provide model systems for the examination of conflicts among parties with different genetic interests. As such, they have provided the best tests of inclusive fitness theory. However, much remains unknown about in which party's favour such conflicts are resolved, partly as a result of the only recent advent of the molecular tools needed to examine the outcome of these conflicts. Two key conflicts in social insect colonies are over control of the reproductive sex ratio and the production of male offspring. Most studies have examined only one of these conflicts but in reality they occur in tandem and may influence each other. Using microsatellite analyses, the outcome of conflict over sex ratios and male production was examined in the bumble bee, Bombus hypnorum. The genotypes were determined for mother queens, their mates and males for each of 10 colonies. In contrast to other reports of mating frequency in this species, all of the queens were singly mated. The population sex ratio was consistent with queen control, suggesting that queens are winning this conflict. In contrast, workers produced over 20% of all males in queen-right colonies, suggesting that they are more effective in competing over male-production. Combining these results with previous work, it is suggested that worker reproduction is a labile trait that may well impose only small costs on queen fitness.