Seasonal variability of prevalence and occurrence of multiple infections shape the population structure of Crithidia bombi, an intestinal parasite of bumblebees (Bombus spp.)

Seasonally increasing parasite load is associated with microbiome dysbiosis in wild bumblebees

The microbiome is increasingly recognized for its complex relationship with host fitness. Bumblebees are host to a characteristic gut microbiome community that is derived and reinforced through social contact between individuals. The bumblebee microbiome is species-poor, and primarily composed from a small number of core taxa that are associated with the greater tribe of corbiculate bees. Experimental findings support a role for the core bumblebee microbiome in resistance to severe infections by a common trypanosomal parasite, Crithidia bombi. However, most studies have been small in scale, often considering just one or two bumblebee species, or making use of commercially-reared bees. To better understand the microbiome diversity of wild populations, we have deeply sampled field populations of ten sympatric species found throughout central and down east Maine in a three-year microbiome field survey. We have used 16S amplicon sequencing to produce microbiome community profiles, and qPCR to screen samples for infections by Crithidia bombi. The breadth of our dataset has enabled us to test for seasonal and interspecific trends in the microbiome community. Controlling for these external sources of variation, we have identified microbial factors associated with infection and parasite load that support the role of the core microbiome in resistance to severe infection.

Methods matter: the influence of method on infection estimates of the bumblebee parasite Crithidia bombi

The bumblebee gut parasite, Crithidia bombi, is widespread and prevalent in the field. Its interaction with Bombus spp. is a well-established epidemiological model. It is spread faecal-orally between colonies via the shared use of flowers when foraging. Accurately measuring the level of infection in bumblebees is important for assessing its distribution in the field, and also when conducting epidemiological experiments. Studies generally use 1 of 2 methods for measuring infection. One approach measures infection in faeces whereas the other method measures infection in guts. We tested whether the method of measuring infection affected the estimation of infection. Bumblebees were inoculated with a standardized inoculum and infection was measured 1 week later using either the faecal or gut method. We found that when the gut method was used to measure infection intensity estimates were significantly different to and approximately double those from the faecal method. These results have implications for the interpretation of previous study results and for the planning of future studies. Given the importance of bumblebees as pollinators, the impact of C. bombi on bumblebee health, and its use as an epidemiological model, we call on researchers to move towards consistent quantification of infections to enable future comparisons and meta-analyses of studies.

Transmission of a bumblebee parasite is robust despite parasite exposure to extreme temperatures

All organisms are exposed to fluctuating environmental conditions, such as temperature. How individuals respond to temperature affects their interactions with one another. Changes to the interaction between parasites and their hosts can have a large effect on disease dynamics. The gut parasite, Crithidia bombi, can be highly prevalent in the bumblebee, Bombus terrestris, and is an established epidemiological model. The parasite is transmitted between bumblebees via flowers, exposing it to a range of environmental temperatures prior to infection. We investigated whether incubation duration and temperature exposure, prior to infection, affects parasite infectivity. Prior to inoculation in B. terrestris, C. bombi was incubated at 10, 20, 30, 40 or 50°C for either 10 or 60 min. These times were chosen to reflect the length of time that the parasite remains infective when outside the host and the rate of floral visitation in bumblebees. Prevalence and infection intensity were measured in bees 1 week later. Incubation duration and the interaction between incubation temperature and duration affected the prevalence of C. bombi at 50°C, resulting in no infections after 60 min. Below 50°C, C. bombi prevalence was not affected by incubation temperature or duration. Extreme temperatures induced morphological changes in C. bombi cells; however, infection intensity was not affected by incubation duration or temperature. These results highlight that this parasite is robust to a wide range of temperatures. The parasite was not infective after being exposed to 50°C for 60 min, such temperatures likely exceed the flight abilities of bumblebees, and thus the potential for transmission. This study shows the importance of understanding the effects of environmental conditions on both hosts and parasites, which is needed to predict transmission under different environmental conditions.

Prevalence and Distribution of Three Bumblebee Pathogens from the Czech Republic

Bumblebees are significant pollinators for both wild plants and economically important crops. Due to the worldwide decrease in pollinators, it is crucial to monitor the prevalence and distribution of bumblebee pathogens. Field-caught bumblebee workers and males were examined for the presence of three pathogens during the summer months of the years 2015-2020 (Bombus terrestris/lucorum) and 2015-2017 (Bombus lapidarius). The greatest prevalence was in the case of Crithidia bombi, where significantly more workers (57%) of B. terrestris/lucorum were infected than males (41%). Infection was also confirmed in 37% of B. lapidarius workers. The average prevalence was very similar in the case of Nosema bombi in workers (25%) and males (22%) of B. terrestris/lucorum. In the case of B. lapidarius, 17% of the workers were infected. The lowest number of infected individuals was for Apicystis bombi and the prevalence of infection was significantly higher for males (22%) than workers (8%) of B. terrestris/lucorum. Only 3% of workers and 4% of males of B. terrestris/lucorum were simultaneously infected with three types of pathogens, but no worker was infected with only a combination of N. bombi and A. bombi. The greatest prevalence of C. bombi was found in urban or woodland areas.

Bee Trypanosomatids: First Steps in the Analysis of the Genetic Variation and Population Structure of Lotmaria passim, Crithidia bombi and Crithidia mellificae

Trypanosomatids are among the most prevalent parasites in bees but, despite the fact that their impact on the colonies can be quite important and that their infectivity may potentially depend on their genotypes, little is known about the population diversity of these pathogens. Here we cloned and sequenced three non-repetitive single copy loci (DNA topoisomerase II, glyceraldehyde-3-phosphate dehydrogenase and RNA polymerase II large subunit, RPB1) to produce new genetic data from Crithidia bombi, C. mellificae and Lotmaria passim isolated from honeybees and bumblebees. These were analysed by applying population genetic tools in order to quantify and compare their variability within and between species, and to obtain information on their demography and population structure. The general pattern for the three species was that (1) they were subject to the action of purifying selection on nonsynonymous variants, (2) the levels of within species diversity were similar irrespective of the host, (3) there was evidence of recombination among haplotypes and (4) they showed no haplotype structuring according to the host. C. bombi exhibited the lowest levels of synonymous variation (π_S= 0.06 ± 0.04 %) - and a mutation frequency distribution compatible with a population expansion after a bottleneck - that contrasted with the extensive polymorphism displayed by C. mellificae (π_S= 2.24 ± 1.00 %), which likely has a more ancient origin. L. passim showed intermediate values (π_S= 0.40 ± 0.28 %) and an excess of variants a low frequencies probably linked to the spread of this species to new geographical areas.

Pathogens Spillover from Honey Bees to Other Arthropods

Honey bees, and pollinators in general, play a major role in the health of ecosystems. There is a consensus about the steady decrease in pollinator populations, which raises global ecological concern. Several drivers are implicated in this threat. Among them, honey bee pathogens are transmitted to other arthropods populations, including wild and managed pollinators. The western honey bee, Apis mellifera, is quasi-globally spread. This successful species acted as and, in some cases, became a maintenance host for pathogens. This systematic review collects and summarizes spillover cases having in common Apis mellifera as the mainteinance host and some of its pathogens. The reports are grouped by final host species and condition, year, and geographic area of detection and the co-occurrence in the same host. A total of eighty-one articles in the time frame 1960-2021 were included. The reported spillover cases cover a wide range of hymenopteran host species, generally living in close contact with or sharing the same environmental resources as the honey bees. They also involve non-hymenopteran arthropods, like spiders and roaches, which are either likely or unlikely to live in close proximity to honey bees. Specific studies should consider host-dependent pathogen modifications and effects on involved host species. Both the plasticity of bee pathogens and the ecological consequences of spillover suggest a holistic approach to bee health and the implementation of a One Health approach.

The potential for parasite spill-back from commercial bumblebee colonies: a neglected threat to wild bees?

Commercially-reared bumblebee colonies provide pollination services to numerous crop species globally. These colonies may harbour parasites which can spill-over to wild bee species. However, the potential for parasites to spread from wild to commercial bumblebees, which could then lead to parasite spill-back, is poorly understood. To investigate this, parasite-free commercial Bombus terrestris audax colonies, which are used commercially for strawberry pollination, were placed into seasonal strawberry crops for either 6- or 8-week blocks across two key time periods, early spring and early summer. Bumblebees were removed from colonies weekly and screened for the presence of parasites. In the early spring placement, only one parasite, the highly virulent neogregarine Apicystis bombi, was detected at a low prevalence (0.46% across all bees screened). In contrast, all colonies placed in the crop in the early summer became infected. A trypanosome, Crithidia bombi, and A. bombi were the most prevalent parasites across all samples, reaching peak prevalence in screened bees of 39.39% and 18.18% respectively at the end of the experimental period. The prevalence of A. bombi was greater than most UK records from wild bumblebees, suggesting that commercial colonies could enhance levels of A. bombi infection in wild bees through spill-back. Studies on larger geographical scales with different commercial colony densities are required to fully assess spill-back risk. However, seasonal management, to minimise spill-back opportunities, and treatment of commercial colonies to prevent infection, could be implemented to manage the potential risks of parasite spill-back to wild bees. Implications for insect conservation Our results show that commercial bumblebee populations do pick up infections, most likely from wild bees, and that these infections can reach prevalences where they may pose a threat to wild bees via parasite spill-back. More research is required to clarify the extent of this potential threat.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10841-021-00322-x.

More is less: mass-flowering fruit tree crops dilute parasite transmission between bees

Parasites influence wild bee population dynamics and are regarded as one of the main drivers of wild bee decline. Most of these parasites are mainly transmitted between bee species via the use of shared floral resources. Disturbance of the plant-pollinator network at a location can hence disturb the transmission of these parasites. Expansion and intensification of agriculture, another major driver of wild bee decline, often disturbs local plant-pollinator networks by altering the availability and diversity of floral resources. Mass-flowering crops are an extreme example as they provide an abundance of floral resources for a short period of time, substantially altering the present plant-pollinator network. This likely has repercussions on parasite transmission in the pollinator community. Using the bloom of mass-flowering crops we tested the hypothesis that an increase in floral resources can dilute parasite transmission in the pollinator community. To test this, we analysed the presence of parasites in the pollen of the brood cell provisions of Osmia spp., collected from trap nests placed in apple and sweet cherry orchards. We collected pollen at several time intervals during and after mass bloom, and found that pollen collected during mass bloom had significantly lower parasite prevalence compared with pollen collected after mass bloom. Furthermore, using pollen barcoding data we found that the presence of MFCs in pollen was a good predictor for lower parasite prevalence. Taken together, our results indicate that an increase in flower availability can reduce parasite transmission between bees.

Pollinator parasites and the evolution of floral traits

The main selective force driving floral evolution and diversity is plant-pollinator interactions. Pollinators use floral signals and indirect cues to assess flower reward, and the ensuing flower choice has major implications for plant fitness. While many pollinator behaviors have been described, the impact of parasites on pollinator foraging decisions and plant-pollinator interactions have been largely overlooked. Growing evidence of the transmission of parasites through the shared-use of flowers by pollinators demonstrate the importance of behavioral immunity (altered behaviors that enhance parasite resistance) to pollinator health. During foraging bouts, pollinators can protect themselves against parasites through self-medication, disease avoidance, and grooming. Recent studies have documented immune behaviors in foraging pollinators, as well as the impacts of such behaviors on flower visitation. Because pollinator parasites can affect flower choice and pollen dispersal, they may ultimately impact flower fitness. Here, we discuss how pollinator immune behaviors and floral traits may affect the presence and transmission of pollinator parasites, as well as how pollinator parasites, through these immune behaviors, can impact plant-pollinator interactions. We further discuss how pollinator immune behaviors can impact plant fitness, and how floral traits may adapt to optimize plant fitness in response to pollinator parasites. We propose future research directions to assess the role of pollinator parasites in plant-pollinator interactions and evolution, and we propose better integration of the role of pollinator parasites into research related to pollinator optimal foraging theory, floral diversity and agricultural practices.

Microbiome Structure Influences Infection by the Parasite Crithidia bombi in Bumble Bees

Recent declines in bumble bee populations are of great concern and have prompted critical evaluations of the role of pathogen introductions and host resistance in bee health. One factor that may influence host resilience when facing infection is the gut microbiota. Previous experiments with Bombus terrestris, a European bumble bee, showed that the gut microbiota can protect against Crithidia bombi, a widespread trypanosomatid parasite of bumble bees. However, the particular characteristics of the microbiome responsible for this protective effect have thus far eluded identification. Using wild and commercially sourced Bombus impatiens, an important North American pollinator, we conducted cross-wise microbiota transplants to naive hosts of both backgrounds and challenged them with a Crithidia parasite. As with B. terrestris, we find that microbiota-dependent protection against Crithidia operates in B. impatiens Lower Crithidia infection loads were experimentally associated with high microbiome diversity, large gut bacterial populations, and the presence of Apibacter, Lactobacillus Firm-5, and Gilliamella spp. in the gut community. These results indicate that even subtle differences between gut community structures can have a significant impact on a microbiome's ability to defend against parasite infections.IMPORTANCE Many wild bumble bee populations are under threat due to human activity, including through the introduction of pathogens via commercially raised bees. Recently, it was found that the bumble bee gut microbiota can help defend against a common parasite, Crithidia bombi, but the particular factors contributing to this protection are unknown. Using both wild and commercially raised bees, we conducted microbiota transplants to show that microbiome diversity, total gut bacterial load, and the presence of certain core members of the microbiota may all impact bee susceptibility to Crithidia infection. Bee origin (genetic background) was also a factor. Finally, by examining this phenomenon in a previously uninvestigated bee species, our study demonstrates that microbiome-mediated resistance to Crithidia is conserved across multiple bumble bee species. These findings highlight how intricate interactions between hosts, microbiomes, and parasites can have wide-ranging consequences for the health of ecologically important species.

Pollen extracts and constituent sugars increase growth of a trypanosomatid parasite of bumble bees

Phytochemicals produced by plants, including at flowers, function in protection against plant diseases, and have a long history of use against trypanosomatid infection. Floral nectar and pollen, the sole food sources for many species of insect pollinators, contain phytochemicals that have been shown to reduce trypanosomatid infection in bumble and honey bees when fed as isolated compounds. Nectar and pollen, however, consist of phytochemical mixtures, which can have greater antimicrobial activity than do single compounds. This study tested the hypothesis that pollen extracts would inhibit parasite growth. Extracts of six different pollens were tested for direct inhibitory activity against cell cultures of the bumble bee trypanosomatid gut parasite Crithidia bombi. Surprisingly, pollen extracts increased parasite growth rather than inhibiting it. Pollen extracts contained high concentrations of sugars, mainly the monosaccharides glucose and fructose. Experimental manipulations of growth media showed that supplemental monosaccharides (glucose and fructose) increased maximum cell density, while a common floral phytochemical (caffeic acid) with inhibitory activity against other trypanosomatids had only weak inhibitory effects on Crithidia bombi. These results indicate that, although pollen is essential for bees and other pollinators, pollen may promote growth of intestinal parasites that are uninhibited by pollen phytochemicals and, as a result, can benefit from the nutrients that pollen provides.

Sexual healing: mating induces a protective immune response in bumblebees

The prevalence of sexual, as opposed to clonal, reproduction given the many costs associated with sexual recombination has been an enduring question in evolutionary biology. In addition to these often discussed costs, there are further costs associated with mating, including the induction of a costly immune response, which leaves individuals prone to infection. Here, we test whether mating results in immune activation and susceptibility to a common, ecologically important, parasite of bumblebees. We find that mating does result in immune activation as measured by gene expression of known immune genes, but that this activation improves resistance to this parasite. We conclude that although mating can corrupt immunity in some systems, it can also enhance immunity in others.

Early gut colonizers shape parasite susceptibility and microbiota composition in honey bee workers

Microbial symbionts living within animal guts are largely composed of resident bacterial species, forming communities that often provide benefits to the host. Gut microbiomes of adult honey bees (Apis mellifera) include core residents such as the betaproteobacterium Snodgrassella alvi, alongside transient parasites such as the protozoan Lotmaria passim To test how these species affect microbiome composition and host physiology, we administered S alvi and/or L passim inocula to newly emerged worker bees from four genetic backgrounds (GH) and reared them in normal (within hives) or stressed (protein-deficient, asocial) conditions. Microbiota acquired by normal bees were abundant but quantitatively differed across treatments, indicating treatment-associated dysbiosis. Pretreatment with S. alvi made normal bees more susceptible to L. passim and altered developmental and detoxification gene expression. Stressed bees were more susceptible to L. passim and were depauperate in core microbiota, yet supplementation with S. alvi did not alter this susceptibility. Microbiomes were generally more variable by GH in stressed bees, which also showed opposing and comparatively reduced modulation of gene expression responses to treatments compared with normal bees. These data provide experimental support for a link between altered gut microbiota and increased parasite and pathogen prevalence, as observed from honey bee colony collapse disorder.

Genome of Leptomonas pyrrhocoris: a high-quality reference for monoxenous trypanosomatids and new insights into evolution of Leishmania

Many high-quality genomes are available for dixenous (two hosts) trypanosomatid species of the genera Trypanosoma, Leishmania, and Phytomonas, but only fragmentary information is available for monoxenous (single-host) trypanosomatids. In trypanosomatids, monoxeny is ancestral to dixeny, thus it is anticipated that the genome sequences of the key monoxenous parasites will be instrumental for both understanding the origin of parasitism and the evolution of dixeny. Here, we present a high-quality genome for Leptomonas pyrrhocoris, which is closely related to the dixenous genus Leishmania. The L. pyrrhocoris genome (30.4 Mbp in 60 scaffolds) encodes 10,148 genes. Using the L. pyrrhocoris genome, we pinpointed genes gained in Leishmania. Among those genes, 20 genes with unknown function had expression patterns in the Leishmania mexicana life cycle suggesting their involvement in virulence. By combining differential expression data for L. mexicana, L. major and Leptomonas seymouri, we have identified several additional proteins potentially involved in virulence, including SpoU methylase and U3 small nucleolar ribonucleoprotein IMP3. The population genetics of L. pyrrhocoris was also addressed by sequencing thirteen strains of different geographic origin, allowing the identification of 1,318 genes under positive selection. This set of genes was significantly enriched in components of the cytoskeleton and the flagellum.

Possible Synergistic Effects of Thymol and Nicotine Against Crithidia bombi Parasitism in Bumble Bees

Floral nectar contains secondary compounds with antimicrobial properties that can affect not only plant-pollinator interactions, but also interactions between pollinators and their parasites. Although recent work has shown that consumption of plant secondary compounds can reduce pollinator parasite loads, little is known about the effects of dosage or compound combinations. We used the generalist pollinator Bombus impatiens and its obligate gut parasite Crithidia bombi to study the effects of nectar chemistry on host-parasite interactions. In two experiments we tested (1) whether the secondary compounds thymol and nicotine act synergistically to reduce parasitism, and (2) whether dietary thymol concentration affects parasite resistance. In both experiments, uninfected Bombus impatiens were inoculated with Crithidia and then fed particular diet treatments for 7 days, after which infection levels were assessed. In the synergism experiment, thymol and nicotine alone and in combination did not significantly affect parasite load or host mortality. However, the thymol-nicotine combination treatment reduced log-transformed parasite counts by 30% relative to the control group (P = 0.08). For the experiment in which we manipulated thymol concentration, we found no significant effect of any thymol concentration on Crithidia load, but moderate (2 ppm) thymol concentrations incurred a near-significant increase in mortality (P = 0.054). Our results tentatively suggest the value of a mixed diet for host immunity, yet contrast with research on the antimicrobial activity of dietary thymol and nicotine in vertebrate and other invertebrate systems. We suggest that future research evaluate genetic variation in Crithidia virulence, multi-strain competition, and Crithidia interactions with the gut microbe community that may mediate antimicrobial activities of secondary compounds.

Testing Dose-Dependent Effects of the Nectar Alkaloid Anabasine on Trypanosome Parasite Loads in Adult Bumble Bees

The impact of consuming biologically active compounds is often dose-dependent, where small quantities can be medicinal while larger doses are toxic. The consumption of plant secondary compounds can be toxic to herbivores in large doses, but can also improve survival in parasitized herbivores. In addition, recent studies have found that consuming nectar secondary compounds may decrease parasite loads in pollinators. However, the effect of compound dose on bee survival and parasite loads has not been assessed. To determine how secondary compound consumption affects survival and pathogen load in Bombus impatiens, we manipulated the presence of a common gut parasite, Crithidia bombi, and dietary concentration of anabasine, a nectar alkaloid produced by Nicotiana spp. using four concentrations naturally observed in floral nectar. We hypothesized that increased consumption of secondary compounds at concentrations found in nature would decrease survival of uninfected bees, but improve survival and ameliorate parasite loads in infected bees. We found medicinal effects of anabasine in infected bees; the high-anabasine diet decreased parasite loads and increased the probability of clearing the infection entirely. However, survival time was not affected by any level of anabasine concentration, or by interactive effects of anabasine concentration and infection. Crithidia infection reduced survival time by more than two days, but this effect was not significant. Our results support a medicinal role for anabasine at the highest concentration; moreover, we found no evidence for a survival-related cost of anabasine consumption across the concentration range found in nectar. Our results suggest that consuming anabasine at the higher levels of the natural range could reduce or clear pathogen loads without incurring costs for healthy bees.

Sex, horizontal transmission, and multiple hosts prevent local adaptation of Crithidia bombi, a parasite of bumblebees (Bombus spp.)

Local adaptation within host-parasite systems can evolve by several non-exclusive drivers (e.g., host species-genetic adaptation; ecological conditions-ecological adaptation, and time-temporal adaptation). Social insects, especially bumblebees, with an annual colony life history not only provide an ideal system to test parasite transmission within and between different host colonies, but also parasite adaptation to specific host species and environments. Here, we study local adaptation in a multiple-host parasite characterized by high levels of horizontal transmission. Crithidia bombi occurs as a gut parasite in several bumblebee species. Parasites were sampled from five different host species in two subsequent years. Population genetic tools were used to test for the several types of adaptation. Although we found no evidence for local adaptation of the parasite toward host species, there was a slight temporal differentiation of the parasite populations, which might have resulted from severe bottlenecks during queen hibernation. Parasite populations were in Hardy-Weinberg equilibrium and showed no signs of linkage disequilibrium suggesting that sexual reproduction is an alternative strategy in this otherwise clonal parasite. Moreover, high levels of multiple infections were found, which might facilitate sexual genetic exchange. The detection of identical clones in different host species suggested that horizontal transmission occurs between host species and underpins the lack of host-specific adaptation.