Differential Expression of Immune Defences Is Associated with Specific Host-Parasite Interactions in Insects

Recent ecological studies in invertebrates show that the outcome of an infection is dependent on the specific pairing of host and parasite. Such specificity contrasts the long-held view that invertebrate innate immunity depends on a broad-spectrum recognition system. An important question is whether this specificity is due to the immune response rather than some other interplay between host and parasite genotypes. By measuring the expression of putative bumblebee homologues of antimicrobial peptides in response to infection by their gut trypanosome Crithidia bombi, we demonstrate that expression differences are associated with the specific interactions.     

Temperature dependence of parasitic infection and gut bacterial communities in bumble bees

High temperatures (e.g., fever) and gut microbiota can both influence host resistance to infection. However, effects of temperature-driven changes in gut microbiota on resistance to parasites remain unexplored. We examined the temperature dependence of infection and gut bacterial communities in bumble bees infected with the trypanosomatid parasite Crithidia bombi. Infection intensity decreased by over 80% between 21 and 37°C. Temperatures of peak infection were lower than predicted based on parasite growth in vitro, consistent with mismatches in thermal performance curves of hosts, parasites and gut symbionts. Gut bacterial community size and composition exhibited slight but significant, non-linear, and taxon-specific responses to temperature. Abundance of total gut bacteria and of Orbaceae, both negatively correlated with infection in previous studies, were positively correlated with infection here. Prevalence of the bee pathogen-containing family Enterobacteriaceae declined with temperature, suggesting that high temperature may confer protection against diverse gut pathogens. Our results indicate that resistance to infection reflects not only the temperature dependence of host and parasite performance, but also temperature-dependent activity of gut bacteria. The thermal ecology of gut parasite-symbiont interactions may be broadly relevant to infectious disease, both in ectothermic organisms that inhabit changing climates, and in endotherms that exhibit fever-based immunity.     

Diet and phylogeny shape the gut microbiota of Antarctic seals: a comparison of wild and captive animals

The gut microbiota of mammals underpins the metabolic capacity and health of the host. Our understanding of what influences the composition of this community has been limited primarily to evidence from captive and terrestrial mammals. Therefore, the gut microbiota of southern elephant seals, Mirounga leonina, and leopard seals, Hydrurga leptonyx, inhabiting Antarctica were compared with captive leopard seals. Each seal exhibited a gut microbiota dominated by four phyla: Firmicutes (41.5 ± 4.0%), Fusobacteria (25.6 ± 3.9%), Proteobacteria (17.0 ± 3.2%) and Bacteroidetes (14.1 ± 1.7%). Species, age, sex and captivity were strong drivers of the composition of the gut microbiota, which can be attributed to differences in diet, gut length and physiology and social interactions. Differences in particular prey items consumed by seal species could contribute to the observed differences in the gut microbiota. The longer gut of the southern elephant seal provides a habitat reduced in available oxygen and more suitable to members of the phyla Bacteroidetes compared with other hosts. Among wild seals, 16 ‘core’ bacterial community members were present in the gut of at least 50% of individuals. As identified between southern elephant seal mother–pup pairs, ‘core’ members are passed on via vertical transmission from a young age and persist through to adulthood. Our study suggests that these hosts have co‐evolved with their gut microbiota and core members may provide some benefit to the host, such as developing the immune system. Further evidence of their strong evolutionary history is provided with the presence of 18 shared ‘core’ members in the gut microbiota of related seals living in the Arctic. The influence of diet and other factors, particularly in captivity, influences the composition of the community considerably. This study suggests that the gut microbiota has co‐evolved with wild mammals as is evident in the shared presence of ‘core’ members.     

An interaction between host and microbe genotypes determines colonization success of a key bumble bee gut microbiota member

There has been a proliferation of studies demonstrating an organism's health is influenced by its microbiota. However, factors influencing beneficial microbe colonization and the evolution of these relationships remain understudied relative to host–pathogen interactions. Vertically transmitted beneficial microbes are predicted to show high levels of specificity in colonization, including genotype matching, which may transpire through coevolution. We investigate how host and bacterial genotypes influence colonization of a core coevolved microbiota member in bumble bees. The hindgut colonizing Snodgrassella alvi confers direct benefits, but, as an early colonizer, also facilitates the further development of a healthy microbiota. Due to predominantly vertical transmission promoting tight evolution between colonization factors of bacteria and host lineages, we predict that genotype‐by‐genotype interactions will determine successful colonization. Germ‐free adult bees from seven bumble bee colonies (host genotypic units) were inoculated with one of six genetically distinct strains of S. alvi. Subsequent colonization within host and microbe genotypes combinations ranged from 0 to 100%, and an interaction between host and microbe genotypes determined colonization success. This novel finding of a genotype‐by‐genotype interaction determining colonization in an animal host‐beneficial microbe system has implications for the ecological and evolutionary dynamics of host and microbe, including associated host‐fitness benefits.     

Probing Mixed-Genotype Infections II: High Multiplicity in Natural Infections of the Trypanosomatid,Crithidia bombi,in Its Host,Bombus spp

Mixed-genotype infections have major consequences for many essential elements of host-parasite interactions. With genetic exchange between co-infecting parasite genotypes increased diversity among parasite offspring and the emergence of novel genotypes from infected hosts is possible. We here investigated mixed- genotype infections using the host, Bombus spp. and its trypanosome parasite Crithidia bombi as our study case. The natural infections of C. bombi were genotyped with a novel method for a representative sample of workers and spring queens in Switzerland. We found that around 60% of all infected hosts showed mixed-genotype infections with an average of 2.47±0.22 (S.E.) and 3.65±1.02 genotypes per worker or queen, respectively. Queens, however, harboured up to 29 different genotypes. Based on the genotypes of co-infecting strains, these could be putatively assigned to either ‘primary’ and ‘derived’ genotypes - the latter resulting from genetic exchange among the primary genotypes. High genetic relatedness among co-infecting derived but not primary genotypes supported this scenario. Co-infection in queens seems to be a major driver for the diversity of genotypes circulating in host populations.     

Gut microbial diversity across a contact zone for California voles: Implications for lineage divergence of hosts and mitonuclear mismatch in the assembly of the mammalian gut microbiome

Gut microbial diversity is thought to reflect the co‐evolution of microbes and their hosts as well as current host‐specific attributes such as genetic background and environmental setting. To explore interactions among these parameters, we characterized variation in gut microbiome composition of California voles (Microtus californicus) across a contact zone between two recently diverged lineages of this species. Because this contact zone contains individuals with mismatched mitochondrial‐nuclear genomes (cybrids), it provides an important opportunity to explore how different components of the genotype contribute to gut microbial diversity. Analyses of bacterial 16S rRNA sequences and joint species distribution modelling revealed that host genotypes and genetic differentiation among host populations together explained more than 50% of microbial community variation across our sampling transect. The ranked importance (most to least) of factors contributing to gut microbial diversity in our study populations were: genome‐wide population differentiation, local environmental conditions, and host genotypes. However, differences in microbial communities among vole populations (β‐diversity) did not follow patterns of lineage divergence (i.e., phylosymbiosis). Instead, among‐population variation was best explained by the spatial distribution of hosts, as expected if the environment is a primary source of gut microbial diversity (i.e., dispersal limitation hypothesis). Across the contact zone, several bacterial taxa differed in relative abundance between the two parental lineages as well as among individuals with mismatched mitochondrial and nuclear genomes. Thus, genetic divergence among host lineages and mitonuclear genomic mismatches may also contribute to microbial diversity by altering interactions between host genomes and gut microbiota (i.e., hologenome speciation hypothesis).     

Host ecology determines the relative fitness of virus genotypes in mixed-genotype nucleopolyhedrovirus infections

Mixed‐genotype infections are common in many natural host–parasite interactions. Classical kin‐selection models predict that single‐genotype infections can exploit host resources prudently to maximize fitness, but that selection favours rapid exploitation when co‐infecting genotypes share limited host resources. However, theory has outpaced evidence: we require empirical studies of pathogen genotypes that naturally co‐infect hosts. Do genotypes actually compete within hosts? Can host ecology affect the outcome of co‐infection? We posed both questions by comparing traits of infections in which two baculovirus genotypes were fed to hosts alongside inocula of the same or a different genotype. The host, Panolis flammea, is a herbivore of Pinus sylvestris and Pi. contorta. The pathogen, PfNPV (a nucleopolyhedrovirus), occurs naturally as mixtures of genotypes that differ, when isolated, in pathogenicity, speed of kill and yield. Single‐genotype infection traits failed to predict the ‘winning’ genotypes in co‐infections. Co‐infections infected and caused lethal disease in more hosts, and produced high yields, relative to single‐genotype infections. The need to share with nonkin did not cause fitness costs to either genotype. In fact, in hosts feeding on Pi. sylvestris, one genotype gained increased yields in mixed‐genotype infections. These results are discussed in relation to theory surrounding adaptive responses to competition with nonkin for limited resources.     

New insights into the dynamics between reef corals and their associated dinoflagellate endosymbionts from population genetic studies

The mutualistic symbioses between reef‐building corals and micro‐algae form the basis of coral reef ecosystems, yet recent environmental changes threaten their survival. Diversity in host‐symbiont pairings on the sub‐species level could be an unrecognized source of functional variation in response to stress. The Caribbean elkhorn coral, Acropora palmata, associates predominantly with one symbiont species (Symbiodinium ‘fitti’), facilitating investigations of individual‐level (genotype) interactions. Individual genotypes of both host and symbiont were resolved across the entire species’ range. Most colonies of a particular animal genotype were dominated by one symbiont genotype (or strain) that may persist in the host for decades or more. While Symbiodinium are primarily clonal, the occurrence of recombinant genotypes indicates sexual recombination is the source of this genetic variation, and some evidence suggests this happens within the host. When these data are examined at spatial scales spanning the entire distribution of A. palmata, gene flow among animal populations was an order of magnitude greater than among populations of the symbiont. This suggests that independent micro‐evolutionary processes created dissimilar population genetic structures between host and symbiont. The lower effective dispersal exhibited by the dinoflagellate raises questions regarding the extent to which populations of host and symbiont can co‐evolve during times of rapid and substantial climate change. However, these findings also support a growing body of evidence, suggesting that genotype‐by‐genotype interactions may provide significant physiological variation, influencing the adaptive potential of symbiotic reef corals to severe selection.     

Impact of climate change on parasite infection of an important pollinator depends on host genotypes

Climate change is predicted to affect host–parasite interactions, and for some hosts, parasite infection is expected to increase with rising temperatures. Global population declines of important pollinators already have been attributed to climate change and parasitism. However, the role of climate in driving parasite infection and the genetic basis for pollinator hosts to respond often remain obscure. Based on decade-long field data, we investigated the association between climate and Nosema bombi (Microsporidia) infection of buffed-tailed bumblebees (Bombus terrestris), and whether host genotypes play a role. For this, we genotyped 876 wild bumblebee queens and screened for N. bombi infection of those queens between 2000 and 2010. We recorded seven climate parameters during those 11 years and tested for correlations between climate and infection prevalence. Here we show that climatic factors drive N. bombi infection and that the impact of climate depends on mitochondrial DNA cytochrome oxidase I (COI) haplotypes of the host. Infection prevalence was correlated with climatic variables during the time when queens emerge from hibernation. Remarkably, COI haplotypes best predict this association between climatic factors and infection. In particular, two host haplotypes (“A” and “B”) displayed phenotypic plasticity in response to climatic variation: Temperature was positively correlated with infection of host haplotype B, but not haplotype A. The likelihood of infection of haplotype A was associated with moisture, conferring greater resistance to parasite infection during wetter years. In contrast, infection of haplotype B was unrelated to moisture. To the best of our knowledge, this is the first study that identifies specific host genotypes that confer differential parasite resistance under variable climatic conditions. Our results underscore the importance of mitochondrial haplotypes to ward off parasites in a changing climate. More broadly, this also suggests that COI may play a pertinent role in climate change adaptations of insect pollinators.     

Host genetics and diet,but not immunoglobulin A expression,converge to shape compositional features of the gut microbiome in an advanced intercross population of mice

Background

Individuality in the species composition of the vertebrate gut microbiota is driven by a combination of host and environmental factors that have largely been studied independently. We studied the convergence of these factors in a G₁₀ mouse population generated from a cross between two strains to search for quantitative trait loci (QTLs) that affect gut microbiota composition or ileal Immunoglobulin A (IgA) expression in mice fed normal or high-fat diets.

Results

We found 42 microbiota-specific QTLs in 27 different genomic regions that affect the relative abundances of 39 taxa, including four QTL that were shared between this G₁₀ population and the population previously studied at G₄. Several of the G₁₀ QTLs show apparent pleiotropy. Eight of these QTLs, including four at the same site on chromosome 9, show significant interaction with diet, implying that diet can modify the effects of some host loci on gut microbiome composition. Utilization patterns of IghV variable regions among IgA-specific mRNAs from ileal tissue are affected by 54 significant QTLs, most of which map to a segment of chromosome 12 spanning the Igh locus. Despite the effect of genetic variation on IghV utilization, we are unable to detect overlapping microbiota and IgA QTLs and there is no significant correlation between IgA variable pattern utilization and the abundance of any of the taxa from the fecal microbiota.

Conclusions

We conclude that host genetics and diet can converge to shape the gut microbiota, but host genetic effects are not manifested through differences in IgA production.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-014-0552-6) contains supplementary material, which is available to authorized users.     

Quantifying the coevolutionary potential of multistep immune defenses

Coevolutionary models often assume host infection by parasites depends on a single bout of molecular recognition. As detailed immunological studies accumulate, however, it becomes increasingly apparent that the outcome of host–parasite interactions more generally depends on complex multiple step infection processes. For example, in plant and animal innate immunity, recognition steps are followed by downstream effector steps that kill recognized parasites, with the outcome depending on an escalatory molecular arms race. Here, we explore the consequences of such multistep infection processes for coevolution using a genetically explicit model. Model analyses reveal that polymorphism is much greater at recognition loci than effector loci, that host–genotype by parasite–genotype (G_h × G_p) interactions are larger for the recognition step, and that the recognition step contributes more to local adaptation than the effector step. These results suggest that (1) local adaptation is more likely when fitness measures are related to recognition versus downstream effectors, (2) effector loci, while mechanistically important, are less likely to harbor the G_h × G_p variation that fuels coevolution, and (3) recognition loci are better candidates for genomic hotspots of coevolution.     

THE CELLULAR IMMUNE RESPONSE OF DAPHNIA MAGNA UNDER HOST–PARASITE GENETIC VARIATION AND VARIATION IN INITIAL DOSE

In invertebrate–parasite systems, the likelihood of infection following parasite exposure is often dependent on the specific combination of host and parasite genotypes (termed genetic specificity). Genetic specificity can maintain diversity in host and parasite populations and is a major component of the Red Queen hypothesis. However, invertebrate immune systems are thought to only distinguish between broad classes of parasite. Using a natural host–parasite system with a well‐established pattern of genetic specificity, the crustacean Daphnia magna and its bacterial parasite Pasteuria ramosa, we found that only hosts from susceptible host–parasite genetic combinations mounted a cellular response following exposure to the parasite. These data are compatible with the hypothesis that genetic specificity is attributable to barrier defenses at the site of infection (the gut), and that the systemic immune response is general, reporting the number of parasite spores entering the hemocoel. Further supporting this, we found that larger cellular responses occurred at higher initial parasite doses. By studying the natural infection route, where parasites must pass barrier defenses before interacting with systemic immune responses, these data shed light on which components of invertebrate defense underlie genetic specificity.     

Major Histocompatibility Complex class IIb polymorphism influences gut microbiota composition and diversity

Animals harbour diverse communities of symbiotic bacteria, which differ dramatically among host individuals. This heterogeneity poses an immunological challenge: distinguishing between mutualistic and pathogenic members of diverse and host‐specific microbial communities. We propose that Major Histocompatibility class II (MHC) genotypes contribute to recognition and regulation of gut microbes, and thus, MHC polymorphism contributes to microbial variation among hosts. Here, we show that MHC IIb polymorphism is associated with among‐individual variation in gut microbiota within a single wild vertebrate population of a small fish, the threespine stickleback. We sampled stickleback from Cedar Lake, on Vancouver Island, and used next‐generation sequencing to genotype the sticklebacks’ gut microbiota (16S sequencing) and their MHC class IIb exon 2 sequences. The presence of certain MHC motifs was associated with altered relative abundance (increase or decrease) of some microbial Families. The effect sizes are modest and entail a minority of microbial taxa, but these results represent the first indication that MHC genotype may affect gut microbiota composition in natural populations (MHC‐microbe associations have also been found in a few studies of lab mice). Surprisingly, these MHC effects were frequently sex‐dependent. Finally, hosts with more diverse MHC motifs had less diverse gut microbiota. One implication is that MHC might influence the efficacy of therapeutic strategies to treat dysbiosis‐associated disease, including the outcome of microbial transplants between healthy and diseased patients. We also speculate that macroparasite‐driven selection on MHC has the potential to indirectly alter the host gut microbiota, and vice versa.     

Chemical modulators of the innate immune response alter gypsy moth larval susceptibility to <Emphasis Type="Italic">Bacillus thuringiensis</Emphasis>

Background  

The gut comprises an essential barrier that protects both invertebrate and vertebrate animals from invasion by microorganisms. Disruption of the balanced relationship between indigenous gut microbiota and their host can result in gut bacteria eliciting host responses similar to those caused by invasive pathogens. For example, ingestion of Bacillus thuringiensis by larvae of some species of susceptible Lepidoptera can result in normally benign enteric bacteria exerting pathogenic effects.     

Complex genotype interactions influence social fitness during the developmental phase of the social amoeba Dictyostelium discoideum

When individuals interact, phenotypic variation can be partitioned into direct genetic effects (DGEs) of the individuals’ own genotypes, indirect genetic effects (IGEs) of their social partners’ genotypes and epistatic interactions between the genotypes of interacting individuals (‘genotype‐by‐genotype (G×G) epistasis’). These components can all play important roles in evolutionary processes, but few empirical studies have examined their importance. The social amoeba Dictyostelium discoideum provides an ideal system to measure these effects during social interactions and development. When starved, free‐living amoebae aggregate and differentiate into a multicellular fruiting body with a dead stalk that holds aloft viable spores. By measuring interactions among a set of natural strains, we quantify DGEs, IGEs and G×G epistasis affecting spore formation. We find that DGEs explain most of the phenotypic variance (57.6%) whereas IGEs explain a smaller (13.3%) but highly significant component. Interestingly, G×G epistasis explains nearly a quarter of the variance (23.0%), highlighting the complex nature of genotype interactions. These results demonstrate the large impact that social interactions can have on development and suggest that social effects should play an important role in developmental evolution in this system.     

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.     

The potential for arms race and Red Queen coevolution in a protist host–parasite system

The dynamics and consequences of host–parasite coevolution depend on the nature of host genotype‐by‐parasite genotype interactions (G × G) for host and parasite fitness. G × G with crossing reaction norms can yield cyclic dynamics of allele frequencies (“Red Queen” dynamics) while G × G where the variance among host genotypes differs between parasite genotypes results in selective sweeps (“arms race” dynamics). Here, we investigate the relative potential for arms race and Red Queen coevolution in a protist host–parasite system, the dinoflagellate Alexandrium minutum and its parasite Parvilucifera sinerae. We challenged nine different clones of A. minutum with 10 clones of P. sinerae in a fully factorial design and measured infection success and host and parasite fitness. Each host genotype was successfully infected by four to ten of the parasite genotypes. There were strong G × Gs for infection success, as well as both host and parasite fitness. About three quarters of the G × G variance components for host and parasite fitness were due to crossing reaction norms. There were no general costs of resistance or infectivity. We conclude that there is high potential for Red Queen dynamics in this host–parasite system.     

Protein-poor diet reduces host-specific immune gene expression in Bombus terrestris

Parasites infect hosts non-randomly as genotypes of hosts vary in susceptibility to the same genotypes of parasites, but this specificity may be modulated by environmental factors such as nutrition. Nutrition plays an important role for any physiological investment. As immune responses are costly, resource limitation should negatively affect immunity through trade-offs with other physiological requirements. Consequently, nutritional limitation should diminish immune capacity in general, but does it also dampen differences among hosts? We investigated the effect of short-term pollen deprivation on the immune responses of our model host Bombus terrestris when infected with the highly prevalent natural parasite Crithidia bombi. Bumblebees deprived of pollen, their protein source, show reduced immune responses to infection. They failed to upregulate a number of genes, including antimicrobial peptides, in response to infection. In particular, they also showed less specific immune expression patterns across individuals and colonies. These findings provide evidence for how immune responses on the individual-level vary with important elements of the environment and illustrate how nutrition can functionally alter not only general resistance, but also alter the pattern of specific host–parasite interactions.     

New insights into the interactions between Blastocystis,the gut microbiota,and host immunity

The human gut microbiota is a diverse and complex ecosystem that is involved in beneficial physiological functions as well as disease pathogenesis. Blastocystis is a common protistan parasite and is increasingly recognized as an important component of the gut microbiota. The correlations between Blastocystis and other communities of intestinal microbiota have been investigated, and, to a lesser extent, the role of this parasite in maintaining the host immunological homeostasis. Despite recent studies suggesting that Blastocystis decreases the abundance of beneficial bacteria, most reports indicate that Blastocystis is a common component of the healthy gut microbiome. This review covers recent finding on the potential interactions between Blastocystis and the gut microbiota communities and its roles in regulating host immune responses.     

Genetic variation for maternal effects on parasite susceptibility

The expression of infectious disease is increasingly recognized to be impacted by maternal effects, where the environmental conditions experienced by mothers alter resistance to infection in offspring, independent of heritability. Here, we studied how maternal effects (high or low food availability to mothers) mediated the resistance of the crustacean Daphnia magna to its bacterial parasite Pasteuria ramosa. We sought to disentangle maternal effects from the effects of host genetic background by studying how maternal effects varied across 24 host genotypes sampled from a natural population. Under low‐food conditions, females produced offspring that were relatively resistant, but this maternal effect varied strikingly between host genotypes, i.e. there were genotype by maternal environment interactions. As infection with P. ramosa causes a substantial reduction in host fecundity, this maternal effect had a large effect on host fitness. Maternal effects were also shown to impact parasite fitness, both because they prevented the establishment of the parasites and because even when parasites did establish in the offspring of poorly fed mothers, and they tended to grow more slowly. These effects indicate that food stress in the maternal generation can greatly influence parasite susceptibility and thus perhaps the evolution and coevolution of host–parasite interactions.