The evolution and ecology of multiple antipredator defences.

Prey seldom rely on a single type of antipredator defence, often using multiple defences to avoid predation. In many cases, selection in different contexts may favour the evolution of multiple defences in a prey. However, a prey may use multiple defences to protect itself during a single predator encounter. Such "defence portfolios" that defend prey against a single instance of predation are distributed across and within successive stages of the predation sequence (encounter, detection, identification, approach (attack), subjugation and consumption). We contend that at present, our understanding of defence portfolio evolution is incomplete, and seen from the fragmentary perspective of specific sensory systems (e.g., visual) or specific types of defences (especially aposematism). In this review, we aim to build a comprehensive framework for conceptualizing the evolution of multiple prey defences, beginning with hypotheses for the evolution of multiple defences in general, and defence portfolios in particular. We then examine idealized models of resource trade-offs and functional interactions between traits, along with evidence supporting them. We find that defence portfolios are constrained by resource allocation to other aspects of life history, as well as functional incompatibilities between different defences. We also find that selection is likely to favour combinations of defences that have synergistic effects on predator behaviour and prey survival. Next, we examine specific aspects of prey ecology, genetics and development, and predator cognition that modify the predictions of current hypotheses or introduce competing hypotheses. We outline schema for gathering data on the distribution of prey defences across species and geography, determining how multiple defences are produced, and testing the proximate mechanisms by which multiple prey defences impact predator behaviour. Adopting these approaches will strengthen our understanding of multiple defensive strategies.

Early-life immune expression profiles predict later-life health and fitness in a wild rodent.

Individuals differ in the nature of the immune responses they produce, affecting disease susceptibility and ultimately health and fitness. These differences have been hypothesized to have an origin in events experienced early in life that then affect trajectories of immune development and responsiveness. Here, we investigate how early-life immune expression profiles influence life history outcomes in a natural population of field voles, Microtus agrestis, in which we are able to monitor variation between and within individuals through time by repeat sampling of individually marked animals. We analysed the co-expression of 20 immune genes in early life to create a correlation network consisting of three main clusters, one of which (containing Gata3, Il10 and Il17) was associated with later-life reproductive success and susceptibility to chronic bacterial (Bartonella) infection. More detailed analyses supported associations between early-life expression of Il17 and reproductive success later in life, and of Il10 expression early in life and later infection with Bartonella. We also found significant association between an Il17 genotype and the early-life expression of Il10. Our results demonstrate that immune expression profiles can be manifested during early life with effects that persist through adulthood and that shape the variability among individuals in susceptibility to infection and fitness widely seen in natural populations.

Effects of an IgE receptor polymorphism acting on immunity, susceptibility to infection, and reproduction in a wild rodent.

The genotype of an individual is an important predictor of their immune function, and subsequently, their ability to control or avoid infection and ultimately contribute offspring to the next generation. However, the same genotype, subjected to different intrinsic and/or extrinsic environments, can also result in different phenotypic outcomes, which can be missed in controlled laboratory studies. Natural wildlife populations, which capture both genotypic and environmental variability, provide an opportunity to more fully understand the phenotypic expression of genetic variation. We identified a synonymous polymorphism in the high-affinity Immunoglobulin E (IgE) receptor (GC and non-GC haplotypes) that has sex-dependent effects on immune gene expression, susceptibility to infection, and reproductive success of individuals in a natural population of field voles (Microtus agrestis). We found that the effect of the GC haplotype on the expression of immune genes differed between sexes. Regardless of sex, both pro-inflammatory and anti-inflammatory genes were more highly relatively expressed in individuals with the GC haplotype than individuals without the haplotype. However, males with the GC haplotype showed a stronger signal for pro-inflammatory genes, while females showed a stronger signal for anti-inflammatory genes. Furthermore, we found an effect of the GC haplotype on the probability of infection with a common microparasite, Babesia microti, in females - with females carrying the GC haplotype being more likely to be infected. Finally, we found an effect of the GC haplotype on reproductive success in males - with males carrying the GC haplotype having a lower reproductive success. This is a rare example of a polymorphism whose consequences we are able to follow across immunity, infection, and reproduction for both males and females in a natural population.

Living with chronic infection: Persistent immunomodulation during avirulent haemoparasitic infection in a wild rodent.

Apicomplexans are a protozoan phylum of obligate parasites which may be highly virulent during acute infections, but may also persist as chronic infections which appear to have little fitness cost. Babesia microti is an apicomplexan haemoparasite that, in immunocompromised individuals, can cause severe, potentially fatal disease. However, in its natural host, wild field voles (Microtus agrestis), it exhibits chronic infections that have no detectable impact on survival or female fecundity. How is damage minimized, and what is the impact on the host's immune state and health? We examine the differences in immune state (here represented by expression of immune-related genes in multiple tissues) associated with several common chronic infections in a population of wild field voles. While some infections show little impact on immune state, we find strong associations between immune state and B. microti. These include indications of clearance of infected erythrocytes (increased macrophage activity in the spleen) and activity likely associated with minimizing damage from the infection (anti-inflammatory and antioxidant activity in the blood). By analysing gene expression from the same individuals at multiple time points, we show that the observed changes are a response to infection, rather than a risk factor. Our results point towards continual investment to minimize the damage caused by the infection. Thus, we shed light on how wild animals can tolerate some chronic infections, but emphasize that this tolerance does not come without a cost.

Mapping the evolution of accurate Batesian mimicry of social wasps in hoverflies.

Hoverflies (Diptera: Syrphidae) provide an excellent opportunity to study the evolution of Batesian mimicry, where defenseless prey avoid predation by evolving to resemble defended "model" species. Although some hoverflies beautifully resemble their hymenopteran models, others seem to be poor mimics or are apparently nonmimetic. The reasons for this variation are still enigmatic despite decades of research. Here, we address this issue by mapping social-wasp mimicry across the phylogeny of Holarctic hoverflies. Using the "distance transform" technique, we calculate an objective measure of the abdominal pattern similarity between 167 hoverfly species and a widespread putative model, the social wasp, Vespula germanica. We find that good wasp mimicry has evolved several times, and may have also been lost, leading to the presence of nonmimics deep within clades of good mimics. Body size was positively correlated with similarity to the model, supporting previous findings that smaller species are often poorer mimics. Additionally, univoltine species were less accurate wasp mimics than multivoltine and bivoltine species. Hence, variation in the accuracy of Batesian mimics may reflect variation in the opportunity for selection caused by differences in prey value or signal perception (influenced by body size) and phenology or generation time (influenced by voltinism).

Transcriptome-wide analysis reveals different categories of response to a standardised immune challenge in a wild rodent.

Individuals vary in their immune response and, as a result, some are more susceptible to infectious disease than others. Little is known about the nature of this individual variation in natural populations, or which components of immune pathways are most responsible, but defining this underlying landscape of variation is an essential first step to understanding the drivers of this variation and, ultimately, predicting the outcome of infection. We describe transcriptome-wide variation in response to a standardised immune challenge in wild field voles. We find that genes (hereafter 'markers') can be categorised into a limited number of types. For the majority of markers, the response of an individual is dependent on its baseline expression level, with significant enrichment in this category for conventional immune pathways. Another, moderately sized, category contains markers for which the responses of different individuals are also variable but independent of their baseline expression levels. This category lacks any enrichment for conventional immune pathways. We further identify markers which display particularly high individual variability in response, and could be used as markers of immune response in larger studies. Our work shows how a standardised challenge performed on a natural population can reveal the patterns of natural variation in immune response.

Immune state is associated with natural dietary variation in wild mice Mus musculus domesticus.

The ability, propensity and need to mount an immune response vary both among individuals and within a single individual over time.A wide array of parameters has been found to influence immune state in carefully controlled experiments, but we understand much less about which of these parameters are important in determining immune state in wild populations.Diet can influence immune responses, for example when nutrient availability is limited. We therefore predict that natural dietary variation will play a role in modulating immune state, but this has never been tested.We measured carbon and nitrogen stable isotope ratios in an island population of house mice Mus musculus domesticus as an indication of dietary variation, and the expression of a range of immune-related genes to represent immune state.After accounting for potential confounding influences such as age, sex and helminth load, we found a significant association between carbon isotope ratio and levels of immune activity in the mesenteric lymph nodes, particularly in relation to the inflammatory response.This association demonstrates the important interplay between diet and an animal's response to immune challenges, and therefore potentially its susceptibility to disease. A plain language summary is available for this article.

Geographical location influences the composition of the gut microbiota in wild house mice (Mus musculus domesticus) at a fine spatial scale.

The composition of the mammalian gut microbiota can be influenced by a multitude of environmental variables such as diet and infections. Studies investigating the effect of these variables on gut microbiota composition often sample across multiple separate populations and habitat types. In this study we explore how variation in the gut microbiota of the house mouse (Mus musculus domesticus) on the Isle of May, a small island off the east coast of Scotland, is associated with environmental and biological factors. Our study focuses on the effects of environmental variables, specifically trapping location and surrounding vegetation, as well as the host variables sex, age, body weight and endoparasite infection, on the gut microbiota composition across a fine spatial scale in a freely interbreeding population. We found that differences in gut microbiota composition were significantly associated with the trapping location of the host, even across this small spatial scale. Sex of the host showed a weak association with microbiota composition. Whilst sex and location could be identified as playing an important role in the compositional variation of the gut microbiota, 75% of the variation remains unexplained. Whereas other rodent studies have found associations between gut microbiota composition and age of the host or parasite infections, the present study could not clearly establish these associations. We conclude that fine spatial scales are important when considering gut microbiota composition and investigating differences among individuals.

Physiological, but not fitness, effects of two interacting haemoparasitic infections in a wild rodent.

In contrast to the conditions in most laboratory studies, wild animals are routinely challenged by multiple infections simultaneously, and these infections can interact in complex ways. This means that the impact of a parasite on its host's physiology and fitness cannot be fully assessed in isolation, and requires consideration of the interactions with other co-infections. Here we examine the impact of two common blood parasites in the field vole (Microtus agrestis): Babesia microti and Bartonella spp., both of which have zoonotic potential. We collected longitudinal and cross-sectional data from four populations of individually tagged wild field voles. This included data on biometrics, life history, ectoparasite counts, presence/absence of microparasites, immune markers and, for a subset of voles, more detailed physiological and immunological measurements. This allowed us to monitor infections over time and to estimate components of survival and fecundity. We confirm, as reported previously, that B. microti has a preventative effect on infection with Bartonella spp., but that the reverse is not true. We observed gross splenomegaly following B. microti infection, and an increase in IL-10 production together with some weight loss following Bartonella spp. infection. However, these animals appeared otherwise healthy and we detected no impact of infection on survival or fecundity due to the two haemoparasite taxa. This is particularly remarkable in the case of B. microti which induces apparently drastic long-term changes to spleen sizes, but without major adverse effects. Our work sheds light on the ecologies of these important zoonotic agents, and more generally on the influence that interactions among multiple parasites have on their hosts in the wild.

A candidate tolerance gene identified in a natural population of field voles (Microtus agrestis).

The animal immune response has hitherto been viewed primarily in the context of resistance only. However, individuals can also employ a tolerance strategy to maintain good health in the face of ongoing infection. To shed light on the genetic and physiological basis of tolerance, we use a natural population of field voles, Microtus agrestis, to search for an association between the expression of the transcription factor Gata3, previously identified as a marker of tolerance in this system, and polymorphism in 84 immune and nonimmune genes. Our results show clear evidence for an association between Gata3 expression and polymorphism in the Fcer1a gene, with the explanatory power of this polymorphism being comparable to that of other nongenetic variables previously identified as important predictors of Gata3 expression. We also uncover the possible mechanism behind this association using an existing protein-protein interaction network for the mouse model rodent, Mus musculus, which we validate using our own expression network for M. agrestis. Our results suggest that the polymorphism in question may be working at the transcriptional level, leading to changes in the expression of the Th2-related genes, Tyrosine-protein kinase BTK and Tyrosine-protein kinase TXK, and hence potentially altering the strength of the Th2 response, of which Gata3 is a mediator. We believe our work has implications for both treatment and control of infectious disease.

Why many Batesian mimics are inaccurate: evidence from hoverfly colour patterns.

Mimicry is considered a classic example of the elaborate adaptations that natural selection can produce, yet often similarity between Batesian (harmless) mimics and their unpalatable models is far from perfect. Variation in mimetic accuracy is a puzzle, as natural selection should favour mimics that are hardest to distinguish from their models. Numerous hypotheses exist to explain the persistence of inaccurate mimics, but most have rarely or never been tested against empirical observations from wild populations. One reason for this is the difficulty in measuring pattern similarity, a key aspect of mimicry. Here, we use a recently developed method, based on the distance transform of binary images, to quantify pattern similarity both within and among species for a group of hoverflies and their hymenopteran models. This allowed us to test three key hypotheses regarding inaccurate mimicry. Firstly, we tested the prediction that selection should be more relaxed in less accurate mimics, but found that levels of phenotypic variation are similar across most hoverfly species. Secondly, we found no evidence that mimics have to compromise between accuracy to multiple model species. However, we did find that darker-coloured hoverflies are less accurate mimics, which could lead to a trade-off between mimicry and thermoregulation in temperate regions. Our results shed light on a classic problem concerning the limitations of natural selection.