Genotype‐by‐environment interactions for cuticular hydrocarbon expression in Drosophila simulans

Genotype‐by‐environment interactions (G × Es) describe genetic variation for phenotypic plasticity. Recent interest in the role of these interactions in sexual selection has identified G × Es across a diverse range of species and sexual traits. Additionally, theoretical work predicts that G × Es in sexual traits could help to maintain genetic variation, but could also disrupt the reliability of these traits as signals of mate quality. However, empirical tests of these theoretical predictions are scarce. We reared iso‐female lines of Drosophila simulans across two axes of environmental variation (diet and temperature) in a fully factorial design and tested for G × Es in the expression of cuticular hydrocarbons (CHCs), a multivariate sexual trait in this species. We find sex‐specific environmental, genetic and G × E effects on CHC expression, with G × Es for diet in both male and female CHC profile and a G × E for temperature in females. We also find some evidence for ecological crossover in these G × Es, and by quantifying variance components, genetic correlations and heritabilities, we show the potential for these G × Es to help maintain genetic variation and cause sexual signal unreliability in D. simulans CHC profiles.     

Mate‐finding failure as an important cause of Allee effects along the leading edge of an invading insect population

The movement of humans and goods has facilitated the arrival of non‐native insects, some of which successfully establish and cause negative consequences to the composition, services, and functioning of ecosystems. The gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), is currently invading North American forests at variable rates, spreading by local and long‐distance movement in a process known as stratified dispersal. Newly arriving colonizers often occur considerably ahead of the population front, and a key question is the degree to which they successfully establish. Prior research has highlighted mate‐finding failures in sparse populations as a cause of an Allee effect (positive density dependence). We explored this mechanism by measuring the relationship between female mating success and background male moth densities along the gypsy moth western front in Northern Wisconsin (USA) over 2 years. The mating results were then compared with analogous previous studies in southern Wisconsin, and the southern front in West Virginia and Virginia (USA). Mate‐finding failures in low‐density populations were consistently observed to be density‐dependent across all years and locations. Mate‐finding failures in low‐density populations have important ramifications to invasive species management, particularly in predicting species invasiveness, preventing successful establishment by small founder populations, and concentrating eradication efforts where they are most likely to succeed.     

The evolution of sexual dimorphism and its potential impact on host–pathogen coevolution

Sex and infection are intimately linked. Many diseases are spread by sexual contact, males are thought to evolve exaggerated sexual signals to demonstrate their immune robustness, and pathogens have been shown to direct the evolution of recombination. In all of these examples, infection is influencing the evolution of male and female fitness, but less is known about how sex differences influence pathogen fitness. A defining characteristic of sexual dimorphism is not only divergent phenotypes, but also a complex genetic architecture involving changes in genetic correlations among shared fitness traits, and differences in the accumulation of mutations—all of which may affect selection on an invading pathogen. Here, we outline the implications that the genetics of sexual dimorphism can have for host–pathogen coevolution and argue that male–female differences influence more than just the environment that a pathogen experiences.     

Effects of pathogen exposure on life‐history variation in the gypsy moth (Lymantria dispar)

Investment in host defences against pathogens may lead to trade‐offs with host fecundity. When such trade‐offs arise from genetic correlations, rates of phenotypic change by natural selection may be affected. However, genetic correlations between host survival and fecundity are rarely quantified. To understand trade‐offs between immune responses to baculovirus exposure and fecundity in the gypsy moth (Lymantria dispar), we estimated genetic correlations between survival probability and traits related to fecundity, such as pupal weight. In addition, we tested whether different virus isolates have different effects on male and female pupal weight. To estimate genetic correlations, we exposed individuals of known relatedness to a single baculovirus isolate. To then evaluate the effect of virus isolate on pupal weight, we exposed a single gypsy moth strain to 16 baculovirus isolates. We found a negative genetic correlation between survival and pupal weight. In addition, virus exposure caused late‐pupating females to be identical in weight to males, whereas unexposed females were 2–3 times as large as unexposed males. Finally, we found that female pupal weight is a quadratic function of host mortality across virus isolates, which is likely due to trade‐offs and compensatory growth processes acting at high and low mortality levels, respectively. Overall, our results suggest that fecundity costs may strongly affect the response to selection for disease resistance. In nature, baculoviruses contribute to the regulation of gypsy moth outbreaks, as pathogens often do in forest‐defoliating insects. We therefore argue that trade‐offs between host life‐history traits may help explain outbreak dynamics.     

Evolution and maintenance of microbe‐mediated protection under occasional pathogen infection

Every host is colonized by a variety of microbes, some of which can protect their hosts from pathogen infection. However, pathogen presence naturally varies over time in nature, such as in the case of seasonal epidemics. We experimentally coevolved populations of Caenorhabditis elegans worm hosts with bacteria possessing protective traits (Enterococcus faecalis), in treatments varying the infection frequency with pathogenic Staphylococcus aureus every host generation, alternating host generations, every fifth host generation, or never. We additionally investigated the effect of initial pathogen presence at the formation of the defensive symbiosis. Our results show that enhanced microbe‐mediated protection evolved during host‐protective microbe coevolution when faced with rare infections by a pathogen. Initial pathogen presence had no effect on the evolutionary outcome of microbe‐mediated protection. We also found that protection was only effective at preventing mortality during the time of pathogen infection. Overall, our results suggest that resident microbes can be a form of transgenerational immunity against rare pathogen infection.     

Within‐host interactions shape virulence‐related traits of trematode genotypes

Within‐host interactions between co‐infecting parasites can significantly influence the evolution of key parasite traits, such as virulence (pathogenicity of infection). The type of interaction is expected to predict the direction of selection, with antagonistic interactions favouring more virulent genotypes and synergistic interactions less virulent genotypes. Recently, it has been suggested that virulence can further be affected by the genetic identity of co‐infecting partners (G × G interactions), complicating predictions on disease dynamics. Here, we used a natural host–parasite system including a fish host and a trematode parasite to study the effects of G × G interactions on infection virulence. We exposed rainbow trout (Oncorhynchus mykiss) either to single genotypes or to mixtures of two genotypes of the eye fluke Diplostomum pseudospathaceum and estimated parasite infectivity (linearly related to pathogenicity of infection, measured as coverage of eye cataracts) and relative cataract coverage (controlled for infectivity). We found that both traits were associated with complex G × G interactions, including both increases and decreases from single infection to co‐infection, depending on the genotype combination. In particular, combinations where both genotypes had low average infectivity and relative cataract coverage in single infections benefited from co‐infection, while the pattern was opposite for genotypes with higher performance. Together, our results show that infection outcomes vary considerably between single and co‐infections and with the genetic identity of the co‐infecting parasites. This can result in variation in parasite fitness and consequently impact evolutionary dynamics of host–parasite interactions.     

Interactions between host sex and age of exposure modify the virulence–transmission trade‐off

The patterns of immunity conferred by host sex or age represent two sources of host heterogeneity that can potentially shape the evolutionary trajectory of disease. With each host sex or age encountered, a pathogen's optimal exploitative strategy may change, leading to considerable variation in expression of pathogen transmission and virulence. To date, these host characteristics have been studied in the context of host fitness alone, overlooking the effects of host sex and age on the fundamental virulence–transmission trade‐off faced by pathogens. Here, we explicitly address the interaction of these characteristics and find that host sex and age at exposure to a pathogen affect age‐specific patterns of mortality and the balance between pathogen transmission and virulence. When infecting age‐structured male and female Daphnia magna with different genotypes of Pasteuria ramosa, we found that infection increased mortality rates across all age classes for females, whereas mortality only increased in the earliest age class for males. Female hosts allowed a variety of trade‐offs between transmission and virulence to arise with each age and pathogen genotype. In contrast, this variation was dampened in males, with pathogens exhibiting declines in both virulence and transmission with increasing host age. Our results suggest that differences in exploitation potential of males and females to a pathogen can interact with host age to allow different virulence strategies to coexist, and illustrate the potential for these widespread sources of host heterogeneity to direct the evolution of disease in natural populations.     

Ample genetic variation but no evidence for genotype specificity in an all‐parthenogenetic host–parasitoid interaction

Antagonistic coevolution between hosts and parasites can result in negative frequency‐dependent selection and may thus be an important mechanism maintaining genetic variation in populations. Negative frequency‐dependence emerges readily if interactions between hosts and parasites are genotype‐specific such that no host genotype is most resistant to all parasite genotypes, and no parasite genotype is most infective on all hosts. Although there is increasing evidence for genotype specificity in interactions between hosts and pathogens or microparasites, the picture is less clear for insect host–parasitoid interactions. Here, we addressed this question in the black bean aphid (Aphis fabae) and its most important parasitoid Lysiphlebus fabarum. Because both antagonists are capable of parthenogenetic reproduction, this system allows for powerful tests of genotype × genotype interactions. Our test consisted of exposing multiple host clones to different parthenogenetic lines of parasitoids in all combinations, and this experiment was repeated with animals from four different sites. All aphids were free of endosymbiotic bacteria known to increase resistance to parasitoids. We observed ample genetic variation for host resistance and parasitoid infectivity, but there was no significant host clone × parasitoid line interaction, and this result was consistent across the four sites. Thus, there is no evidence for genotype specificity in the interaction between A. fabae and L. fabarum, suggesting that the observed variation is based on rather general mechanisms of defence and attack.     

Fluctuating temperatures alter environmental pathogen transmission in a Daphnia–pathogen system

Environmental conditions are rarely constant, but instead vary spatially and temporally. This variation influences ecological interactions and epidemiological dynamics, yet most experimental studies examine interactions under constant conditions. We examined the effects of variability in temperature on the host–pathogen relationship between an aquatic zooplankton host (Daphnia laevis) and an environmentally transmitted fungal pathogen (Metschnikowia bicuspidata). We manipulated temperature variability by exposing all populations to mean temperatures of 20°C for the length of the experiments, but introducing periods of 1, 2, and 4 hr each day where the populations were exposed to 28°C followed by periods of the same length (1, 2, and 4 hr, respectively) where the populations were exposed to 12°C. Three experiments were performed to assess the role of thermal variability on Daphnia–pathogen interactions, specifically with respect to: (1) host infection prevalence and intensity; (2) free‐living pathogen survival; and (3) host foraging ecology. We found that temperature variability affected host filtering rate, which is closely related to pathogen transmission in this system. Further, infection prevalence was reduced as a function of temperature variability, while infection intensity was not influenced, suggesting that pathogen transmission was influenced by temperature variability, but the growth of pathogen within infected hosts was not. Host survival was reduced by temperature variability, but environmental pathogen survival was unaffected, suggesting that zooplankton hosts were more sensitive than the fungal pathogen to variable temperatures. Together, these experiments suggest that temperature variability may influence host demography and host–pathogen interactions, providing a link between host foraging ecology and pathogen transmission.     

Combining genome‐wide association study and FST‐based approaches to identify targets of Borrelia‐mediated selection in natural rodent hosts

Recent advances in high‐throughput sequencing technologies provide opportunities to gain novel insights into the genetic basis of phenotypic trait variation. Yet to date, progress in our understanding of genotype–phenotype associations in nonmodel organisms in general and natural vertebrate populations in particular has been hampered by small sample sizes typically available for wildlife populations and a resulting lack of statistical power, as well as a limited ability to control for false‐positive signals. Here we propose to combine a genome‐wide association study (GWAS) and F_ST‐based approach with population‐level replication to partly overcome these limitations. We present a case study in which we used this approach in combination with genotyping‐by‐sequencing (GBS) single nucleotide polymorphism (SNP) data to identify genomic regions associated with Borrelia afzelii resistance or susceptibility in the natural rodent host of this Lyme disease‐causing spirochete, the bank vole (Myodes glareolus). Using this combined approach we identified four consensus SNPs located in exonic regions of the genes Slc26a4, Tns3, Wscd1 and Espnl, which were significantly associated with the voles’ Borrelia infectious status within and across populations. Functional links between host responses to bacterial infections and most of these genes have previously been demonstrated in other rodent systems, making them promising new candidates for the study of evolutionary host responses to Borrelia emergence. Our approach is applicable to other systems and may facilitate the identification of genetic variants underlying disease resistance or susceptibility, as well as other ecologically relevant traits, in wildlife populations.     

Predators mediate the effects of a fungal pathogen on prey: an experiment with grasshoppers,wolf spiders,and fungal pathogens

1. Prey interact with multiple kinds of enemies such as predators, parasites, and pathogens. Interactions among enemies can alter prey dynamics but they are often studied separately. 2. During the summers of 2005–2006, we conducted a field experiment to examine interactions among grasshoppers, spider predators, and a lethal fungal pathogen of grasshoppers. Grasshopper nymphs were stocked into field enclosures. Predation was manipulated by adding spiders to enclosures on day 1, day 5, or day 10 of the experiment, or no spiders were added. We monitored grasshopper survival and grasshopper mortality from fungal pathogens for 4 weeks. 3. Fungal pathogens were abundant in 2005 but not in 2006, probably because of favourable weather conditions in 2005. When fungal pathogens were abundant, spider presence reduced grasshopper mortality from fungal pathogens, but only when spiders were present early in the experiment (added on day 1 or day 5). 4. The outcome of predator–prey interactions varied between years, probably as a result of differences in pathogen prevalence. In 2005, spider presence reduced the number of deaths from the pathogen, leading to a slight trend of increased grasshopper density. However, in 2006, when pathogens were not an important source of mortality, spider predation was compensatory.     

1H‐NMR screening for the high‐throughput determination of genotype and environmental effects on the content of asparagine in wheat grain

Free asparagine in cereals is known to be the precursor of acrylamide, a neurotoxic and carcinogenic product formed during cooking processes. Thus, the development of crops with lower asparagine is of considerable interest to growers and the food industry. In this study, we describe the development and application of a rapid ¹H‐NMR‐based analysis of cereal flour, that is, suitable for quantifying asparagine levels, and hence acrylamide‐forming potential, across large numbers of samples. The screen was applied to flour samples from 150 bread wheats grown at a single site in 2005, providing the largest sample set to date. Additionally, screening of 26 selected cultivars grown for two further years in the same location and in three additional European locations in the third year (2007) provided six widely different environments to allow estimation of the environmental (E) and G x E effects on asparagine levels. Asparagine concentrations in the 150 genotypes ranged from 0.32 to 1.56 mg/g dry matter in wholemeal wheat flours. Asparagine levels were correlated with plant height and therefore, due to recent breeding activities to produce semi‐dwarf varieties, a negative relationship with the year of registration of the cultivar was also observed. The multisite study indicated that only 13% of the observed variation in asparagine levels was heritable, whilst the environmental contribution was 36% and the GxE component was 43%. Thus, compared to some other phenotypic traits, breeding for low asparagine wheats presents a difficult challenge.     

Next‐generation sequencing sheds light on intricate regulation of insect gut microbiota

Next‐generation sequencing (NGS) technologies are getting cheaper and easier and hence becoming readily accessible for many researchers in biological disciplines including ecology. In this issue of Molecular Ecology, Sudakaran et al. (2012) show how the NGS revolution contributes to our better and more comprehensive understanding of ecological interactions between gut symbiotic microbiota and the host organism. Using the European red firebug Pyrrhocoris apterus as a model system, they demonstrated that the gut microbiota consists of a small number of major bacterial phylotypes plus other minor bacterial associates. The major bacteria are localized in a specific anoxic section of the midgut and quantitatively account for most of the gut microbiota irrespective of host's geographic populations. The specific gut microbiota is established through early nymphal development of the host insect. Interestingly, the host feeding on different food, namely linden seeds, sunflower seeds or wasp larvae, scarcely affected the symbiont composition, suggesting homoeostatic control over the major symbiotic microbiota in the anoxic section of the midgut. Some of the minor components of the gut microbiota, which conventional PCR/cloning/sequencing approaches would have failed to detect, were convincingly shown to be food‐derived. These findings rest on the robust basis of high‐throughput sequencing data, and some of them could not be practically obtained by conventional molecular techniques, highlighting the significant impact of NGS approaches on ecological aspects of host–symbiont interactions in a nonmodel organism.     

Movement can mediate temporal mismatches between resource availability and biological events in host–pathogen interactions

Global change is shifting the timing of biological events, leading to temporal mismatches between biological events and resource availability. These temporal mismatches can threaten species’ populations. Importantly, temporal mismatches not only exert strong pressures on the population dynamics of the focal species, but can also lead to substantial changes in pairwise species interactions such as host–pathogen systems. We adapted an established individual‐based model of host–pathogen dynamics. The model describes a viral agent in a social host, while accounting for the host''s explicit movement decisions. We aimed to investigate how temporal mismatches between seasonal resource availability and host life‐history events affect host–pathogen coexistence, that is, disease persistence. Seasonal resource fluctuations only increased coexistence probability when in synchrony with the hosts’ biological events. However, a temporal mismatch reduced host–pathogen coexistence, but only marginally. In tandem with an increasing temporal mismatch, our model showed a shift in the spatial distribution of infected hosts. It shifted from an even distribution under synchronous conditions toward the formation of disease hotspots, when host life history and resource availability mismatched completely. The spatial restriction of infected hosts to small hotspots in the landscape initially suggested a lower coexistence probability due to the critical loss of susceptible host individuals within those hotspots. However, the surrounding landscape facilitated demographic rescue through habitat‐dependent movement. Our work demonstrates that the negative effects of temporal mismatches between host resource availability and host life history on host–pathogen coexistence can be reduced through the formation of temporary disease hotspots and host movement decisions, with implications for disease management under disturbances and global change.     

A peptide resource for the analysis of Staphylococcus aureus in host–pathogen interaction studies

Staphylococcus aureus is an opportunistic human pathogen, which can cause life‐threatening disease. Proteome analyses of the bacterium can provide new insights into its pathophysiology and important facets of metabolic adaptation and, thus, aid the recognition of targets for intervention. However, the value of such proteome studies increases with their comprehensiveness. We present an MS–driven, proteome‐wide characterization of the strain S. aureus HG001. Combining 144 high precision proteomic data sets, we identified 19 109 peptides from 2088 distinct S. aureus HG001 proteins, which account for 72% of the predicted ORFs. Peptides were further characterized concerning pI, GRAVY, and detectability scores in order to understand the low peptide coverage of 8.7% (19 109 out of 220 245 theoretical peptides). The high quality peptide‐centric spectra have been organized into a comprehensive peptide fragmentation library (SpectraST) and used for identification of S. aureus‐typic peptides in highly complex host–pathogen interaction experiments, which significantly improved the number of identified S. aureus proteins compared to a MASCOT search. This effort now allows the elucidation of crucial pathophysiological questions in S. aureus‐specific host–pathogen interaction studies through comprehensive proteome analysis. The S. aureus‐specific spectra resource developed here also represents an important spectral repository for SRM or for data‐independent acquisition MS approaches. All MS data have been deposited in the ProteomeXchange with identifier PXD000702 ( http://proteomecentral.proteomexchange.org/dataset/PXD000702 ).     

Field evidence for indirect interactions between foliar‐feeding insect and root‐feeding nematode communities on Nicotiana tabacum

Abstract 1. As herbivory often elicits systemic changes in plant traits, indirect interactions via induced plant responses may be a pervasive feature structuring herbivore communities. Although the importance of this phenomenon has been emphasised for herbivorous insects, it is unknown if and how induced responses contribute to the organisation of other major phytoparasitic taxa. 2. Survey and experimental field studies were used to investigate the role of plants in linking the dynamics of foliar‐feeding insects and root‐feeding nematodes on tobacco, Nicotiana tabacum. 3. Plant‐mediated interactions between insects and nematodes could largely be differentiated by insect feeding guild, with positive insect–nematode interactions predominating with leaf‐chewing insects (caterpillars) and negative interactions occurring with sap‐feeding insects (aphids). For example, insect defoliation was positively correlated with the abundance of root‐feeding nematodes, but aphids and nematodes were negatively correlated. Experimental field manipulations of foliar insect and nematode root herbivory also tended to support this outcome. 4. Overall, these results suggest that plants indirectly link the dynamics of divergent consumer taxa in spatially distinct ecosystems. This lends support to the growing perception that plants play a critical role in propagating indirect effects among a diverse assemblage of consumers.     

Genome‐wide association studies on the phyllosphere microbiome: Embracing complexity in host–microbe interactions

Environmental sequencing shows that plants harbor complex communities of microbes that vary across environments. However, many approaches for mapping plant genetic variation to microbe‐related traits were developed in the relatively simple context of binary host–microbe interactions under controlled conditions. Recent advances in sequencing and statistics make genome‐wide association studies (GWAS) an increasingly promising approach for identifying the plant genetic variation associated with microbes in a community context. This review discusses early efforts on GWAS of the plant phyllosphere microbiome and the outlook for future studies based on human microbiome GWAS. A workflow for GWAS of the phyllosphere microbiome is then presented, with particular attention to how perspectives on the mechanisms, evolution and environmental dependence of plant–microbe interactions will influence the choice of traits to be mapped.     

Measuring the immune system of the three‐spined stickleback – investigating natural variation by quantifying immune expression in the laboratory and the wild

Current understanding of the immune system comes primarily from laboratory‐based studies. There has been substantial interest in examining how it functions in the wild, but studies have been limited by a lack of appropriate assays and study species. The three‐spined stickleback (Gasterosteus aculeatus L.) provides an ideal system in which to advance the study of wild immunology, but requires the development of suitable immune assays. We demonstrate that meaningful variation in the immune response of stickleback can be measured using real‐time PCR to quantify the expression of eight genes, representing the innate response and Th1‐, Th2‐ and Treg‐type adaptive responses. Assays are validated by comparing the immune expression profiles of wild and laboratory‐raised stickleback, and by examining variation across populations on North Uist, Scotland. We also compare the immune response potential of laboratory‐raised individuals from two Icelandic populations by stimulating cells in culture. Immune profiles of wild fish differed from laboratory‐raised fish from the same parental population, with immune expression patterns in the wild converging relative to those in the laboratory. Innate measures differed between wild populations, whilst the adaptive response was associated with variation in age, relative size of fish, reproductive status and S. solidus infection levels. Laboratory‐raised individuals from different populations showed markedly different innate immune response potential. The ability to combine studies in the laboratory and in the wild underlines the potential of this toolkit to advance our understanding of the ecological and evolutionary relevance of immune system variation in a natural setting.