Invertebrate ecological immunology

Four invertebrate-type lysozyme genes from triangle-shell pearl mussel (Hyriopsis cumingii)

Lysozymes in animals have three types, namely chicken-type, goose-type, and invertebrate-type (i-type) lysozymes and all these 3 types have been found in bivalve mollusks. The i-type lysozymes in mollusks are involved in digestion and innate immunity. In this study, four different lysozyme genes that belong to i-type were identified from Hyriopsis cumingii. The HcLyso1 to HcLyso4 genes encode proteins with 144, 144, 161, and 228 amino acids, respectively, and contain a destabilase domain. HcLyso4 also contains SH3b domain in addition to its destabilase domain. Multiple alignments showed that two catalytic residues of Glu and Asp which were necessary for enzyme activity were present in i-type lysozymes. Phylogenetic analysis using CDS sequences of i-type lysozymes showed that these lysozymes can be divided into mollusk and crustacean clades, and that HcLyso1 to HcLyso4 all belong to the mollusk clades. Although there was no positive selection predicted in i-type lysozymes, some branches suffered rapid evolution. HcLyso1 is mainly expressed in hepatopancreas and can be detected in hemocytes. HcLyso2 is primarily expressed in hepatopancreas and can be detected in hemocytes Whereas, HcLyso3 can be detected mainly in hemocytes, hepatopancreas, gills, and mantle. HcLyso4 is expressed in hemocytes and hepatopancreas. qRT-PCR analysis showed that HcLyso1 to HcLyso4 were all nearly down-regulated by Vibrio or Staphylococcus aureus challenge. Moreover, our research indicated that HcLyso1 to HcLyso4 might play a key role in the innate immunity of mussel.

Detecting genes for variation in parasite burden and immunological traits in a wild population: testing the candidate gene approach

Identifying the genes underlying phenotypic variation in natural populations can provide novel insight into the evolutionary process. The candidate gene approach has been applied to studies of a number of traits in various species, in an attempt to elucidate their genetic basis. Here, we test the application of the candidate gene approach to identify the loci involved in variation in gastrointestinal parasite burden, a complex trait likely to be controlled by many loci, in a wild population of Soay sheep. A comprehensive literature review, Gene Ontology databases, and comparative genomics resources between cattle and sheep were used to generate a list of candidate genes. In a pilot study, these candidates, along with 50 random genes, were then sequenced in two pools of Soay sheep; one with low gastrointestinal nematode burden and the other high, using a NimbleGen sequence capture experiment. Further candidates were identified from single nucleotide polymorphisms (SNPs) that were highly differentiated between high- and low-resistance sheep breeds. A panel of 192 candidate and control SNPs were then typed in 960 individual Soay sheep to examine whether they individually explained variation in parasite burden, as measured as faecal egg count, as well as two immune measures (Teladorsagia circumcincta-specific antibodies and antinuclear antibodies). The cumulative effect of the candidate and control SNPs were estimated by fitting genetic relationship matrices (GRMs) as random effects in animal models of the three traits. No more significant SNPs were identified in the pilot sequencing experiment and association study than expected by chance. Furthermore, no significant difference was found between the proportions of candidate or control SNPs that were found to be significantly associated with parasite burden/immune measures. No significant effect of the candidate or control gene GRMs was found. There is thus little support for the candidate gene approach to the identification of loci explaining variation in parasitological and immunological traits in this population. However, a number of SNPs explained significant variation in multiple traits and significant correlations were found between the proportions of variance explained by individual SNPs across multiple traits. The significant SNPs identified in this study may still, therefore, merit further investigation.

Behavioral Immunity in Insects

Parasites can dramatically reduce the fitness of their hosts, and natural selection should favor defense mechanisms that can protect hosts against disease. Much work has focused on understanding genetic and physiological immunity against parasites, but hosts can also use behaviors to avoid infection, reduce parasite growth or alleviate disease symptoms. It is increasingly recognized that such behaviors are common in insects, providing strong protection against parasites and parasitoids. We review the current evidence for behavioral immunity in insects, present a framework for investigating such behavior, and emphasize that behavioral immunity may act through indirect rather than direct fitness benefits. We also discuss the implications for host-parasite co-evolution, local adaptation, and the evolution of non-behavioral physiological immune systems. Finally, we argue that the study of behavioral immunity in insects has much to offer for investigations in vertebrates, in which this topic has traditionally been studied.

Maternal effects in disease resistance: poor maternal environment increases offspring resistance to an insect virus

Maternal effects can be adaptive and because of their intrinsic time delays may have important effects on population dynamics. In vertebrates, and increasingly invertebrates, it is well established that offspring defence is in part determined by maternal parasite exposure. It has also been suggested that there may be indirect maternal effects on immunity mediated by other components of the maternal environment, including density and resource availability. Here, we examine the effect maternal resource availability has on the immunity of offspring in an insect-virus system. We use five different maternal resource levels and examine immunity in the offspring both directly, by challenge with a virus, and by measuring a major component of the immune system, across three offspring environments. Both the direct infection assay and the measure of immunocompetence show clearly that offspring from mothers in poor environments are more resistant to parasites. This may result from life-history optimization of mothers in poor environments, or because the poor environment acts as a cue for higher disease risk in the next generation. This emphasizes the importance of maternal effects on disease resistance, mediated through indirect environmental factors that will have important implications to both the ecological and evolutionary dynamics of host-parasite interactions.

Influences of Plant Traits on Immune Responses of Specialist and Generalist Herbivores

Specialist and generalist insect herbivore species often differ in how they respond to host plant traits, particularly defensive traits, and these responses can include weakened or strengthened immune responses to pathogens and parasites. Accurate methods to measure immune response in the presence and absence of pathogens and parasites are necessary to determine whether susceptibility to these natural enemies is reduced or increased by host plant traits. Plant chemical traits are particularly important in that host plant metabolites may function as antioxidants beneficial to the immune response, or interfere with the immune response of both specialist and generalist herbivores. Specialist herbivores that are adapted to process and sometimes accumulate specific plant compounds may experience high metabolic demands that may decrease immune response, whereas the metabolic demands of generalist species differ due to more broad-substrate enzyme systems. However, the direct deleterious effects of plant compounds on generalist herbivores may weaken their immune responses. Further research in this area is important given that the ecological relevance of plant traits to herbivore immune responses is equally important in natural systems and agroecosystems, due to potential incompatibility of some host plant species and cultivars with biological control agents of herbivorous pests.

Transgenerational effects of heavy metal pollution on immune defense of the blow fly Protophormia terraenovae

Recently environmental conditions during early parental development have been found to have transgenerational effects on immunity and other condition-dependent traits. However, potential transgenerational effects of heavy metal pollution have not previously been studied. Here we show that direct exposure to heavy metal (copper) upregulates the immune system of the blow fly, Protophormia terraenovae, reared in copper contaminated food. In the second experiment, to test transgenerational effects of heavy metal, the parental generation of the P. terraenovae was reared in food supplemented with copper, and the immunocompetence of their offspring, reared on uncontaminated food, was measured. Copper concentration used in this study was, in the preliminary test, found to have no effect on mortality of the flies. Immunity was tested on the imago stage by measuring encapsulation response against an artificial antigen, nylon monofilament. We found that exposure to copper during the parental development stages through the larval diet resulted in immune responses that were still apparent in the next generation that was not exposed to the heavy metal. We found that individuals reared on copper-contaminated food developed more slowly compared with those reared on uncontaminated food. The treatment groups did not differ in their dry body mass. However, parental exposure to copper did not have an effect on the development time or body mass of their offspring. Our study suggests that heavy metal pollution has positive feedback effect on encapsulation response through generations which multiplies the harmful effects of heavy metal pollution in following generations.

Eicosanoids: Exploiting Insect Immunity to Improve Biological Control Programs

Insects, like all invertebrates, express robust innate, but not adaptive, immune reactions to infection and invasion. Insect immunity is usually resolved into three major components. The integument serves as a physical barrier to infections. Within the hemocoel, the circulating hemocytes are the temporal first line of defense, responsible for clearing the majority of infecting bacterial cells from circulation. Specific cellular defenses include phagocytosis, microaggregation of hemocytes with adhering bacteria, nodulation and encapsulation. Infections also stimulate the humoral component of immunity, which involves the induced expression of genes encoding antimicrobial peptides and activation of prophenoloxidase. These peptides appear in the hemolymph of challenged insects 6-12 hours after the challenge. Prostaglandins and other eicosanoids are crucial mediators of innate immune responses. Eicosanoid biosynthesis is stimulated by infection in insects. Inhibition of eicosanoid biosynthesis lethally renders experimental insects unable to clear bacterial infection from hemolymph. Eicosanoids mediate specific cell actions, including phagocytosis, microaggregation, nodulation, hemocyte migration, hemocyte spreading and the release of prophenoloxidase from oenocytoids. Some invaders have evolved mechanisms to suppress insect immunity; a few of them suppress immunity by targeting the first step in the eicosanoid biosynthesis pathways, the enzyme phospholipase A₂. We proposed research designed to cripple insect immunity as a technology to improve biological control of insects. We used dsRNA to silence insect genes encoding phospholipase A₂, and thereby inhibited the nodulation reaction to infection. The purpose of this article is to place our view of applying dsRNA technologies into the context of eicosanoid actions in insect immunity. The long-term significance of research in this area lies in developing new pest management technologies to contribute to food security in a world with a rapidly growing human population.

Endocrine control of mucosal immunity in the female reproductive tract: impact of environmental disruptors

The complexity of the human female reproductive tract (FRT) with its multiple levels of hormonally controlled immune protection has only begun to be understood. Dissecting the functions and roles of the immune system in the FRT is complicated by the differential hormonal regulation of its distinct anatomical structures that vary throughout the menstrual cycle. Although many fundamental mechanisms of steroid regulation of reproductive tract immune function have been determined, the effects of exogenous synthetic steroids or endocrine disruptors on immune function and disease susceptibility in the FRT have yet to be evaluated in detail. There is increasing evidence that environmental or synthetic molecules can alter normal immune function. This review provides an overview of the innate and adaptive immune systems, the current status of immune function in the FRT and the potential risks of environmental or pharmacological molecules that may perturb this system.

Clot Formation in the Sipunculid Worm Themiste petricola: A Haemostatic and Immune Cellular Response

Clot formation in the sipunculid Themiste petricola, a coelomate nonsegmented marine worm without a circulatory system, is a cellular response that creates a haemostatic mass upon activation with sea water. The mass with sealing properties is brought about by homotypic aggregation of granular leukocytes present in the coelomic fluid that undergo a rapid process of fusion and cell death forming a homogenous clot or mass. The clot structure appears to be stabilized by abundant F-actin that creates a fibrous scaffold retaining cell-derived components. Since preservation of fluid within the coelom is vital for the worm, clotting contributes to rapidly seal the body wall and entrap pathogens upon injury, creating a matrix where wound healing can take place in a second stage. During formation of the clot, microbes or small particles are entrapped. Phagocytosis of self and non-self particles shed from the clot occurs at the clot neighbourhood, demonstrating that clotting is the initial phase of a well-orchestrated dual haemostatic and immune cellular response.

Evaluation of the nutritive value of maize for honey bees

In modern managed agro-ecosystems, the supply of adequate food from blooming crops is limited to brief periods. During periods of pollen deficiencies, bees are forced to forage on alternative crops, such as maize. However, pollen of maize is believed to be a minor food source for bees as it is thought to be lacking in proteins and essential amino acids. This study was conducted to verify this assumption. In maize, a strikingly low concentration of histidine was found, but the amount of all other essential amino acids was greater than that of mixed pollen. The performance and the immunocompetence of bees consuming a pure maize pollen diet (A) was compared to bees feeding on a polyfloral pollen diet (B) and to bees feeding on an artificial substitute of pollen (C). Consumption of diets A and C were linked to a reduction in brood rearing and lifespan. However, no immunological effects were observed based on two parameters of the humoral immunity.

Diet quality can play a critical role in defense efficacy against parasitoids and pathogens in the Glanville fritillary (Melitaea cinxia)

Numerous herbivorous insect species sequester noxious chemicals from host plants that effectively defend against predators, and against parasitoids and pathogens. Sequestration of these chemicals may be expensive and involve a trade off with other fitness traits. Here, we tested this hypothesis. We reared Glanville fritillary butterfly (Melitaea cinxia L.) larvae on plant diets containing low- and high-levels of iridoid glycosides (IGs) (mainly aucubin and catalpol) and tested: 1) whether IGs affect the herbivore's defense against parasitoids (measured as encapsulation rate) and bacterial pathogens (measured as herbivore survival); 2) whether parasitoid and bacterial defenses interact; and 3) whether sequestration of the plant's defense chemicals incurs any life history costs. Encapsulation rates were stronger when there were higher percentages of catalpol in the diet. Implanted individuals had greater amounts of IGs in their bodies as adults. This suggests that parasitized individuals may sequester more IGs, increase their feeding rate after parasitism, or that there is a trade off between detoxification efficiency and encapsulation rate. Larval survival after bacterial infection was influenced by diet, but probably not by diet IG content, as changes in survival did not correlate linearly with the levels of IGs in the diet. However, M. cinxia larvae with good encapsulation abilities were better defended against bacteria. We did not find any life history costs of diet IG concentration for larvae. These results suggest that the sequestering of plant defense chemicals can help herbivorous insects to defend against parasitoids.

Constitutive immune function in European starlings, Sturnus vulgaris, is decreased immediately after an endurance flight in a wind tunnel

Life-history theory predicts that animals face a trade-off in energy allocation between performing strenuous exercise, such as migratory flight, and mounting an immune response. We experimentally tested this prediction by studying immune function in European starlings, Sturnus vulgaris, flown in a wind tunnel. Specifically, we predicted that constitutive immune function decreases in response to training and, additionally, in response to immediate exercise. We compared constitutive immune function among three groups: (1) ‘untrained’ birds that were kept in cages and were not flown; (2) ‘trained’ birds that received flight training over a 15 day period and performed a 1-4 h continuous flight, after which they rested for 48 h before being sampled; and (3) ‘post-flight’ birds that differed from the ‘trained’ group only in being sampled immediately after the final flight. A bird in our trained group represents an individual during migration that has been resting between migratory flights for at least 2 days. A bird in our post-flight group represents an individual that has just completed a migratory flight and has not yet had time to recover. Three of our four indicators (haptoglobin, agglutination and lysis) showed the predicted decrease in immune function in the post-flight group, and two indicators (haptoglobin, agglutination) showed the predicted decreasing trend from the untrained to trained to post-flight group. Haptoglobin levels were negatively correlated with flight duration. No effect of training or flight was detected on leukocyte profiles. Our results suggest that in European starlings, constitutive immune function is decreased more as a result of immediate exercise than of exercise training. Because of the recent emergence of avian-borne diseases, understanding the trade-offs and challenges faced by long-distance migrants has gained a new level of relevance and urgency.

Quantitative genetics of immunity and life history under different photoperiods

Insects with complex life-cycles should optimize age and size at maturity during larval development. When inhabiting seasonal environments, organisms have limited reproductive periods and face fundamental decisions: individuals that reach maturity late in season have to either reproduce at a small size or increase their growth rates. Increasing growth rates is costly in insects because of higher juvenile mortality, decreased adult survival or increased susceptibility to parasitism by bacteria and viruses via compromised immune function. Environmental changes such as seasonality can also alter the quantitative genetic architecture. Here, we explore the quantitative genetics of life history and immunity traits under two experimentally induced seasonal environments in the cricket Gryllus bimaculatus. Seasonality affected the life history but not the immune phenotypes. Individuals under decreasing day length developed slower and grew to a bigger size. We found ample additive genetic variance and heritability for components of immunity (haemocyte densities, proPhenoloxidase activity, resistance against Serratia marcescens), and for the life history traits, age and size at maturity. Despite genetic covariance among traits, the structure of G was inconsistent with genetically based trade-off between life history and immune traits (for example, a strong positive genetic correlation between growth rate and haemocyte density was estimated). However, conditional evolvabilities support the idea that genetic covariance structure limits the capacity of individual traits to evolve independently. We found no evidence for G × E interactions arising from the experimentally induced seasonality.

Immune response increases predation risk

Why do individuals have an imperfect immune system? Most studies suggest trade-offs associated with immunity and metabolism, and neglect ecological factors, such as predation. We provide one of the first experimental studies demonstrating a context-dependent survival cost to immune activation. In the presence of a predator, immune-challenged male field crickets showed significantly lower survival than controls, whilst there was no difference in a predator-free environment. Immune-challenged males spent more time outside their burrows and reacted slower to a simulated predator attack. We conclude that some costs of immunity are expressed via increased susceptibility to predation, indicating the importance of integrating the ecological context when investigating optimal investment in immunity.

The modulation of catecholamines to the immune response against bacteria Vibrio anguillarum challenge in scallop Chlamys farreri

Catecholamines are pivotal signal molecules in the neuroendocrine-immune regulatory network, and implicated in the modulation of immune response. In the present study, the activities of some immune-related enzymes and the concentration of catecholamines were determined in circulating haemolymph of scallops Chlamys farreri after bacteria Vibrio anguillarum challenge. The activities of superoxide dismutase (SOD), catalase (CAT) and lysozyme (LYZ) increased significantly and reached 610 U mg(-1) at 12 h, 37.6 U mg(-1) at 6 h and 261.5 U mg(-1) at 6 h after bacteria challenge, respectively. The concentration of norepinephrine, epinephrine and dopamine also increased significantly and reached 114.9 ng mL(-1) at 12 h, 86.9 ng mL(-1) at 24 h and 480.4 pg mL(-1) at 12 h after bacteria challenge, respectively. Meanwhile, the activities of these immune-related enzymes in haemolymph were monitored in those scallops which were challenged by bacteria V. anguillarum and stimulated simultaneously with norepinephrine, epinephrine and adrenoceptor antagonist. The injection of norepinephrine and epinephrine repressed significantly the induction of bacteria challenge on the activities of immune-related enzymes, and they were reduced to about half of that in the control groups. The blocking of α and β-adrenoceptor by antagonist only repressed the increase of CAT and LYZ activities significantly, while no significant effect was observed on the increase of SOD activities. The collective results indicated that scallop catecholaminergic neuroendocrine system could be activated by bacteria challenge to release catecholamines after the immune response had been triggered, and the immune response against bacteria challenge could been negatively modulated by norepinephrine, epinephrine, and adrenoceptor antagonist. This information is helpful to further understand the immunomodulation of catecholamines in scallops.

Development of gene expression markers of acute heat-light stress in reef-building corals of the genus Porites

Coral reefs are declining worldwide due to increased incidence of climate-induced coral bleaching, which will have widespread biodiversity and economic impacts. A simple method to measure the sub-bleaching level of heat-light stress experienced by corals would greatly inform reef management practices by making it possible to assess the distribution of bleaching risks among individual reef sites. Gene expression analysis based on quantitative PCR (qPCR) can be used as a diagnostic tool to determine coral condition in situ. We evaluated the expression of 13 candidate genes during heat-light stress in a common Caribbean coral Porites astreoides, and observed strong and consistent changes in gene expression in two independent experiments. Furthermore, we found that the apparent return to baseline expression levels during a recovery phase was rapid, despite visible signs of colony bleaching. We show that the response to acute heat-light stress in P. astreoides can be monitored by measuring the difference in expression of only two genes: Hsp16 and actin. We demonstrate that this assay discriminates between corals sampled from two field sites experiencing different temperatures. We also show that the assay is applicable to an Indo-Pacific congener, P. lobata, and therefore could potentially be used to diagnose acute heat-light stress on coral reefs worldwide.

Clone-specific immune reactions in a trematode-crustacean system

Variability of immune responses is an essential aspect of ecological immunology, yet how much of this variability is due to differences among parasite genotypes remains unknown. Here, variation in immune response of the crab, Macrophthalmus hirtipes, is examined as a function of experimental exposure to 10 clonal cercarial lineages of the trematode Maritrema novaezealandensis. Our goals were (1) to assess the variability of the host immune reaction elicited by 10 parasite clones, (2) to test if the heterozygosity–fitness correlation, whereby organisms with higher heterozygosities achieve a higher fitness than those with lower heterozygosities, applies to heterozygous parasites eliciting weak immune responses, and (3) to see how concomitant infections by other macroparasites influence the crab's immune response to cercariae. Parasite clones were distinguished and heterozygosities calculated using 20 microsatellite markers. We found that exposure to cercariae resulted in increased haemocyte counts, and that although interclonal differences in immune response elicited were detected, parasite heterozygosity did not correlate with host immune response. Additionally, the presence of other pre-existing parasites in hosts did not influence their immune response following experimental exposure to cercariae. Overall, the existence of variability in immune response elicited by different parasite clones is promising for future ecological immunology studies using this system.

The sub-lethal effects of repeated freezing in the woolly bear caterpillar Pyrrharctia isabella

Repeated freeze-thaw cycles are common and are increasing in frequency with climate change in many temperate locations, yet understanding of their impact on freeze-tolerant insects is extremely limited. We investigated the effects of repeated freezing and thawing on the freeze-tolerant final instar caterpillars of the moth Pyrrharctia isabella (Lepidoptera: Arctiidae) by subjecting individuals to either a single sustained 35 h freeze or five 7 h freezes. Sub-lethal effects were quantified with changes in three broad groups of measures: (1) cold hardiness, (2) metabolic rate and energy reserves and (3) survival after challenge with fungal spores. Repeated freeze-thaw cycles increased mortality to almost 30% and increased tissue damage in Malpighian tubules and hemocytes. Repeated freezing increased caterpillar glycerol concentration by 0.82 mol l(-1). There were no changes in metabolic rate or energy reserves with repeated freezing. For the first time, we report increased survival after immune challenge in caterpillars after freezing and suggest that this may be linked to wounding during freezing. We suggest that little repair of freezing damage is possible in P. isabella caterpillars and repeated freeze-thaw cycles may present significant challenges to survival in this species.

How does variability of immune system genes affect placentation?

Formation of the placenta is a crucial step in mammalian pregnancy. Apart from its function in ensuring an optimal supply of nutrients and oxygen to the fetus, the placenta is also the interface at which allo-recognition of invading trophoblast cells by the maternal immune system can potentially occur. We summarise here the “state of the art” on how variability of immune system genes that code for major histocompatibility complex (MHC) molecules and natural killer receptors (NKR) may impact on human placentation. MHC and NKR are the most polymorphic human genes. Our recent reports point out that specific combinations of fetal MHC and maternal NKR genes in humans correlate with the risk of pre-eclampsia, recurrent miscarriage (RM) and fetal growth restriction (FGR). Research in this field is still at an early stage and future studies in mouse and humans will be needed before the results can be translated to clinical applications. We discuss our recent work, as well as the opportunities offered by mouse genetics, to understand the cellular and molecular mechanisms underlying immune interactions at the maternal-fetal interface.

Immunological function in marine invertebrates: responses to environmental perturbation

The inception of ecological immunology has led to an increase in the number of studies investigating the impact of environmental stressors on host immune defence mechanisms. This in turn has led to an increased understanding of the importance of invertebrate groups for immunological research. This review discusses the advances made within marine invertebrate ecological immunology over the past decade. By demonstrating the environmental stressors tested, the immune parameters typically investigated, and the species that have received the greatest level of investigation, this review provides a critical assessment of the field of marine invertebrate ecological immunology. In highlighting the methodologies employed within this field, our current inability to understand the true ecological significance of any immune dysfunction caused by environmental stressors is outlined. Additionally, a number of examples are provided in which studies successfully demonstrate a measure of immunocompetence through alterations in disease resistance and organism survival to a realized pathogenic threat. Consequently, this review highlights the potential to advance our current understanding of the ecological and evolutionary significance of environmental stressor related immune dysfunction. Furthermore, the potential for the advancement of our understanding of the immune system of marine invertebrates, through the incorporation of newly emerging and novel molecular techniques, is emphasized.

Cellular immunosenescence in adult male crickets, Gryllus assimilis

Ecological immunity studies in invertebrates, particularly insects, have generated new insights into trade-offs between immune functions and other physiological parameters. These studies document physiologically directed reallocations of immune costs to other high-cost areas of physiology. Immunosenescence, recognized as the age-related deterioration of immune functions, is another mechanism of radically altering immune systems. We investigated the hypothesis that aging brings on immunosenescence in adult males of the cricket, Gryllus assimilis. Our data show that the intensity of melanotic nodule formation decreased with adult age from after 3-week post-adult emergence. Circulating hemocyte populations similarly decreased from about 5,000 hemocytes/µl hemolymph to about 1,000 hemocytes/µl hemolymph. The numbers of damaged hemocytes in circulation increased from less than 10% at 1-week post-adult emergence to approximately 60% by 3-week post-adult emergence. The composition of hemocyte types changed with age, with increasing proportions of granulocytes and decreasing proportions of plasmatocytes. The declines in nodule formation were not linked to the adult age of sexual behaviors, which begin shortly after entering adulthood in this species. We infer that age-related senescence, rather than cost reallocations, may account for observed declines in various parameters of immune functions in insects, as seen in other animals.

Non-immunological defense in an evolutionary framework

After parasite infection, invertebrates activate immune system-based defenses such as encapsulation and the signaling pathways of the innate immune system. However, hosts are often able to defend against parasites without using these mechanisms. The non-immunological defenses, such as behaviors that prevent or combat infection, symbiont-mediated defense, and fecundity compensation, are often ignored but can be important in host-parasite dynamics. We review recent studies showing that heritable variation in these traits exists among individuals, and that they are costly to activate and maintain. We also discuss findings from genome annotation and expression studies to show how immune system-based and non-immunological defenses interact. Placing these studies into an evolutionary framework emphasizes their importance for future studies of host-parasite coevolution.

Potential for evolutionary coupling and decoupling of larval and adult immune gene expression

Almost all studies of the immune system of animals with metamorphosis have focused on either larval or on adult immunity, implicitly assuming that these traits are either perfectly correlated or evolutionarily independent. In this study, we use 80 crosses among 21 Drosophila melanogaster lines to investigate the degree and constancy of genetic correlation in immune system activity between larvae and adults. The constitutive transcription of Diptericin, a gene encoding a defensive antimicrobial peptide, was controlled by the same genetic factors in larvae and adults, with variation in expression determined exclusively by nonadditive genetic effects. This contrasted with another peptide-encoding gene, Drosomycin, in which larval transcription was highly variable and determined by additive effects but adult transcription genetically invariant. We found no evidence for a fitness cost to the transcription of these genes in our study. The shared genetic control of larval and adult Diptericin transcription stands in contrast to predictions of the adaptive decoupling hypothesis, which states that distinct life-stages should permit the independent evolution of larval and adult phenotypes. Importantly, genetic correlations between larval and adult immunities imply that parasite pressure on one life-stage can drive the evolution of immunity (and resistance) in the other life-stage.

Low cost antiviral activity of Plodia interpunctella haemolymph in vivo demonstrated by dose dependent infection

Given the ubiquity of infectious disease it is important to understand the way in which hosts defend themselves and any costs that they may pay for this defence. Despite this, we know relatively little about insect immune responses to viruses when compared to their well-characterized responses to other pathogens. In particular it is unclear whether there is significant haemocoelic response to viral infection. Here we directly examine this question by examining whether there is a dose-dependency in infection risk when a DNA virus is injected directly into the haemocoel. Infection from direct injection into the haemocoel showed a clear dose dependency that is indicative of an active intrahaemocoelic immune response to DNA viruses in insects. In contrast to the natural oral infection route, we found no measurable sublethal effects in the survivors from direct injection. This suggests that the immune responses in the haemocoel are less costly than those that occur earlier.

Wild immunology

In wild populations, individuals are regularly exposed to a wide range of pathogens. In this context, organisms must elicit and regulate effective immune responses to protect their health while avoiding immunopathology. However, most of our knowledge about the function and dynamics of immune responses comes from laboratory studies performed on inbred mice in highly controlled environments with limited exposure to infection. Natural populations, on the other hand, exhibit wide genetic and environmental diversity. We argue that now is the time for immunology to be taken into the wild. The goal of 'wild immunology' is to link immune phenotype with host fitness in natural environments. To achieve this requires relevant measures of immune responsiveness that are both applicable to the host-parasite interaction under study and robustly associated with measures of host and parasite fitness. Bringing immunology to nonmodel organisms and linking that knowledge host fitness, and ultimately population dynamics, will face difficult challenges, both technical (lack of reagents and annotated genomes) and statistical (variation among individuals and populations). However, the affordability of new genomic technologies will help immunologists, ecologists and evolutionary biologists work together to translate and test our current knowledge of immune mechanisms in natural systems. From this approach, ecologists will gain new insight into mechanisms relevant to host health and fitness, while immunologists will be given a measure of the real-world health impacts of the immune factors they study. Thus, wild immunology can be the missing link between laboratory-based immunology and human, wildlife and domesticated animal health.

Prostaglandins and their receptors in insect biology

We treat the biological significance of prostaglandins (PGs) and their known receptors in insect biology. PGs and related eicosanoids are oxygenated derivatives of arachidonic acid (AA) and two other C20 polyunsaturated fatty acids. PGs are mostly appreciated in the context of biomedicine, but a growing body of literature indicates the biological significance of these compounds extends throughout the animal kingdom, and possibly beyond. The actions of most PGs are mediated by specific receptors. Biomedical research has discovered a great deal of knowledge about PG receptors in mammals, including their structures, pharmacology, molecular biology and cellular locations. Studies of PG receptors in insects lag behind the biomedical background, however, recent results hold the promise of accelerated research in this area. A PG receptor has been identified in a class of lepidopteran hemocytes and experimentally linked to the release of prophenoloxidase. PGs act in several crucial areas of insect biology. In reproduction, a specific PG, PGE(2), releases oviposition behavior in most crickets and a few other insect species; PGs also mediate events in egg development in some species, which may represent all insects. PGs play major roles in modulating fluid secretion in Malpighian tubules, rectum and salivary glands, although, again, this has been studied in only a few insect species that may represent the Class. Insect immunity is a very complex defense system. PGs and other eicosanoids mediate a large number of immune reactions to infection and invasion. We conclude that research into PGs and their receptors in insects will lead to important advances in our understanding of insect biology.

Successfully resisting a pathogen is rarely costly in Daphnia magna

BACKGROUND

A central hypothesis in the evolutionary ecology of parasitism is that trade-offs exist between resistance to parasites and other fitness components such as fecundity, growth, survival, and predator avoidance, or resistance to other parasites. These trade-offs are called costs of resistance. These costs fall into two broad categories: constitutive costs of resistance, which arise from a negative genetic covariance between immunity and other fitness-related traits, and inducible costs of resistance, which are the physiological costs incurred by hosts when mounting an immune response. We sought to study inducible costs in depth using the crustacean Daphnia magna and its bacterial parasite Pasteuria ramosa.

RESULTS

We designed specific experiments to study the costs induced by exposure to this parasite, and we re-analysed previously published data in an effort to determine the generality of such costs. However, despite the variety of genetic backgrounds of both hosts and parasites, and the different exposure protocols and environmental conditions used in these experiment, this work showed that costs of exposure can only rarely be detected in the D. magna-P. ramosa system.

CONCLUSIONS

We discuss possible reasons for this lack of detectable costs, including scenarios where costs of resistance to parasites might not play a major role in the co-evolution of hosts and parasites.

Fitness correlates of heritable variation in antibody responsiveness in a wild mammal

A functional immune system is important for survival in natural environments, where individuals are frequently exposed to parasites. Yet strong immune responses may have fitness costs if they deplete limited energetic resources or cause autoimmune disease. We have found associations between fitness and heritable self-reactive antibody responsiveness in a wild population of Soay sheep. The occurrence of self-reactive antibodies correlated with overall antibody responsiveness and was associated with reduced reproduction in adults of both sexes. However, in females, the presence of self-reactive antibodies was positively associated with adult survival during harsh winters. Our results highlight the complex effects of natural selection on immune responsiveness and suggest that fitness trade-offs may maintain immunoheterogeneity, including genetic variation in autoimmune susceptibility.

Immunocompetence of the red turpentine beetle, Dendroctonus valens LeConte (Coleoptera: Curculionidae, Scolytinae): variation between developmental stages and sexes in populations in China

Immune defense imposes fitness costs as well as benefits, so organisms should optimize, not maximize, their immune function through their life cycle. We investigated this issue in the red turpentine beetle, Dendroctonus valens LeConte (Coleoptera: Curculionidae, Scolytinae), which is a pine-killing invasive beetle in China, though it is usually considered as a secondary pest in its native range of North America. We hypothesized that pathogen pressure may affect these beetles differently throughout their life history. We measured the insect's immunocompetence throughout life, determining encapsulation ability and phenoloxidase activity in larval stages, pupae and adults. Pupae had the highest encapsulation ability, but encapsulation was not different between final instar larvae and adults. Phenoloxidase (PO) activity was highest in final instar larvae and pupae, followed by the second instar larvae and adults. Total phenoloxidase activity increased significantly from the second instar larval stage to pupae, and then decreased in adults. Although the second instar larvae had the lowest phenoloxidase activity, more than 90% of total PO existed in the hemolymph in the form of the active enzyme, as compared with pupae, in which over 60% of PO occurred as a proenzyme. Both active PO and total PO were much higher in females than in males, though no significant differences were detected between the encapsulation ability of male and female adults. This result suggests the existence of a sexual dimorphism of immunocompetence in D. valens adults. Variations in immunocompetence across developmental stages suggest that D. valens adopts diverse investment strategies in immunocompetence during different stages. Potential reasons for variation in immunocompetence among developmental stages and between the sexes of D. valens are discussed.

The coevolution of virulence: tolerance in perspective

Coevolutionary interactions, such as those between host and parasite, predator and prey, or plant and pollinator, evolve subject to the genes of both interactors. It is clear, for example, that the evolution of pollination strategies can only be understood with knowledge of both the pollinator and the pollinated. Studies of the evolution of virulence, the reduction in host fitness due to infection, have nonetheless tended to focus on parasite evolution. Host-centric approaches have also been proposed—for example, under the rubric of “tolerance”, the ability of hosts to minimize virulence without necessarily minimizing parasite density. Within the tolerance framework, however, there is room for more comprehensive measures of host fitness traits, and for fuller consideration of the consequences of coevolution. For example, the evolution of tolerance can result in changed selection on parasite populations, which should provoke parasite evolution despite the fact that tolerance is not directly antagonistic to parasite fitness. As a result, consideration of the potential for parasite counter-adaptation to host tolerance—whether evolved or medially manipulated—is essential to the emergence of a cohesive theory of biotic partnerships and robust disease control strategies.

Phenoloxidase but not lytic activity reflects resistance against Pasteuria ramosa in Daphnia magna

The field of ecological immunology strongly relies on indicators of immunocompetence. Two major indicators in invertebrates, the activity of phenoloxidase (PO) and lytic activity have recently been questioned in studies showing that, across a natural range of baseline levels, these indicators did not predict resistance against a manipulated challenge with natural parasites. We confirmed this finding by showing that baseline levels of PO and lytic activity in the host Daphnia magna were not related to spore load of the parasite Pasteuria ramosa. Yet, PO levels in infected hosts did predict spore load, indicating PO activity can be useful as an indicator of immunocompetence in this model parasite-host system.

Transgenerational immune priming as cryptic parental care

Eggs are relatively large and can provide offspring with resources that improve their survival. While such maternal effects are common, it has been difficult to imagine what, other than genes, individual offspring could receive from their fathers. The study by Roth et al. (2009a)suggests that we should look more closely. Their experiments show that red flour beetle fathers can transfer specific biochemical information to their offspring, priming their immune system to combat pathogens better. When mothers do the same, the offspring get a double dose of protection. This discovery alerts us to re-evaluate the importance of cryptic parental care.

The effects of age and social interactions on innate immunity in a leaf-cutting ant

Both developmental and environmental factors shape investment in costly immune defences. Social insect workers have different selection pressures on their innate immune system compared to non-social insects because workers do not reproduce and their longevity affects the fitness of relatives. Furthermore, hygienic behavioural defences found in social insects can result in increased survival after fungal infection, although it is not known if there is modulation in physiological immune defence associated with group living vs. solitary living. Here we investigated whether physiological immune defence is affected by both age and the short-term presence or absence of nestmates in the leaf-cutting ant Acromyrmex octospinosus. We predicted that older ants would show immune senescence and that group living may result in prophylactic differences in immune defence compared to solitarily kept ants. We kept old and young workers alone or in nestmate groups for 48h and assayed a key innate immune system enzyme, expressing phenoloxidase (PO) and its stored precursor (proPO), a defence that acts immediately, i.e. it is constitutive. Short-term solitary living did not affect PO or proPO levels relative to group living controls and we found no evidence for immunosenescence in proPO. However, we found a significant increase in active PO in older workers, which is consistent with two non-mutually exclusive explanations: it could be an adaptive response or indicative of immunosenescence. Our results suggest that future studies of immunosenescence should consider both active immune effectors in the body, such as PO, and the stored potential to express immune defences, such as proPO.

Parasitic infection protects wasp larvae against a bacterial challenge

Host antibacterial defense after Strepsiptera parasitization is a complex and rather unexplored topic. The way how these parasites interact with bacteria invading into the host insect during an infection is completely unknown. In the present study we demonstrate that larvae of the paper wasp Polistes dominulus are more efficient at eliminating bacteria when they are parasitized by the strepsipteran insect Xenos vesparum. We looked at the expression levels of the antimicrobial peptide defensin and we screened for the activity of other hemolymph components by using a zone of inhibition assay. Transcription of defensin is triggered by parasitization, but also by mechanical injury (aseptic injection). Inhibitory activity in vitro against the Gram positive bacterium Staphylococcus aureus is not influenced by the presence of the parasite in the wasp or by a previous immune challenge, suggesting a constitutive power of killing this bacterium by wasp hemolymph. Our results suggest either direct involvement of the parasite or that defensin and further immune components not investigated in this paper, for example other antimicrobial peptides, could play a role in fighting off bacterial infections in Polistes.

Prior residency does not always pay off--co-infections in Daphnia

The epidemiological and ecological processes which govern the success of multiple-species co-infections are as yet unresolved. Here we investigated prior versus late residency within hosts, meaning which parasite contacts the host first, to determine if the outcomes of intra-host competition are altered. We infected a single genotype of the waterflea Daphnia galeata with both the intestinal protozoan Caullerya mesnili and the haemolymph fungus Metschnikowia sp. (single genotype of each parasite species), as single infections, simultaneous co-infections and as sequential co-infections, with each parasite given 4 days prior residency. Simultaneous co-infections were significantly more virulent than both single infections and sequential co-infections, as measured by a decreased host life span and fecundity. Further, in addition to the Daphnia host, the parasites also suffered fitness decreases in simultaneous co-infections, as measured by spore production. The sequential co-infections, however, had mixed effects: C. mesnili benefited from prior residency, whereas Metschnikowia sp. experienced a decline in fitness. Our results show that multiple-species co-infections of Daphnia may be more virulent than single infections, and that prior residency does not always provide a competitive advantage.