The evolution of female-biased genital diversity in bedbugs (Cimicidae)

Rapid genitalia evolution is believed to be mainly driven by sexual selection. Recently, noncopulatory genital functions have been suggested to exert stronger selection pressure on female genitalia than copulatory functions. In bedbugs (Cimicidae), the impact of the copulatory function can be isolated from the noncopulatory impact. Unlike in other taxa, female copulatory organs have no function in egg-laying or waste-product expulsion. Males perform traumatic mating by piercing the female integument, thereby imposing antagonistic selection on females and suspending selection to morphologically match female genitalia. We found the location of the copulatory organ evolved rapidly, changing twice between dorsal and ventral sides, and several times along the anteroposterior and the left-right axes. Male genital length and shape varied much less, did not appear to follow the positional changes seen in females, and showed no evidence for coevolution. Female genitalia position evolved 1.5 times faster than male genital length and shape and showed little neutral or geographic signals. Instead, we propose that nonmorphological male traits, such as mating behavior, may drive female genitalia morphology in this taxon. Models of genitalia evolution may benefit from considering morphological genital responses to nonmorphological stimuli, such as male mating behavior or copulatory position.

Social cues trigger differential immune investment strategies in a non-social insect, Tenebrio molitor

Social immunization (SI) is a horizontal transfer of immunity that protects naive hosts against infection following exposure to infected nestmates. While mainly documented in eusocial insects, non-social species also share similar ecological features which favour the development of group-level immunity. Here, we investigate SI in Tenebrio molitor by pairing naive females with a pathogen-challenged conspecific for 72 h before measuring a series of immune and fitness traits. We found no evidence for SI, as beetles who cohabited with a live pathogen-challenged conspecific were not better protected against bacterial challenge. However, exposure to a heat-killed-bacteria-challenged conspecific appeared to increase pathogen tolerance, which manifested in differential fitness investment. Our results together suggest that T. molitor do respond to immune-related cues in the social environment, despite not showing a classic immunization response as predicted.

Cuticular colour reflects underlying architecture and is affected by a limiting resource

Central to the basis of ecological immunology are the ideas of costs and trade-offs between immunity and life history traits. As a physical barrier, the insect cuticle provides a key resistance trait, and Tenebrio molitor shows phenotypic variation in cuticular colour that correlates with resistance to the entomopathogenic fungus Metarhizium anisopliae. Here we first examined whether there is a relationship between cuticular colour variation and two aspects of cuticular architecture that we hypothesised may influence resistance to fungal invasion through the cuticle: its thickness and its porosity. Second, we tested the hypothesis that tyrosine, a semi-essential amino acid required for immune defence and cuticular melanisation and sclerotisation, can act as a limiting resource by supplementing the larval diet and subsequently examining adult cuticular colouration and thickness. We found that stock beetles and beetles artificially selected for extremes of cuticular colour had thicker less porous cuticles when they were darker, and thinner more porous cuticles when they were lighter, showing that colour co-varies with two architectural cuticular features. Experimental supplementation of the larval diet with tyrosine led to the development of darker adult cuticle and affected thickness in a sex-specific manner. However, it did not affect two immune traits. The results of this study provide a mechanism for maintenance of cuticular colour variation in this species of beetle; darker cuticles are thicker, but their production is potentially limited by resource constraints and differential investments in resistance mechanisms between the sexes.

Traumatic insemination in terrestrial arthropods

Traumatic insemination is a bizarre form of mating practiced by some invertebrates in which males use hypodermic genitalia to penetrate their partner's body wall during copulation, frequently bypassing the female genital tract and ejaculating into their blood system. The requirements for traumatic insemination to evolve are stringent, yet surprisingly it has arisen multiple times within invertebrates. In terrestrial arthropods traumatic insemination is most prevalent in the true bug infraorder Cimicomorpha, where it has evolved independently at least three times. Traumatic insemination is thought to occur in the Strepsiptera and has recently been recorded in fruit fly and spider lineages. We review the putative selective pressures that may have led to the evolution of traumatic insemination across these lineages, as well as the pressures that continue to drive divergence in male and female reproductive morphology and behavior. Traumatic insemination mechanisms and attributes are compared across independent lineages.

Human fine body hair enhances ectoparasite detection

Although we are relatively naked in comparison with other primates, the human body is covered in a layer of fine hair (vellus and terminal hair) at a relatively high follicular density. There are relatively few explanations for the evolutionary maintenance of this type of human hair. Here, we experimentally test the hypothesis that human fine body hair plays a defensive function against ectoparasites (bed bugs). Our results show that fine body hair enhances the detection of ectoparasites through the combined effects of (i) increasing the parasite's search time and (ii) enhancing its detection.

The ontogeny of immunity in the honey bee, Apis mellifera L. following an immune challenge

The honey bee, Apis mellifera, is an ideal system for investigating ontogenetic changes in the immune system, because it combines holometabolous development within a eusocial caste system. As adults, male and female bees are subject to differing selective pressures: worker bees (females) exhibit temporal polyethism, while the male drones invest in mating. They are further influenced by changes in the threat of pathogen infection at different life stages. We investigated the immune response of workers and drones at all developmental phases, from larvae through to late stage adults, assaying both a constitutive (phenoloxidase, PO activity) and induced (antimicrobial peptide, AMP) immune response. We found that larval bees have low levels of PO activity. Adult workers produced stronger immune responses than drones, and a greater plasticity in immune investment. Immune challenge resulted in lower levels of PO activity in adult workers, which may be due to the rapid utilisation and a subsequent failure to replenish the constitutive phenoloxidase. Both adult workers and drones responded to an immune challenge by producing higher titres of AMPs, suggesting that the cost of this response prohibits its constant maintenance. Both castes showed signs of senescence in immune investment in the AMP response. Different sexes and life stages therefore alter their immune system management based on the combined factors of disease risk and life history.

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.

Examining the relationship between hemolymph phenoloxidase and resistance to a DNA virus, Plodia interpunctella granulosis virus (PiGV)

We have a detailed understanding of invertebrate immune responses to bacteria and fungal pathogens, but we know less about how insects respond to virus challenge. Phenoloxidase (PO) functions as an important immune response against many parasites and pathogens and is routinely used as a measure of immune competance. We examine the role of haemolymph PO activity in Plodia interpuncetella's response to its natural granulosis virus (PiGV). Larvae were challenged with virus by both oral inoculation of occluded virus (the natural infection route) and direct intrahaemocoelic injection of budded virus. Haemolymph was collected at time points post-viral challenge using a novel method that allows the volume of haemolymph to be quanitified. The haemolmyph was collected without killing the larvae so that haemolymph samples from individuals that developed viral disease could be distinguished from samples collected from those that fought off infection. The level of haemolymph PO activity in resistant larvae did not differ from control larvae. Therefore we have no evidence that PO is involved in resistance to virus in the haemocoel whether larvae are challenged naturally by oral innoculation or directly by intraheamocoelic injection. Phenoloxidase may therefore not be a relevant metric of immunocompetence for viral infection.

Bacteriolytic activity in the ejaculate of an insect

The rapid evolution of ejaculate components is considered to be largely driven by sexual selection. Less attention has been paid to the fact that sperm and microorganisms frequently meet; we consequently predict selection for substances that protect a male's ejaculate. We report, for the first time, bacteriolytic activity (lysozyme-like immune activity [LLA]) in the ejaculate of an animal, the common bedbug Cimex lectularius. We also show that in almost half the males LLA in the seminal fluid exceeded LLA in the hemolymph. We detected no antimicrobial peptide activity in seminal fluid. Because lysozymes degrade only bacteria, our results suggest that sperm-microbe interactions are probably important in the evolution of ejaculate components and thereby provide a route for natural selection to account for some of the diversity of seminal components.

Sensitivity to bites by the bedbug, Cimex lectularius

Bedbugs are a public health problem and can cause significant economic losses, but little is known about the effects of bites on humans. We reviewed case reports and published papers on bedbug bites to assess the empirical basis of the commonly cited figure that only approximately 80% of the population are sensitive to bedbug bites. We found the sensitivity estimate to be based on only one study carried out 80 years ago. However, this study did not account for the now well-established fact that only repeated exposure to external allergens leads to skin reactions. In our sample, 18 of 19 persons showed a skin reaction after bedbug exposure, but in most cases only after repeated controlled exposure. With repeated exposure, the latency between bite and skin reactions decreased from approximately 10 days to a few seconds. Our results are relevant for the hospitality industry, where apparently increasing infestation rates are likely to lead to an increase in the number of tourists and hotel employees exposed to bedbugs. Medical and public health professionals may expect to see an increase in the prevalence of people with bedbug bite sensitivity. The significance of the delayed reaction time of skin to bites may also have implications in litigation cases where people seek compensation.

Introduction. Ecological immunology

An organism's fitness is critically reliant on its immune system to provide protection against parasites and pathogens. The structure of even simple immune systems is surprisingly complex and clearly will have been moulded by the organism's ecology. The aim of this review and the theme issue is to examine the role of different ecological factors on the evolution of immunity. Here, we will provide a general framework of the field by contextualizing the main ecological factors, including interactions with parasites, other types of biotic as well as abiotic interactions, intraspecific selective constraints (life-history trade-offs, sexual selection) and population genetic processes. We then elaborate the resulting immunological consequences such as the diversity of defence mechanisms (e.g. avoidance behaviour, resistance, tolerance), redundancy and protection against immunopathology, life-history integration of the immune response and shared immunity within a community (e.g. social immunity and microbiota-mediated protection). Our review summarizes the concepts of current importance and directs the reader to promising future research avenues that will deepen our understanding of the defence against parasites and pathogens.

Antimicrobial defense and persistent infection in insects

During 400 million years of existence, insects have rarely succumbed to the evolution of microbial resistance against their potent antimicrobial immune defenses. We found that microbial clearance after infection is extremely fast and that induced antimicrobial activity starts to increase only when most of the bacteria (99.5%) have been removed. Our experiments showed that those bacteria that survived exposure to the insect's constitutive immune response were subsequently more resistant to it. These results imply that induced antimicrobial compounds function primarily to protect the insect against the bacteria that persist within their body, rather than to clear microbial infections. These findings suggest that understanding of the management of antimicrobial peptides in natural systems might inform medical treatment strategies that avoid the risk of drug resistance.

Situation exploitation: higher male mating success when female resistance is reduced by feeding

Optimal male and female mating rates rarely coincide. Males often shift the rate in their favor by either increased signaling and by overcoming female resistance to copulation. The concept of sensory exploitation posits that males produce signals that mimic naturally selected benefits and so deceitfully attract females. However, males also have to overcome female resistance to actual copulation. Males may do so by copulating during situations when the female's ability to resist is decreased because of competing naturally selected demands. Males of the common bedbug, Cimex lectularius, an obligate blood feeder, mate at a rate, and in a manner that is harmful to females. Females have to feed regularly to produce eggs, and during feeding female body volume increases by 300%. Choice trials using unfed and either fed or experimentally enlarged but unfed females showed that the increased postfeeding body volume of females attracted more male mating attempts, strongly reduced female resistance to male mating attempts and resulted in a net increase in female mating rate. Our results, therefore, suggest that males have increased mating success in a situation that females cannot avoid because it is naturally selected. Such "situation exploitation" of low resistance may be a common phenomenon.

Temporal patterns in immune responses to a range of microbial insults (Tenebrio molitor)

Much work has elucidated the pathways and mechanisms involved in the production of insect immune effector systems. However, the temporal nature of these responses with respect to different immune insults is less well understood. This study investigated the magnitude and temporal variation in phenoloxidase and antimicrobial activity in the mealworm beetle Tenebrio molitor in response to a number of different synthetic and real immune elicitors. We found that antimicrobial activity in haemolymph increased rapidly during the first 48h after a challenge and was maintained at high levels for at least 14 days. There was no difference in the magnitude of responses to live or dead Escherichia coli or Bacillus subtilis. While peptidoglylcan also elicited a long-lasting antimicrobial response, the response to LPS was short lived. There was no long-lasting upregulation of phenoloxidase activity, suggesting that this immune effector system is not involved in the management of microbial infections over a long time scale.

Estimating the mean abundance and feeding rate of a temporal ectoparasite in the wild: Afrocimex constrictus (Heteroptera: Cimicidae)

The feeding frequency of blood-feeding invertebrates in the wild is largely unknown but is an important predictor for the potential of disease transmission and for estimating the effects blood feeding may have on the host population. We present a method to estimate the mean feeding frequency per individual parasite from the frequency distribution of fed and unfed individuals in the wild. We used three populations of the cimicid species, Afrocimex constrictus, that parasitises the fruit bat Rousettus aegyptiacus. We found that the area occupied by a bug refugium was a good predictor of the number of bugs in that refugia. The estimated parasite population sizes ranged from ca. 25,000 to 3 million bugs. Their mean abundance was 1-15 bugs per host individual. Preventing feeding by bugs in their natural habitat showed that bugs took approximately 20 days to return to an unfed stage. A formula is presented by which the distribution of digestion stages in the samples was used to calculate that A. constrictus feeds approximately every 7-10 days. The dry weight of a full blood meal was approximated as 13.3 mg. Therefore A. constrictus is estimated to draw an average of 1-28 microL blood per host per day. We suggest that any of our methods can be adjusted to be used in other haematophagous insects to estimate host and parasite population size, mean parasite abundance and blood meal size as well as mean feeding frequency in the wild, including the bed bug species that parasitise humans.

Self-harm caused by an insect's innate immunity

It has been a long-held assumption that the innate immune system of insects causes self-harm when used to combat an immune insult. We show empirically that this assumption is correct. Invertebrate innate immunity relies heavily on effector systems which, on activation, produce cytotoxins that kill pathogens. Reliance on these robust, fast-acting, generic killing mechanisms ensures a potent and rapid response to pathogen invasion, but has the potential disadvantage of causing self-damage. We show that the innate immune response against an immune insult produces measurable phenotypic and functional damage to self-tissue in the beetle Tenebrio molitor. This type of self-harm (autoreactivity) and the life-history implications that arise from it are important to understand evolutionary phenomena such as the dynamics between hosts and parasites as well as the nature of immune system costs.

Biology of the bed bugs (Cimicidae)

The cimicids, or bed bugs, belong to a highly specialized hematophagous taxon that parasitizes primarily humans, birds, and bats. Their best-known member is the bed bug, Cimex lectularius. This group demonstrates some bizarre but evolutionarily important biology. All members of the family Cimicidae show traumatic insemination and a suite of female adaptations to this male trait. Cimicids therefore constitute an ideal model system for examining the extreme causes and consequences of sexual selection. Our dual goal in re-examining the extensive literature on this group is to identify issues relevant to pest control, such as dispersal ecology and the recent global spread, and to understand the selective forces that have shaped the unique aspects of this insect's biology.

Functional consequences of blood clotting in insects

Recent in vitro studies have revealed several important aspects of the biochemical and cellular processes involved in insect blood clotting. However, in vivo empirical studies of the functional consequences of clotting are lacking, despite the role of coagulation in wound-healing, preventing infection, and its homology with vertebrate wound repair. Here we present results of the in vivo effects of haemolymph coagulation and its consequences on the spatial disposition of immune activity, in the American cockroach Periplaneta americana. Our results demonstrate that clotting: (1) localises immune effectors in the vicinity of a breach of the cuticle; (2) restricts the spread of invasive particles across the haemocoel, and (3) is greater when wounding is associated with non-self. Our results demonstrate that haemolymph coagulation has major functional consequences, the most important of which is the compartmentalisation of the open haemocoel.

Modulation of sexual signalling by immune challenged male mealworm beetles (Tenebrio molitor, L.): evidence for terminal investment and dishonesty

Organisms partition resources into life-history traits in order to maximise fitness over their expected lifespan. For the males of many species fitness is determined by qualitative and quantitative aspects of costly sexual signals: The notion that epigamic traits are costly forms the cornerstone of those theories that propose parasites drive sexual selection. Consequently studies examining this notion assume sexual signalling is honest (i.e. driven by cost) when they seek to identify correlations or causal links between male immune function and attractiveness. We demonstrate that immune challenged males of the mealworm beetle, Tenebrio molitor, increased their investment in epigamic pheromone signals: these males became significantly more attractive to females whilst increasing the activity of a key immune effector system. In other words males increase terminal reproductive effort (invest in attractiveness) in response to a survival threat (immune insult). Consequently the signal preferred by the female is dishonest when considering the male's condition.

Trauma, disease and collateral damage: conflict in cimicids

The bed bugs and bat bugs (Hemiptera: Cimicidae) are unusual in being a gonochorist (separate male and female genders) taxon with obligate traumatic insemination. Males of all the species in this family have a lanceolate paramere (intromittent organ) which they use to pierce the female's body wall and inseminate directly into her haemocoel, despite the presence of a functional female genital tract. Mating is tightly linked to the feeding cycle in Cimex lectularius, the common bed bug. In this paper, I examine key aspects of the reproductive anatomy and behaviour of C. lectularius that underpin the nature of the conflict over mating rate in this species. I then examine the consequences of traumatic insemination for female fitness and examine potential mechanisms that might underpin those costs. Finally, the collateral consequences of the male reproductive tactic on other males of C. lectularius and the African bat bug, Afrocimex constrictus are examined.

Immune function and parasite resistance in male and polymorphic female Coenagrion puella

Background

Colour polymorphisms are widespread and one of the prime examples is the colour polymorphism in female coenagrionid damselflies: one female morph resembles the male colour (andromorph) while one, or more, female morphs are described as typically female (gynomorph). However, the selective pressures leading to the evolution and maintenance of this polymorphism are not clear. Here, based on the hypothesis that coloration and especially black patterning can be related to resistance against pathogens, we investigated the differences in immune function and parasite resistance between the different female morphs and males.

Results

Our studies of immune function revealed no differences in immune function between the female morphs but between the sexes in adult damselflies. In an experimental infection females infected shortly after emergence showed a higher resistance against a fungal pathogen than males, however female morphs did not differ in resistance. In a field sample of adult damselflies we did not find differences in infection rates with watermites and gregarines.

Conclusion

With respect to resistance and immune function 'andromorph' blue females of Coenagrion puella do not resemble the males. Therefore the colour polymorphism in coenagrionid damselflies is unlikely to be maintained by differences in immunity.

Immune function responds to selection for cuticular colour in Tenebrio molitor

Cuticular colour in the mealworm beetle (Tenebrio molitor) is a quantitative trait, varying from tan to black. Population level variation in cuticular colour has been linked to pathogen resistance in this species and in several other insects: darker individuals are more resistant to pathogens. Given that cuticular colour has a heritable component, we have taken an experimental evolution approach: we selected 10 lines for black and 10 lines for tan adult cuticular phenotypes over at least six generations and measured the correlated responses to selection in a range of immune effector systems. Our results show that two immune parameters related to resistance (haemocyte density and pre-immune challenge activity of phenoloxidase (PO)) were significantly higher in selection lines of black beetles compared to tan lines. This may help to explain increased resistance to pathogens in darker individuals. Cuticular colour is dependent upon melanin production, which requires the enzyme PO that is present in its inactive form inside haemocytes. Thus, the observed correlated response to selection upon cuticular colour and immune variables probably results from these traits' shared dependence on melanin production.

An advantage for young sperm in the house cricket Acheta domesticus

We show that males of the house cricket Acheta domesticus regularly expel sperm packages (spermatophores) independently of copulation and at a rate that is not affected by the presence of females. We then show for the first time that the age of sperm affects their likelihood of being stored by females after copulation; younger sperm were overrepresented in the female sperm storage organ and therefore in the sperm population used for fertilization. Our results suggest that the reproductive success of males may increase if they deliver ejaculates with young sperm, and the results may explain why the males of several species are regularly observed to discard ejaculates. Our results also suggest that phenomena such as female multiple mating, paternity bias, and/or exaggerated ejaculate sizes may be related to the advantage both genders gain by using young sperm.

Selection on insect immunity in the wild

The strength of selection on immune function in wild populations has only been examined in a few vertebrate species. We report the results from a study measuring selection on a key insect immune enzyme, phenoloxidase (PO), in a wild population of the damselfly Calopteryx xanthostoma. We followed individually marked males from the pre-reproductive adult phase and recorded their lifetime mating success. We found positive selection on PO activity in response to an immune insult, but no selection on wing-spot quality, a trait actively displayed to females during courtship. We suggest that positive selection on PO activity in the year of study may be explained by annual fluctuations in parasite loads.

Dietary glycerol and adult access to water: effects on fecundity and longevity in the almond moth

The quality of food eaten by larval insects will affect traits such as gamete production, fat reserves, muscle bulk and body size in the adult. Moreover, larvae also depend on high moisture content in the diet for survival. The almond moth (Ephestia cautella) (W.) (Lepidoptera; Pyralidae) does not feed as an adult although it continues to drink water. We tested the idea that an almond moth could compensate for a low-water diet as a larva by increasing its water intake as an adult. We reared larvae on two different food sources with different moisture regimes; standard laboratory diet with glycerol (relatively wet) and standard diet without glycerol (relatively dry). Half the adult moths from each treatment were given water to drink before their first and only mating. Our results show that wet larval diets (i.e. containing glycerol) significantly decreased fecundity (total number of eggs laid and the proportion of hatched larvae), whilst it significantly increased male and female longevity. The interaction effect of water access for adult males and females was significant, independent of the glycerol in the larval diet. Longevity in females that were not presented with water as adults was slightly higher if mated with a male that had had access to water, suggesting a mating donation of water. However, females that received water as adults showed a decreased longevity if mated with a male who had also had access to water as an adult, indicating a negative effect of water if received by both males and females. In addition, when the larval diet included glycerol, increased number of eggs laid decreased female longevity, whilst an absence of glycerol in the larval diet resulted in low female longevity that was unlinked with fecundity. Glycerol is used in many artificial insect diets and the fact that it shows a strong effect on key life-history traits (reproductive output and longevity in this species), merits careful re-examination of its effects on these important traits in other laboratory models. We also discuss the possibility that larval diet can affect female reproductive decisions.

Adaptive innate immunity? Responsive-mode prophylaxis in the mealworm beetle, Tenebrio molitor

A primary infection by a parasite may indicate a higher risk of being reinfected in the near future (since infection may indicate that enemies are becoming more abundant). Acquired immunity does not exist in invertebrates despite the fact that they also face increased risks of reinfection following primary exposure. However, when subjected to immune insult, insects can produce immune responses that persist for long enough to provide prophylaxis. Because these immune responses are costly, persistence must be maintained through a selective advantage. We tested for the possibility that these long-lasting immune responses provided increased resistance to later infections by experimentally mimicking a primary immune insult (pre-challenge) in larvae of the mealworm beetle, Tenebrio molitor, with lipopolysaccharides (LPS) prior to early or late exposure to spores of the entomopathogenic fungus Metarhizium anisopliae. We found that pre-challenged larvae produced a long-lasting antimicrobial response, which provided a survival benefit when the larvae were exposed to fungal infection. These results suggest that the observed response is functionally "adaptive".

Reducing a cost of traumatic insemination: female bedbugs evolve a unique organ

The frequent wounding of female bedbugs (Cimex lectularius: Cimicidae) during copulation has been shown to decrease their fitness, but how females have responded to this cost in evolutionary terms is unclear. The evolution of a unique anatomical structure found in female bedbugs, the spermalege, into which the male's intromittent organ passes during traumatic insemination, is a possible counteradaptation to harmful male traits. Several functions have been proposed for this organ, and we test two hypotheses related to its role in sexual conflict. We examine the hypotheses that the spermalege functions to (i) defend against pathogens introduced during traumatic insemination; and (ii) reduce the costs of wound healing during traumatic insemination. Our results support the 'defence against pathogens' hypothesis, suggesting that the evolution of this unique cimicid organ resulted, at least partly, from selection to reduce the costs of mating-associated infection. We found no evidence that the spermalege reduces the costs of wound healing.

Examining costs of induced and constitutive immune investment in Tenebrio molitor

Central to the conceptual basis of ecological immunity is the notion that immune effector systems are costly to produce, run, and/or maintain. Using the mealworm beetle, Tenebrio molitor, as a model we investigated two aspects of the costs of innate immunity. We conducted an experiment designed to identify the cost of an induced immune response, and the cost of constitutive investment in immunity, as well as potential interactions. The immune traits under consideration were the encapsulation response and prophylactic cuticular melanization, which are mechanistically linked by the melanin-producing phenoloxidase cascade. If immunity is costly, we predicted reduced longevity and/or fecundity as a consequence of investment in either immune trait. We found a measurable longevity cost associated with producing an inducible immune response (encapsulation). In contrast to other studies, this cost was expressed under ad libitum feeding conditions. We found no measurable costs for constitutive investment in immunity (prophylactic investment in cuticular colour).

Invertebrate ecological immunology

Ecological immunology is a rapidly expanding field that examines the causes and consequences of variation in immune function in the context of evolution and of ecology. Millions of invertebrate species rely solely on innate immunity, compared with only 45,000 vertebrate species that rely additionally on an acquired immune system. Despite this difference in diversity, most studies of ecological immunology focus on vertebrates. Here we review recent progress derived largely from the mechanistic analysis of invertebrate innate immunity. Using this empirical base, we pose general questions in areas that are of central importance for the development of ecological immunology.

A matter of taste: direct detection of female mating status in the bedbug

Males of the bedbug, Cimex lectularius, traumatically inseminate females by inserting a needle-like intromittent organ (penis) through the female's abdominal wall after she has fed. We demonstrate that: (i) mating duration determines ejaculate size; (ii) a female's first copulation in a bout of copulations always lasts longer than subsequent copulations; (iii) the intromittent organ bears sensillae; (iv) males use their intromittent organ to 'taste' whether their current mate has recently copulated; and (v) the consequence of detecting female mating status is the reduction of copulation duration and ejaculate size. We discuss why male bedbugs might show this pattern of ejaculate-size adjustment.

Copulation corrupts immunity: a mechanism for a cost of mating in insects

There are well documented costs of mating in insects but little evidence for underlying mechanisms. Here, we provide experimental evidence for a hormone-based mechanism that reduces immunity as a result of mating. We examined the mealworm beetle Tenebrio molitor and show that (i) mating reduces a major humoral immune effector-system (phenoloxidase) in both sexes, and (ii) that this down-regulation is mediated by juvenile hormone. Because both juvenile hormone and phenoloxidase have highly conserved functions across all insects, the identified mechanism is similarly likely to be highly conserved. The positive physiological function of mating-induced juvenile hormone secretion is gamete and accessory gland production: we propose that its negative effects on immune function are the consequence of physiological antagonism. Therefore, we have identified a physiological tradeoff between mating and immunity. Our results suggest that increasing mating success can result in increasing periods of immune suppression, which in turn implies that reproductively successful individuals may be more vulnerable to infection by, and the negative fitness effects of, pathogens.

Traumatic insemination and sexual conflict in the bed bug Cimex lectularius

The bed bug, Cimex lectularius, has a unique mode of copulation termed "traumatic" insemination [Carayon, J. (1966) in Monograph of the Cimicidae, ed. Usinger, R. (Entomol. Soc. Am., Philadelphia), pp. 81-167] during which the male pierces the female's abdominal wall with his external genitalia and inseminates into her body cavity [Carayon, J. (1966) in Monograph of the Cimicidae, ed. Usinger, R. (Entomol. Soc. Am., Philadelphia), pp. 81-167]. Under controlled natural conditions, traumatic insemination was frequent and temporally restricted. We show for the first time, to our knowledge, that traumatic insemination results in (i) last-male sperm precedence, (ii) suboptimal remating frequencies for the maintenance of female fertility, and (iii) reduced longevity and reproductive success in females. Experimental females did not receive indirect benefits from multiple mating. We conclude that traumatic insemination is probably a coercive male copulatory strategy that results in a sexual conflict of interests.

A mechanistic link between parasite resistance and expression of a sexually selected trait in a damselfly

This paper examines a field-based insect system in which a signal trait and an immune effector system responsible for parasite resistance rely on the same melanin-producing enzyme cascade (phenoloxidase, PO). Observations and experiments on males of the calopterygid damselfly Calopteryx splendens xanthostoma revealed that resistance to the prevalent parasite in the study system (a eugregarine protozoan infecting the mid-gut) was correlated with quantitative aspects of the sexually dimorphic melanized wingspot of males, a trait that is produced and fixed before the host comes into contact with the sporozsoites of the parasite. Regulation of PO during experimental immune challenge showed that males with dark, homogenous melanin distribution in their wings showed no change in PO levels 24 h after challenge. By contrast males with lighter and/or more heterogenous melanin distribution in their wings tended to show higher PO levels 2-h after immune challenge. The changes in PO levels occur despite the lack of a relationship between wing-pigment distribution and the cellular encapsulation response. These results suggest a shared, limiting resource may form the mechanistic basis of the trade-off between a condition-dependent signal trait and immune function in this system.

Male calling song provides a reliable signal of immune function in a cricket

A critical prediction of the immunocompetence handicap hypothesis is that the expression of secondary sexual traits should be positively correlated with pathogen resistance ability This correlation is necessary if females are to be able to use a particular sexual trait as an indicator of a male's resistance ability. In this study we document a positive correlation between a sexually selected component of the calling song of male house crickets (the number of syllables per chirp) and haemocyte load, an important determinant of the ability to encapsulate pathogens in insects. The results indicate that, by favouring males which produce more syllables per chirp, females may also select males with higher pathogen resistance ability, potentially generating either direct or indirect selection on female mating preferences.

Density-dependent prophylaxis in the mealworm beetle Tenebrio molitor L. (Coleoptera: Tenebrionidae): cuticular melanization is an indicator of investment in immunity

If there are costs involved with the maintenance of pathogen resistance, then higher investment in this trait is expected when the risk of pathogenesis is high. One situation in which the risk of pathogenesis is elevated is at increased conspecific density. This paper reports the results of a study of density-dependent polyphenism in pathogen resistance and immune function in the mealworm beetle Tenebrio molitor. Beetles reared at high larval densities showed lower mortality when exposed to a generalist entomopathogenic fungus and a higher degree of cuticular melanization than those reared solitarily. The degree of cuticular melanization was a strong indicator of resistance, with darker beetles being more resistant than lighter ones regardless of rearing density. No differences were found between rearing densities in the levels of phenoloxidase, an enzyme key to the insect immune response. The results show that pathogen resistance is phenotypically plastic in T. molitor, suggesting that the maintenance of this trait is costly.

Last male sperm precedence in a damselfly demonstrated by RAPD profiling

We used the random amplified polymorphic DNA technique to determine last male sperm precedence (P2) in the damselfly Calopteryx splendens xanthostoma (Charpentier). We amplified DNA from mothers, putative fathers and from the embryos of individual offspring, and subsequently calculated band-matching coefficients between known first-order relatives (offspring within a clutch) and non-relatives (mothers and fathers) to estimate last-male paternity. The data indicate that, as in other Calopterygidae, P2 is high (0.98) in the bout of oviposition immediately following copulation, despite the fact that the males of this species do not completely remove the sperm of previous males (Siva-Jothy & Hooper 1995).

Unravelling the components that underlie insect reproductive traits using a simple molecular approach

Applications of RAPD technology to questions of paternity and maternity in studies of insect reproductive traits are discussed. We present three case studies where RAPD fingerprinting reveals levels of complexity in insect systems that help us to understand the causal mechanisms underlying observed behaviour. Finally, we consider ways in which RAPD data can be analysed to generate information about kinship.