Sex, war, and disease: the role of parasite infection on weapon development and mating success in a horned beetle (Gnatocerus cornutus)

Comparing ant morphology measurements from microscope and online AntWeb.org 2D z-stacked images

Unprecedented technological advances in digitization and the steadily expanding open-access digital repositories are yielding new opportunities to quickly and efficiently measure morphological traits without transportation and advanced/expensive microscope machinery. A prime example is the AntWeb.org database, which allows researchers from all over the world to study taxonomic, ecological, or evolutionary questions on the same ant specimens with ease. However, the reproducibility and reliability of morphometric data deduced from AntWeb compared to traditional microscope measurements has not yet been tested. Here, we compared 12 morphological traits of 46 Temnothorax ant specimens measured either directly by stereomicroscope on physical specimens or via the widely used open-access software tpsDig utilizing AntWeb digital images. We employed a complex statistical framework to test several aspects of reproducibility and reliability between the methods. We estimated (i) the agreement between the measurement methods and (ii) the trait value dependence of the agreement, then (iii) compared the coefficients of variation produced by the different methods, and finally, (iv) tested for systematic bias between the methods in a mixed modeling-based statistical framework. The stereomicroscope measurements were extremely precise. Our comparisons showed that agreement between the two methods was exceptionally high, without trait value dependence. Furthermore, the coefficients of variation did not differ between the methods. However, we found systematic bias in eight traits: apart from one trait where software measurements overestimated the microscopic measurements, the former underestimated the latter. Our results shed light on the fact that relying solely on the level of agreement between methods can be highly misleading. In our case, even though the software measurements predicted microscope measurements very well, replacing traditional microscope measurements with software measurements, and especially mixing data collected by the different methods, might result in erroneous conclusions. We provide guidance on the best way to utilize virtual specimens (2D z-stacked images) as a source of morphometric data, emphasizing the method's limitations in certain fields and applications.

The Tapeworm Hymenolepis diminuta as an Important Model Organism in the Experimental Parasitology of the 21st Century

The tapeworm Hymenolepis diminuta is a common parasite of the small intestine in rodents but it can also infect humans. Due to its characteristics and ease of maintenance in the laboratory, H. diminuta is also an important model species in studies of cestodiasis, including the search for new drugs, treatments, diagnostics and biochemical processes, as well as its host-parasite interrelationships. A great deal of attention has been devoted to the immune response caused by H. diminuta in the host, and several studies indicate that infection with H. diminuta can reduce the severity of concomitant disease. Here, we present a critical review of the experimental research conducted with the use of H. diminuta as a model organism for over more than two decades (in the 21st century). The present review evaluates the tapeworm H. diminuta as a model organism for studying the molecular biology, biochemistry and immunology aspects of parasitology, as well as certain clinical applications. It also systematizes the latest research on this species. Its findings may contribute to a better understanding of the biology of tapeworms and their adaptation to parasitism, including complex correlations between H. diminuta and invertebrate and vertebrate hosts. It places particular emphasis on its value for the further development of modern experimental parasitology.

Evolution: Natural selection, sexual selection, and the jaws of death

Traits that increase reproductive success in males can have negative fitness consequences in females. A new study shows that natural selection by a predator that targets males with larger secondary sexual traits drives an evolutionary increase in female fitness.

Body size variation in a guild of carrion beetles

Body size is a key biological trait, influencing the biomechanics, physiology, behaviour, and ecology of species. Describing variation in body size within and among co-occurring species within an ecological guild can provide important context for understanding the ecology of species and the structure of ecological communities. Here, we focus on a guild of carrion beetles (Coleoptera: Silphidae) co-occurring in southeastern Ontario, Canada. We examine how body size varies (i) among species, (ii) within species, including among sexes, and (iii) across the active season, contrasting estimates of size based on mass with those based on morphological structure (elytron length). We find that body size varies significantly both within and among species. Five focal species show evidence for sexual dimorphism in size. All focal species show significant seasonal variation in size, but these patterns differ across species, and depend on our estimates of size. Overall, the observed variation in body size is most consistent with widespread environmental constraints on size and (or) diverse selective pressures favouring different sizes within and among species. We discuss possible selective pressures acting on size within and among species; however, the causes and consequences of the variation in body size that we describe remain to be discovered.

Divergent sexual signals reflect costs of local parasites

Many closely related populations are distinguished by variation in sexual signals and this variation is hypothesized to play an important role in reproductive isolation and speciation. Within populations, there is considerable evidence that sexual signals provide information about the incidence and severity of parasite infections, but it remains unclear if variation in parasite communities across space could play a role in initiating or maintaining sexual trait divergence. To test for variation in parasite-associated selection, we compared three barn swallow subspecies with divergent sexual signals. We found that parasite community structure and host tolerance to ecologically similar parasites varied between subspecies. Across subspecies we also found that different parasites were costly in terms of male survival and reproductive success. For each subspecies, the preferred sexual signal(s) were associated with the most costly local parasite(s), indicating that divergent signals are providing relevant information to females about local parasite communities. Across subspecies, the same traits were often associated with different parasites, indicating that parasite-sexual signal links are quite flexible and may evolve relatively quickly. This study provides evidence for (1) variation in parasite communities and (2) different parasite-sexual signal links among three closely related subspecies with divergent sexual signal traits, suggesting that parasites may play an important role in initiating and/or maintaining the divergence of sexual signals among these closely related, yet geographically isolated populations.

Defining an intrasexual male weapon polymorphism in a New Zealand harvestman (Opiliones: Neopilionidae) using traditional and geometric morphometrics

In many species, competition for mates has led to exaggerated male sexually-selected traits. Sexually-selected male weapons are used in male-male combat and include structures like horns, antlers and enlarged teeth. Weapons often vary intraspecifically in size, resulting in either a continuum of weapon sizes or in discrete male polymorphisms. More rarely, complex weapon polymorphisms can also include variation in weapon shape; however, these are difficult to quantify. Here we first use traditional linear morphometrics to describe a weapon trimorphism in the endemic New Zealand harvestman, Forsteropsalis pureoraTaylor, 2013. We identified three male morphs: a small-bodied gamma male with reduced chelicera, a large-bodied beta male with long, slender chelicerae, and a large-bodied alpha male with shorter, but very broad, robust chelicerae. Chelicera length alone failed to fully capture the variation in weapon investment. Using geometric morphometrics, we show that alpha males are different in weapon shape, whereas beta and gamma males have similar weapon shape, but vary in their body size and chelicera length. Additionally, we describe how the chelicerae function during male-male combat from observations of contests. This work demonstrates how combining linear and geometric morphometrics can help to elucidate complex polymorphism.

First Case of Dual Size Asymmetry in an Identical Arthropod Organ: Different Asymmetries of the Combative (Sexual) and Cutting (Non-Sexual) Parts of Mandibles in the Horned Stored-Product Beetle Gnatocerus cornutus (Fabricius, 1798)

Although it is known that separate insect body structures may be asymmetrical within one species, the different functional asymmetries within a single organ as a result of differential selective regimes have not been described. Based on microscopic measurements and SEM photography, we examined the size, shape and asymmetry of the mandibular structures of males and females of the sexually dimorphic broad-horned flour beetle, Gnatocerus cornutus (Tenebrionidae, Coleoptera). It was found that sexual dimorphism only manifests in certain outgrowth parts (horns) of male mandibles, while the remaining cutting parts of the mandibles hold identical morphologies for both sexes. A more interesting finding—since this is the first published case of dual functionally selected asymmetry in an identical arthropod organ—was that the cutting part of the male mandible exhibited directional asymmetry, whereas the outgrowth horn part of the mandible showed a high degree of symmetry. Moreover, there was no relationship between the size and asymmetry of horns. The results indicate different regulatory mechanisms of sexually selected combative horns and the food-functional, more conservative (constrained by hard food and adult long life) cutting parts of mandibles.

Recent advances in understanding the mechanisms of sexually dimorphic plasticity: insights from beetle weapons and future directions

Many traits that are sexually dimorphic, appearing either differently or uniquely in one sex, are also sensitive to an organism's condition. This phenomenon seems to have evolved to limit genetic conflict between traits that are under different selective pressures in each sex. Recent work has shed light on the molecular and developmental mechanisms that govern this condition sensitive growth, and this work has now expanded to encompass both sexual dimorphism as well as conditionally plastic growth, as it seems the two phenomena are linked on a molecular level. In all cases studied the gene doublesex, a conserved regulator of sex differentiation, controls both sexual dimorphism as well as the condition-dependent plastic responses common to these traits. However, the advent of next-generation -omics technologies has allowed researchers to decipher the common and diverged mechanisms of sexually dimorphic plasticity and expand investigations beyond the foundation laid by studies utilizing beetle weapons.

The sociality-health-fitness nexus: synthesis, conclusions and future directions

This theme issue has highlighted the links between sociality, health and fitness in a broad range of organisms, and with approaches that include field and captive studies of animals, comparative and meta-analyses, theoretical modelling and clinical and psychological studies of humans. In this concluding chapter, we synthesize the results of these diverse studies into some of the key concepts discussed in this issue, focusing on risks of infectious disease through social contact, the effects of competition in groups on susceptibility to disease, and the integration of sociality into research on life-history trade-offs. Interestingly, the studies in this issue both support pre-existing hypotheses, and in other ways challenge those hypotheses. We focus on unexpected results, including a lack of association between ectoparasites and fitness and weak results from a meta-analysis of the links between dominance rank and immune function, and place these results in a broader context. We also review relevant topics that were not covered fully in this theme issue, including self-medication and sickness behaviours, society-level defences against infectious disease, sexual selection, evolutionary medicine, implications for conservation biology and selective pressures on parasite traits. We conclude by identifying general open questions to stimulate and guide future research on the links between sociality, health and fitness.

PO-CALC: a novel tool to correct common inconsistencies in the measurement of phenoloxidase activity

A broad range of physiological and evolutionarily studies requires standard and robust methods to assess the strength and activity of an individual's immune defense. In insects, this goal is generally reached by spectrophotometrically measuring (pro-) phenoloxidase activity, an enzymatic and non-specific process activated after wounding and parasite infections. However, the literature surprisingly lacks a standard method to calculate these values from spectrophotometer data and thus to be able to compare results across studies. In this study, we demonstrated that nine methods commonly used to extract phenoloxidase activities (1) provide inconsistent results when tested on the same data sets, at least partly due to their specific sensitivity to the noise regularly present in enzymatic reaction curves. To circumvent this issue, we then (2) developed a novel, free and simple R-based program called PO-CALC and (3) demonstrated the robustness of its calculations for the different types of noises. Overall, we show that PO-CALC corrects overlooked though important inconsistencies in the measurement of phenoloxidase activities, and claim that its broad use would increase the significance and general validity of studies on invertebrate immunity.

Effect of an immune challenge on the functional performance of male weaponry

Theories of parasite-mediated sexual selection predict a positive association between immune function and the expression of sexually selected ornaments. Few studies, however, have investigated how an immune challenge affects the performance of sexually selected weaponry. Male Wellington tree weta (Hemideina crassidens) (Orthoptera: Anostostomatidae) possess enlarged mandibles that are used as weapons in fights for access to females residing in tree galleries. Intense sexual competition appears to have favoured the evolution of alternative male mating strategies in this species as males have a trimorphic phenotype in which weapon size varies across morphotype: 8th instar males have the smallest jaws, 10th instar males have the largest and 9th instar males being intermediate to the other two. After injecting males and females with either lipopolysaccharide (LPS; immune challenge) or saline (control) I measured over a 24h period each weta's body mass to assess whether they responded immunologically to the LPS and their bite force to assess the functional performance of their jaws. Both sexes responded immunologically to the immune-challenge as LPS-injected individuals lost significantly more body mass than saline-injected controls with females losing more mass than males. Female bite force was significantly reduced 8h after LPS-injection whereas male bite force did not significantly decline. Both sexes regained pre-injection functional performance of their jaws 24h after the immune challenge. My results suggest that females trade-off bite force for immune function whereas males do not. This article is part of a Special Issue entitled: insert SI title.

Sexual activity increases resistance against Pseudomonas entomophila in male Drosophila melanogaster

BACKGROUND

Maintenance and deployment cost of immunity is high, therefore, it is expected to trade-off with other high cost traits like sexual activity. Previous studies with Drosophila melanogaster show that male's ability to clear bacteria decreases with increase in sexual activity. We subjected this idea to test using two pathogens (Pseudomonas entomophila and Staphylococcus succinus) and three different populations of Drosophila melanogaster.

RESULTS

We found that sexual activity enhanced male survivorship in a pathogen specific manner. Sexually active males show higher resistance than virgins upon infection with Pseudomonas entomophila. Interestingly, the beneficial effects of sexual activity increased with time of co-habitation with females and declined when access to females was restricted. We observed no change in male survivorship upon experimentally varying the number of sexual interactions.

CONCLUSION

Our results show that the sexual activity-immunity trade-off in males cannot be generalised. The trade-off is potentially mediated through complex interactions between the host, pathogen and the environment experienced by the host.

Hymenolepis diminuta infections in tenebrionid beetles as a model system for ecological interactions between helminth parasites and terrestrial intermediate hosts: a review and meta-analysis

The cestode Hymenolepis diminuta (Cyclophyllidea) uses a variety of insects as its intermediate host, where ingestion of eggs results in development in the hemocoel of a cysticercoid that is infective to a rat definitive host. Species in 2 genera, Tenebrio and Tribolium (Coleoptera: Tenebrionidae) have been used extensively as laboratory intermediate hosts. This review examines experimental studies on ecological aspects of the relationship between H. diminuta and tenebrionid beetles, including the acquisition and establishment of the parasite, host effects on the parasite, and parasite effects on the host. A meta-analysis of infection results from the literature revealed strong relationships across host species and strains between (1) prevalence and intensity of infection, (2) efficiency of cysticercoid production and exposure conditions, and (3) variance in abundance or intensity of infection relative to their respective means. The underlying mechanisms producing these patterns remain elusive. Comparative studies are infrequent, and the use of divergent methodologies hampers comparisons among studies. In spite of these problems, there is much to recommend this as a terrestrial host-parasite model system. It represents those relationships in which mostly minor, but occasionally major, responses to parasitic infection occur, and in which host genetics and environmental conditions can serve as modifying factors. Moreover, this is a tractable experimental system, and is backed by an extensive literature on host biology.

Inter- and intrasexual genetic correlations of exaggerated traits and locomotor activity

Exaggerated traits in males can be costly and therefore can negatively affect fitness. Although these costs are thought to be male specific, traits that have a negative effect due to exaggeration are often shared between the sexes as life-history traits. When there are genetic intersexual correlations for these shared characters, the evolution of the exaggerated traits can impose these costs on nonadorned females through the intersexual correlation. Thus, the exaggerated traits can constrain optimum development of female characters, even if the females lack these exaggerations completely. However, investigation of this pattern has been largely ignored, and thus, it is necessary to investigate genetic architectures of these traits within and across the sexes. Male flour beetles, Gnatocerus cornutus, have enlarged mandibles that are used in male-male competition, but females lack this character completely. Using a traditional full-sib/half-sib breeding design, we detected a negative intrasexual genetic correlation between male weapon size and locomotor activity, but not an intersexual genetic correlation for locomotor activity. After subjecting this weapon to 17 generations of bidirectional selection, we found a correlated response to locomotor activity in the male, whereas there was no correlated response in the female. Our results suggest that the costs of exaggerated traits to locomotion are not imposed on females and would be male specific. This is partly explained by genetic decoupling of locomotor activities across the sexes.

Maternal adjustment of the sex ratio in broods of the broad-horned flour beetle, Gnathocerus cornutus

We report that females of the broad-horned flour beetle, Gnathocerus cornutus, can plastically adjust the sex ratio in their broods in response to environmental quality. Specifically, females reared in nutritionally poor environments produce broods that are 65% female, on average, with the degree of female-bias in some broods approaching 95%. In addition, females reared in nutritionally poor environments lay significantly more eggs than do females reared on standard medium, which produce broods with an even sex ratio. These effects of the mother's environment on size and sex ratio in broods are manifest even when oviposition occurs in the standard nutritional environment; indeed, the degree of female-bias increases with advancing female age despite the availability of nutritional resources to females at the time of egg laying. Our studies rule out sex-specific differences in viability early in larval development as the mechanism for the bias in sex-ratio of broods, since females reared in nutritionally poor environments have broods with hatchability and larval viability comparable to those of nonstressed females. Our studies also rule out an effect of the sire on the sex ratio in broods, since all male mates were reared on standard medium. We discuss our results in the context of theories for the evolution of plastic sex-ratios in the face of environmental deterioration and discuss how plasticity can resolve a long-standing question about the conditions underlying the evolution of biased sex ratios.