Exposure to sexual signals during rearing increases immune defence in adult field crickets

Temporal genomics in Hawaiian crickets reveals compensatory intragenomic coadaptation during adaptive evolution

Theory predicts that compensatory genetic changes reduce negative indirect effects of selected variants during adaptive evolution, but evidence is scarce. Here, we test this in a wild population of Hawaiian crickets using temporal genomics and a high-quality chromosome-level cricket genome. In this population, a mutation, flatwing, silences males and rapidly spread due to an acoustically-orienting parasitoid. Our sampling spanned a social transition during which flatwing fixed and the population went silent. We find long-range linkage disequilibrium around the putative flatwing locus was maintained over time, and hitchhiking genes had functions related to negative flatwing-associated effects. We develop a combinatorial enrichment approach using transcriptome data to test for compensatory, intragenomic coevolution. Temporal changes in genomic selection were distributed genome-wide and functionally associated with the population's transition to silence, particularly behavioural responses to silent environments. Our results demonstrate how 'adaptation begets adaptation'; changes to the sociogenetic environment accompanying rapid trait evolution can generate selection provoking further, compensatory adaptation.

Sex-specific associations between life-history traits and a novel reproductive polymorphism in the Pacific field cricket

Associations between heritable polymorphisms and life-history traits, such as development time or reproductive investment, may play an underappreciated role in maintaining polymorphic systems. This is because selection acting on a particular morph could be bolstered or disrupted by correlated changes in life history or vice versa. In a Hawaiian population of the Pacific field cricket (Teleogryllus oceanicus), a novel mutation (flatwing) on the X-chromosome is responsible for a heritable polymorphism in male wing structure. We used laboratory cricket colonies fixed for male wing morph to investigate whether males and females bearing the flatwing or normal-wing (wild-type) allele differed in their life-history traits. We found that flatwing males developed faster and had heavier testes than normal-wings, whereas flatwing homozygous females developed slower and had lighter reproductive tissues than normal-wing homozygous females. Our results advance our understanding of the evolution of polymorphisms by demonstrating that the genetic change responsible for a reproductive polymorphism can also have consequences for fundamental life-history traits in both males and females.

Immunogenetic and tolerance strategies against a novel parasitoid of wild field crickets

Among the parasites of insects, endoparasitoids impose a costly challenge to host defenses because they use their host's body for the development and maturation of their eggs or larvae, and ultimately kill the host. Tachinid flies are highly specialized acoustically orienting parasitoids, with first instar mobile larvae that burrow into the host's body to feed. We investigated the possibility that Teleogryllus oceanicus field crickets employ postinfestation strategies to maximize survival when infested with the larvae of the parasitoid fly Ormia ochracea. Using crickets from the Hawaiian Islands of Kauai, where the parasitoid is present, and crickets from the Cook Islands (Mangaia), where the parasitoid is absent, we evaluated fitness consequences of infestation by comparing feeding behavior, reproductive capacity, and survival of males experimentally infested with O. ochracea larvae. We also evaluated mechanisms underlying host responses by comparing gene expression in crickets infested with fly larvae for different lengths of time with that of uninfested control crickets. We observed weak population differences in fitness (spermatophore production) and survival (total survival time postinfestation). These responses generally did not show an interaction between population and the number of larva hosts carried or by host body condition. Gene expression patterns also revealed population differences in response to infestation, but we did not find evidence for consistent differences in genes associated with immunity or stress response. One possibility is that any postinfestation evolved resistance does not involve genes associated with these particular functional categories. More likely, these results suggest that coevolution with the fly does not strongly select for either postinfestation resistance or tolerance of parasitoid larvae in male crickets.

Dim artificial light at night reduces the cellular immune response of the black field cricket, Teleogryllus commodus

A functioning immune system is crucial for protection against disease and illness, yet increasing evidence suggests that species living in urban areas could be suffering from immune suppression, due to the presence of artificial light at night (ALAN). This study examined the effects of ecologically relevant levels of ALAN on three key measures of immune function (haemocyte concentration, lytic activity, and phenoloxidase activity) using a model invertebrate species, the Australian black field cricket, Teleogryllus commodus. We reared crickets under an ecologically relevant daily light-cycle consisting of 12 hr bright daylight (2600 lx) followed by either 12 h darkness (0 lx) or dim environmentally relevant ALAN (1, 10, 100 lx), and then assessed immune function at multiple time points throughout adult life using haemolymph samples. We found that the presence of ALAN had a clear negative effect on haemocytes, while the effects on lytic activity and phenoloxidase activity were more complex or largely unaffected by ALAN. Furthermore, the effects of lifelong exposure to ALAN of 1 lx were comparable to those of 10 and 100 lx. Our data suggest that the effects of ALAN could be large and widespread, and such reductions in the core immune response of individuals will likely have greater consequences for fitness and survival under more malign conditions, such as those of the natural environment.

Divergent neurogenomic responses shape social learning of both personality and mate preference

Behavior plays a fundamental role in shaping the origin and fate of species. Mating decisions can act to promote or restrict gene flow, as can personality traits that influence dispersal and niche use. Mate choice and personality are often both learned and therefore influenced by an individual's social environment throughout development. Likewise, the molecular pathways that shape these behaviors may also be co-expressed. In this study on swordtail fish (Xiphophorus birchmanni), we show that female mating preferences for species-typical pheromone cues are entirely dependent on social experience with adult males. Experience with adults also shapes development along the shy-bold personality axis, with shy behaviors arising from exposure to risk-averse heterospecifics as a potential stress-coping strategy. In maturing females, conspecific exposure results in a strong upregulation of olfaction and vision genes compared with heterospecific exposure, as well as immune response genes previously linked to anxiety, learning and memory. Conversely, heterospecific exposure involves an increased expression of genes important for neurogenesis, synaptic plasticity and social decision-making. We identify subsets of genes within the social decision-making network and with known stress-coping roles that may be directly coupled to the olfactory processes females rely on for social communication. Based on these results, we conclude that the social environment affects the neurogenomic trajectory through which socially sensitive behaviors are learned, resulting in adult phenotypes adapted for specific social groupings.

The role of sex and temperature in melanin-based immune function

Sex differences in immunity have been observed across a wide range of species. Still, it remains unclear how sex-specific interactions with the environment are linked to sex differences in immunity. We studied the plasticity of immunological sex differences by focusing on melanin-based traits in the Pacific field cricket (Teleogryllus oceanicus (Le Guillou, 1841)). Insects rely on the pigment melanin for both immune function and coloration of the cuticle; therefore, changes in melanin production for one of these traits may indirectly affect the other. Male crickets use melanized wing structures to chirp. These cuticular structures are missing in females and a songless male morph. Given that the thermal environment influences cuticle melanization, we investigated the interactive effects of sex and developmental temperature on melanin-based immunity. Both immunity and wing cuticle melanism were reduced in individuals that developed under warmer temperatures. Rearing temperature also mediated the extent to which the sexes differed in immune traits. Males had darker cuticles, whereas females expressed greater immune activity, suggesting that sex-specific investment in melanin corresponds with sex differences in immunity. However, the lack of immunological differences between the two male morphs does not support the hypothesis that investment in cuticle melanism affects investment in immunity.

Socially cued seminal fluid gene expression mediates responses in ejaculate quality to sperm competition risk

There is considerable evidence that males will increase the number of sperm ejaculated in response to sperm competition risk. However, whether they have the capacity to adjust seminal fluid components of the ejaculate has received less attention. Male crickets (Teleogryllus oceanicus) have been shown to adjust the viability of sperm in their ejaculate in response to sperm competition risk. Here we show that socially mediated plasticity in sperm viability is probably due, at least in part, to male adjustments in the protein composition of the seminal fluid. Seven seminal fluid protein genes were found to have an increased expression in males exposed to rival calls. Increased expression of these genes was correlated with increased sperm viability in whole ejaculates, and gene knockdown confirmed that at least one of these proteins promotes sperm viability. Our results lend support for recent theoretical models that predict complex responses in male allocation to seminal fluid composition in response to sperm competition risk.

Environmental Adaptations, Ecological Filtering, and Dispersal Central to Insect Invasions

Insect invasions, the establishment and spread of nonnative insects in new regions, can have extensive economic and environmental consequences. Increased global connectivity accelerates rates of introductions, while climate change may decrease the barriers to invader species' spread. We follow an individual-level insect- and arachnid-centered perspective to assess how the process of invasion is influenced by phenotypic heterogeneity associated with dispersal and stress resistance, and their coupling, across the multiple steps of the invasion process. We also provide an overview and synthesis on the importance of environmental filters during the entire invasion process for the facilitation or inhibition of invasive insect population spread. Finally, we highlight important research gaps and the relevance and applicability of ongoing natural range expansions in the context of climate change to gain essential mechanistic insights into insect invasions.

When do trade-offs occur? The roles of energy constraints and trait flexibility in bushcricket populations

In many animal species, the expression of sexually selected traits is negatively correlated with traits associated with survival such as immune function, a relationship termed a 'trade-off'. But an alternative in which sexually selected traits are positively correlated with survival traits is also widespread. We propose that the nature of intertrait relationships is largely determined by overall energy expenditure, energy availability and trait flexibility, with trade-offs expected when individuals are subject to energy constraints. We tested this hypothesis in Ephippiger diurnus, a European bushcricket in which males are distinguished by two prominent sexually selected traits, acoustic calls and a large spermatophore transferred to the female at mating, and where immune function may be critical in survival. Ephippiger diurnus are distributed as small, isolated populations that are differentiated genetically and behaviourally. We analysed songs, spermatophores and the immune function in male individuals from eight populations spanning a range of song types. As predicted, we only found trade-offs in those populations that expended more energy on song and were less flexible in their ability to adjust that expenditure. Ultimately, energy constraints and resulting trade-offs may limit the evolution of song exaggeration in E. diurnus populations broadcasting long calls comprised of multiple 'syllables'.

Constant illumination reduces circulating melatonin and impairs immune function in the cricket Teleogryllus commodus

Exposure to constant light has a range of negative effects on behaviour and physiology, including reduced immune function in both vertebrates and invertebrates. It is proposed that the associated suppression of melatonin (a ubiquitous hormone and powerful antioxidant) in response to the presence of light at night could be an underlying mechanistic link driving the changes to immune function. Here, we investigated the relationship between constant illumination, melatonin and immune function, using a model invertebrate species, the Australian black field cricket, Teleogryllus commodus. Crickets were reared under either a 12 h light: 12 h dark regimen or a constant 24 h light regimen. Circulating melatonin concentration and immune function (haemocyte concentration, lytic activity and phenoloxidase (PO) activity) were assessed in individual adult crickets through the analysis of haemolymph. Constant illumination reduced melatonin and had a negative impact on haemocyte concentrations and lytic activity, but its effect on PO activity was less apparent. Our data provide the first evidence, to our knowledge, of a link between exposure to constant illumination and variation in haemocyte concentration in an invertebrate model, while also highlighting the potential complexity of the immune response following exposure to constant illumination. This study provides insight into the possible negative effect of artificial night-time lighting on the physiology of invertebrates, but whether lower and potentially more ecologically relevant levels of light at night produce comparable results, as has been reported in several vertebrate taxa, remains to be tested.

Females suffer a reduction in the viability of stored sperm following an immune challenge

Despite the ubiquitous nature of sperm storage in invertebrates, relatively little is known about its costs, or the impact that immune activation can have on a female's ability to maintain viable sperm stores. We explored the effects of an immune challenge on sperm storage under food-limited and ad libitum conditions in the field cricket, Teleogryllus oceanicus, by injecting mated adult females with either a LD5 dose of live bacteria or a nonpathogenic immune elicitor [lipopolysaccharide (LPS)] and then scoring the viability of their stored sperm. Females that were infected with bacteria showed a reduction in the viability of stored sperm 48 h after infection; interestingly, this pattern was not evident when females were injected with LPS. Reduction in sperm viability post-infection may reflect a reproductive trade-off between immune function and sperm store maintenance, as only females injected with bacteria showed an elevated antibacterial immune (lytic) response. Alternatively, bacteria may act directly on sperm quality. Dietary manipulations showed that lytic activity in females is condition dependent, irrespective of their immune challenge treatment. Diet affected the ability of females to maintain the viability of stored sperm, suggesting that sperm storage is condition dependent. That bacterial infection associated with a reduction in stored sperm quality has potentially important implications for the outcomes of sperm competition in T. oceanicus and in other species in which females store sperm between matings.

Immune anticipation of mating in Drosophila: Turandot M promotes immunity against sexually transmitted fungal infections

Although it is well known that mating increases the risk of infection, we do not know how females mitigate the fitness costs of sexually transmitted infections (STIs). It has recently been shown that female fruitflies, Drosophila melanogaster, specifically upregulate two members of the Turandot family of immune and stress response genes, Turandot M and Turandot C (TotM and TotC), when they hear male courtship song. Here, we use the Gal4/UAS RNAi gene knockdown system to test whether the expression of these genes provides fitness benefits for females infected with the entomopathogenic fungus, Metarhizium robertsii under sexual transmission. As a control, we also examined the immunity conferred by Dorsal-related immunity factor (Dif), a central component of the Toll signalling pathway thought to provide immunity against fungal infections. We show that TotM, but not TotC or Dif, provides survival benefits to females following STIs, but not after direct topical infections. We also show that though the expression of TotM provides fecundity benefits for healthy females, it comes at a cost to their survival, which helps to explain why TotM is not constitutively expressed. Together, these results show that the anticipatory expression of TotM promotes specific immunity against fungal STIs and suggest that immune anticipation is more common than currently appreciated.

The genomic response to courtship song stimulation in female Drosophila melanogaster

Courtship behaviour involves a complex exchange of signals and responses. These are usually studied at the phenotypic level, and genetic or transcriptional responses to courtship are still poorly understood. Here, we examine the gene-expression changes in Drosophila melanogaster females in response to one of the key male courtship signals in mate recognition, song produced by male wing vibration. Using long oligonucleotide microarrays, we identified several genes that responded differentially to the presence or absence of acoustic courtship stimulus. These changes were modest in both the number of genes involved and fold-changes, but notably dominated by antennal signalling genes involved in olfaction as well as neuropeptides and immune response genes. Second, we compared the expression patterns of females stimulated with synthetic song typical of either conspecific or heterospecific (Drosophila simulans) males. In this case, antennal olfactory signalling and innate immunity genes were also enriched among the differentially expressed genes. We confirmed and investigated the time course of expression differences of two identified immunity genes using real-time quantitative PCR. Our results provide novel insight into specific molecular changes in females in response to courtship song stimulation. These may be involved in both signal perception and interpretation and some may anticipate molecular interactions that occur between the sexes after mating.

Grasshopper calling songs convey information about condition and health of males

Females of the grasshopper Chorthippus biguttulus invest much more in the offspring than do males. As a consequence, females are the more selective sex and exert a sexual selection on males by responding to the songs of certain conspecific males while rejecting others. What kind of information about the sender may a female obtain from a male's song, in addition to its species identity? We searched for correlations between a series of song features and morphometric parameters of individual males. In addition, also the immunocompetence of males was assessed by implanting small pieces of nylon thread. We found significant, positive correlations between certain song characteristics and indicators of male size and immunocompetence. Thus, grasshopper females may--in principle--be able to judge a male's condition and health from the acoustic signals he produces.