The effects of host plant species and larval density on immune function in the polyphagous moth Spodoptera littoralis

Immune functions are costly, and immune investment is usually dependent on the individual''s condition and resource availability. For phytophagous insects, host plant quality has large effects on performance, for example growth and survival, and may also affect their immune function. Polyphagous insects often experience a large variation in quality among different host plant species, and their immune investment may thus vary depending on which host plant species they develop on. Larvae of the polyphagous moth Spodoptera littoralis have previously been found to exhibit density‐dependent prophylaxis as they invest more in certain immune responses in high population densities. In addition, the immune response of S. littoralis has been shown to depend on nutrient quality in experiments with artificial diet. Here, I studied the effects of natural host plant diet and larval density on a number of immune responses to understand how host plant species affects immune investment in generalist insects, and whether the density‐dependent prophylaxis could be mediated by host plant species. While host plant species in general did not mediate the density‐dependent immune expression, particular host plant species was found to increase larval investment in certain functions of the immune system. Interestingly, these results indicate that different host plants may provide a polyphagous species with protection against different kinds of antagonisms. This insight may contribute to our understanding of the relationship between preference and performance in generalists, as well as having applied consequences for sustainable pest management.     

Experimental evolution reveals differences between phenotypic and evolutionary responses to population density

Group living can select for increased immunity, given the heightened risk of parasite transmission. Yet, it also may select for increased male reproductive investment, given the elevated risk of female multiple mating. Trade‐offs between immunity and reproduction are well documented. Phenotypically, population density mediates both reproductive investment and immune function in the Indian meal moth, Plodia interpunctella. However, the evolutionary response of populations to these traits is unknown. We created two replicated populations of P. interpunctella, reared and mated for 14 generations under high or low population densities. These population densities cause plastic responses in immunity and reproduction: at higher numbers, both sexes invest more in one index of immunity [phenoloxidase (PO) activity] and males invest more in sperm. Interestingly, our data revealed divergence in PO and reproduction in a different direction to previously reported phenotypic responses. Males evolving at low population densities transferred more sperm, and both males and females displayed higher PO than individuals at high population densities. These positively correlated responses to selection suggest no apparent evolutionary trade‐off between immunity and reproduction. We speculate that the reduced PO activity and sperm investment when evolving under high population density may be due to the reduced population fitness predicted under increased sexual conflict and/or to trade‐offs between pre‐ and post‐copulatory traits.     

Environmental determinants of population divergence in life‐history traits for an invasive species: climate,seasonality and natural enemies

Invasive species cope with novel environments through both phenotypic plasticity and evolutionary change. However, the environmental factors that cause evolutionary divergence in invasive species are poorly understood. We developed predictions for how different life‐history traits, and plasticity in those traits, may respond to environmental gradients in seasonal temperatures, season length and natural enemies. We then tested these predictions in four geographic populations of the invasive cabbage white butterfly (Pieris rapae) from North America. We examined the influence of two rearing temperatures (20 and 26.7 °C) on pupal mass, pupal development time, immune function and fecundity. As predicted, development time was shorter and immune function was greater in populations adapted to longer season length. Also, phenotypic plasticity in development time was greater in regions with shorter growing seasons. Populations differed significantly in mean and plasticity of body mass and fecundity, but these differences were not associated with seasonal temperatures or season length. Our study shows that some life‐history traits, such as development time and immune function, can evolve rapidly in response to latitudinal variation in season length and natural enemies, whereas others traits did not. Our results also indicate that phenotypic plasticity in development time can also diverge rapidly in response to environmental conditions for some traits.     

Short‐term larval food stress and associated compensatory growth reduce adult immune function in a damselfly

Abstract 1. In animals with a complex life cycle, larval stressors may carry over to the adult stage. Carry‐over effects not mediated through age and size at metamorphosis have rarely been studied. The present study focuses on the poorly documented immune costs of short‐term food stress both in the larval stage and after metamorphosis in the adult stage. 2. The present study quantified immune function [number of haemocytes, activity of prophenoloxidase (proPO) and phenoloxidase (PO)] in an experiment where larvae of the damselfly Lestes viridis were exposed to a transient starvation period. 3. Directly after starvation, immune variables were reduced in starved larvae. Levels of proPO and PO remained low after starvation, even after metamorphosis. In contrast, haemocyte numbers were fully compensated by the end of the larval stage, yet were lower in previously starved animals after metamorphosis. This can be explained as a cost of the observed compensatory growth after starvation. Focusing only on potential costs of larval stressors within the larval stage may therefore be misleading. 4. The here‐identified immunological cost in the adult stage of larval short‐term food stress and associated compensatory growth strongly indicates that physiological costs may explain hidden carry‐over effects bridging metamorphosis. This adds to the increasing awareness that the larval and adult stages in animals with a complex life cycle should be jointly studied, as trade‐offs may span metamorphosis.     

Host use of a hemiparasitic plant: no trade-offs in performance on different hosts

To examine putative specialization of a hemiparasitic plant to the most beneficial host species, we studied genetic variation in performance and trade-offs between performance on different host species in the generalist hemiparasite, Rhinanthus serotinus. We grew 25 maternal half-sib families of the parasite on Agrostis capillaris and Trifolium pratense and without a host in a greenhouse. Biomass and number of flowers of the parasite were the highest when grown on T. pratense. There were significant interactions between host species and R. serotinus seed-family indicating that the differences in performance on the two hosts and without a host varied among the families. However, we found no significant negative correlations between performance of R. serotinus on the host species or between performance on the two hosts and autotrophic performance. Thus, the genetic factors studied here are not likely to affect the evolution of specialization of R. serotinus to the most beneficial host.     

Parallel invasions produce heterogenous patterns of life history adaptation: rapid divergence in an invasive insect

Biotic invasions provide a natural experiment in evolution: when invasive species colonize new ranges, they may evolve new clines in traits in response to environmental gradients. Yet it is not clear how rapidly such patterns can evolve and whether they are consistent between regions. We compare four populations of the invasive cabbage white butterfly (Pieris rapae) from North America and Japan, independently colonized by P. rapae 150 years ago and 300 years ago, respectively. On each continent, we employed a northern and southern population to compare the effects of latitude on body mass, development rate and immune function. For each population, we used a split‐sibling family design in which siblings were reared at either warm (26.7 °C) or cool (20 °C) temperatures to determine reaction norms for each trait. Latitudinal patterns in development time were similar between the two continents. In contrast, there were strong geographical differences in reaction norms for body size, but no consistent effects of latitude; there were no detectable effects of latitude or continent on immune function. These results imply that some life history traits respond consistently to selection along climatic gradients, whereas other traits may respond to local environmental factors, or not at all.     

A test of the relationship between cuticular melanism and immune function in wild‐caught Mormon crickets

Cuticular melanism and innate immune parameters can share common physiological pathways in insects, and this functional connection may contribute to the maintenance of insect colour polymorphisms. However, evidence linking colouration and immune function has been equivocal, particularly when tested in wild populations. The present study investigates phenotypic links between colouration and immune function in migratory Mormon crickets (Anabrus simplex, Haldeman), in which juveniles occur in conspicuous colour variants but mature to become uniformly melanic adults. Wild‐caught insects are used to evaluate the relationship between juvenile colouration and three immune parameters: encapsulation ability, lysozyme‐like activity and phenoloxidase activity. As nymphs, brown crickets are better able to encapsulate an inert implant introduced into the haemocoel than green crickets, although the difference is slight and ceases after they all become darkly‐coloured adults. By contrast, adults that develop from brown nymphs have a higher basal phenoloxidase activity than those that develop from green nymphs, regardless of the fact that all adults are brown. Intrinsic factors other than colouration exert larger effects on immunity: males show stronger encapsulation responses but lower phenoloxidase activity than females, suggesting a sex‐specific trade‐off between these two immune parameters, and adults exhibit higher immune function than nymphs. In summary, modest support is found for a correlation between cuticular melanism and increased immune function in wild Mormon crickets. Additional intrinsic factors such as developmental stage and sex appear to interact with colouration and have a more substantial connection to immune function in the wild.     

Sex‐specific life history responses to nymphal diet quality and immune status in a field cricket

Individual fitness is expected to benefit from earlier maturation at a larger body size and higher body condition. However, poor nutritional quality or high prevalence of disease make this difficult because individuals either cannot acquire sufficient resources or must divert resources to other fitness‐related traits such as immunity. Under such conditions, individuals are expected to mature later at a smaller body size and in poorer body condition. Moreover, the juvenile environment can also produce longer‐term effects on adult fitness by causing shifts in resource allocation strategies that could alter investment in immune function and affect adult lifespan. We manipulated diet quality and immune status of juvenile Texas field crickets, Gryllus texensis, to investigate how poor developmental conditions affect sex‐specific investment in fitness‐related traits. As predicted, a poor juvenile diet was related to smaller mass and body size at eclosion in both sexes. However, our results also reveal sexually dimorphic responses to different facets of the rearing environment: female life history decisions are affected more by diet quality, whereas males are affected more by immune status. We suggest that females respond to decreased nutritional income because this threatens their ability to achieve a large adult body size, whereas male fitness is more dependent on reaching adulthood and so they invest in immunity and survival to eclosion.     

Phenotypic plasticity and the evolution of trade-offs: the quantitative genetics of resource allocation in the wing dimorphic cricket, Gryllus firmus

In the wing dimorphic sand cricket, Gryllus firmus, there is a pronounced trade-off between flight capability and fecundity. This trade-off is found both between morphs and within the macropterous morph, in which fecundity is negatively correlated with the mass of the principle flight muscles, the dorso-longitudinal muscles (DLM). In this paper, we examine how this trade-off is affected by a reduction in food and its genetic basis. We find that the relative fitness of the two wing morphs is not changed although both fecundity and DLM mass are decreased. A quantitative genetic analysis shows that the trade-off function is genetically variable but that most of the variation occurs in the intercept rather than the slope of the function. Analysis further indicates a very high genetic correlation between environments (food ration) supporting the hypothesis of a strong functional constraint between reproduction and flight capability.     

Direct and correlated responses to selection for larval ethanol tolerance in Drosophila melanogaster

Ethanol is an important larval resource and toxin for natural Drosophila melanogaster populations, and ethanol tolerance is genetically variable within and among populations. If ethanol‐tolerant genotypes have relatively low fitness in the absence of ethanol, as suggested by the results of an earlier study, genetic variation for ethanol tolerance could be maintained by variation in ethanol levels among breeding sites. I selected for ethanol tolerance in large laboratory populations by maintaining flies on ethanol‐supplemented media. After 90 generations, the populations were compared with control populations in egg‐to‐adult survival and development rate on ethanol‐supplemented and unsupplemented food. When compared on ethanol‐supplemented food, the ethanol‐selected populations had higher survival and faster development than the control populations, but on unsupplemented food, the populations did not differ in either trait. These results give no evidence for a ‘trade‐off’ between the ability to survive and develop rapidly in the presence of ethanol and the ability to do so in its absence. The effect of physiological induction of ethanol tolerance by exposing eggs to ethanol was also investigated; exposing eggs to ethanol strongly increased subsequent larval survival on ethanol‐supplemented food, but did not affect survival on regular food, and slowed development on both ethanol‐supplemented and regular food, partly by delaying egg hatch.     

Testing for evolutionary trade-offs in a phylogenetic context: ecological diversification and evolution of locomotor performance in emydid turtles

The evolution of ecological trade-offs is an important component of ecological specialization and adaptive radiation. However, the pattern that would show that evolutionary trade-offs have occurred between traits among species has not been clearly defined. In this paper, we propose a phylogeny-based definition of an evolutionary trade-off, and apply it to an analysis of the evolution of trade-offs in locomotor performance in emydid turtles. We quantified aquatic and terrestrial speed and endurance for up to 16 species, including aquatic, semi-terrestrial and terrestrial emydids. Emydid phylogeny was reconstructed from morphological characters and nuclear and mitochondrial DNA sequences. Surprisingly, we find that there have been no trade-offs in aquatic and terrestrial speed among species. Instead, specialization to aquatic and terrestrial habitats seems to have involved trade-offs in speed and endurance. Given that trade-offs between speed and endurance may be widespread, they may underlie specialization to different habitats in many other groups.     

Host‐associated divergence in sympatric host races of the leaf beetle Lochmaea capreae: implications for local adaptation and reproductive isolation

Ecological specialization is widely recognized as a major determinant of the emergence and maintenance of biodiversity. We studied two critical facets of specialization – local adaptation and habitat choice – in the host races of the leaf beetle Lochmaea capreae on willow and birch. Our results revealed that there is asymmetric disruptive selection for host use traits, and host races achieved different adaptive sets of life history traits through association with their host plant. Beetles from each host race exhibited food and oviposition preference for their own host plant. Reciprocal transplant displayed significant variation in host acceptance and performance: all families from the willow race rejected the alternative host plant before initiation of feeding and all died on this host plant. By contrast, all families from the birch race accepted willow for feeding, but they consumed less and performed less well. Intriguingly, families that performed well on birch also performed well on willow, suggesting positive genetic correlation rather than genetic trade‐offs. Our results suggest that the major proximal determinant of host specialization in the willow race is the behavioural acceptance of a plant rather than the toxicity of the food resource. However, in the birch race a combination of behavioural host acceptance and performance may play a role in specialization. Our study sheds light on the mechanisms by which divergent host adaptation might influence the evolution of reproductive isolation between herbivorous populations.     

Natural variation in the contribution of microbial density to inducible immune dynamics

Immune responses evolve to balance the benefits of microbial killing against the costs of autoimmunity and energetic resource use. Models that explore the evolution of optimal immune responses generally include a term for constitutive immunity, or the level of immunological investment prior to microbial exposure, and for inducible immunity, or investment in immune function after microbial challenge. However, studies rarely consider the functional form of inducible immune responses with respect to microbial density, despite the theoretical dependence of immune system evolution on microbe‐ versus immune‐mediated damage to the host. In this study, we analyse antimicrobial peptide (AMP) gene expression from seven wild‐caught flour beetle populations (Tribolium spp.) during acute infection with the virulent bacteria Bacillus thuringiensis (Bt) and Photorhabdus luminescens (P.lum) to demonstrate that inducible immune responses mediated by the humoral IMD pathway exhibit natural variation in both microbe density‐dependent and independent temporal dynamics. Beetle populations that exhibited greater AMP expression sensitivity to Bt density were also more likely to die from infection, while populations that exhibited higher microbe density‐independent AMP expression were more likely to survive P. luminescens infection. Reduction in pathway signalling efficiency through RNAi‐mediated knockdown of the imd gene reduced the magnitude of both microbe‐independent and dependent responses and reduced host resistance to Bt growth, but had no net effect on host survival. This study provides a framework for understanding natural variation in the flexibility of investment in inducible immune responses and should inform theory on the contribution of nonequilibrium host‐microbe dynamics to immune system evolution.     

Responses to a warming world: Integrating life history,immune investment,and pathogen resistance in a model insect species

Environmental temperature has important effects on the physiology and life history of ectothermic animals, including investment in the immune system and the infectious capacity of pathogens. Numerous studies have examined individual components of these complex systems, but little is known about how they integrate when animals are exposed to different temperatures. Here, we use the Indian meal moth (Plodia interpunctella) to understand how immune investment and disease resistance react and potentially trade‐off with other life‐history traits. We recorded life‐history (development time, survival, fecundity, and body size) and immunity (hemocyte counts, phenoloxidase activity) measures and tested resistance to bacterial (E. coli) and viral (Plodia interpunctella granulosis virus) infection at five temperatures (20–30°C). While development time, lifespan, and size decreased with temperature as expected, moths exhibited different reproductive strategies in response to small changes in temperature. At cooler temperatures, oviposition rates were low but tended to increase toward the end of life, whereas warmer temperatures promoted initially high oviposition rates that rapidly declined after the first few days of adult life. Although warmer temperatures were associated with strong investment in early reproduction, there was no evidence of an associated trade‐off with immune investment. Phenoloxidase activity increased most at cooler temperatures before plateauing, while hemocyte counts increased linearly with temperature. Resistance to bacterial challenge displayed a complex pattern, whereas survival after a viral challenge increased with rearing temperature. These results demonstrate that different immune system components and different pathogens can respond in distinct ways to changes in temperature. Overall, these data highlight the scope for significant changes in immunity, disease resistance, and host–parasite population dynamics to arise from small, biologically relevant changes to environmental temperature. In light of global warming, understanding these complex interactions is vital for predicting the potential impact of insect disease vectors and crop pests on public health and food security.     

Heritabilities,social environment effects and genetic correlations of social behaviours in a cooperatively breeding vertebrate

Social animals interact frequently with conspecifics, and their behaviour is influenced by social context, environmental cues and the behaviours of interaction partners, allowing for adaptive, flexible adjustments to social encounters. This flexibility can be limited by part of the behavioural variation being genetically determined. Furthermore, behaviours can be genetically correlated, potentially constraining independent evolution. Understanding social behaviour thus requires carefully disentangling genetic, environmental, maternal and social sources of variations as well as the correlation structure between behaviours. Here, we assessed heritability, maternal, common environment and social effects of eight social behaviours in Neolamprologus pulcher, a cooperatively breeding cichlid. We bred wild‐caught fish in a paternal half‐sibling design and scored ability to defend a resource against conspecifics, to integrate into a group and the propensity to help defending the group territory (“helping behaviour”). We assessed genetic, social and phenotypic correlations within clusters of behaviours predicted to be functionally related, namely “competition,” “aggression,” “aggression‐sociability,” “integration” and “integration‐help.” Helping behaviour and two affiliative behaviours were heritable, whereas there was little evidence for a genetic basis in all other traits. Phenotypic social effects explained part of the variation in a sociable and a submissive behaviour, but there were no maternal or common environment effects. Genetic and phenotypic correlation within clusters was mostly positive. A group's social environment influenced covariances of social behaviours. Genetic correlations were similar in magnitude but usually exceeding the phenotypic ones, indicating that conclusions about the evolution of social behaviours in this species could be provisionally drawn from phenotypic data in cases where data for genetic analyses are unobtainable.     

Future directions in the ontogeny of plant defence: understanding the evolutionary causes and consequences

Plant defence often varies by orders of magnitude as plants develop from the seedling to juvenile to mature and senescent stages. Ontogenetic trajectories can involve switches among defence traits, leading to complex shifting phenotypes across plant lifetimes. While considerable research has characterised ontogenetic trajectories for now hundreds of plant species, we still lack a clear understanding of the molecular, ecological and evolutionary factors driving these patterns. In this study, we identify several non‐mutually exclusive factors that may have led to the evolution of ontogenetic trajectories in plant defence, including developmental constraints, resource allocation costs, multi‐functionality of defence traits, and herbivore selection pressure. Evidence from recent physiological studies is highlighted to shed light on the underlying molecular mechanisms involved in the regulation and activation of these developmental changes. Overall, our goal is to promote new research avenues that would provide evidence for the factors that have promoted the evolution of this complex lifetime phenotype. Future research focusing on the questions and approaches identified here will advance the field and shed light on why defence traits shift so dramatically across plant ontogeny, a widespread but poorly understood ecological pattern.     

On the role of host phenotypic plasticity in host shifting by parasites

Ecological speciation appears to contribute to the diversification of insect herbivores and other parasites, which together comprise a major component of Earth's biodiversity. Host shifts are likely an important step in ecological speciation, and understanding how such shifts occur is critical to forming and testing hypotheses explaining parasite diversity. In this article, I argue that phenotypic variation in hosts arising from environmental variation (phenotypic plasticity) can promote shifts in parasites by bridging both spatiotemporal and phenotypic gaps between ancestral and novel hosts. This hypothesis, which I call the ‘plastic‐bridge hypothesis’, is conceptually distinct from those invoking genetic variation in bridging these gaps. I describe the mechanistic basis of plastic bridges, review circumstantial evidence in support of the hypothesis and suggest strategies for testing it. I use herbivorous insects and their host plants as a model, but the proposed ideas apply to any system fitting a broad definition of a host‐parasite relationship. The plastic‐bridge perspective suggests that parasite diversity is not only due to divergent selection provided by hosts, but also to the intraspecific variation that facilitates shifts between them. This view is timely, as biological invasion and range shifts associated with climate change foster novel interactions between parasites and hosts.     

Genetic and nutritional effects on male traits and reproductive performance in Tribolium flour beetles

In Tribolium flour beetles and other organisms, individuals migrate between heterogeneous environments where they often encounter markedly different nutritional conditions. Under these circumstances, theory suggests that genotype-by-environment interactions (GEI) may be important in facilitating adaptation to new environments and maintaining genetic variation for male traits subject to directional selection. Here, we used a nested half-sib breeding design with Tribolium castaneum to partition the separate and joint effects of male genotype and nutritional environment on phenotypic variation in a comprehensive suite of life-history traits, reproductive performance measures across three sequential sexual selection episodes, and fitness. When male genotypes were tested across three nutritional environments, considerable phenotypic plasticity was found for male mating and insemination success, longevity and traits related to larval development. Our results also revealed significant additive genetic variation for male mating rate, sperm offence ability (P(2)), longevity and total fitness and for several traits reflecting both larval and adult resource use. In addition, we found evidence supporting GEI for sperm defence ability (P(1)), adult longevity and larval development; thus, no single male genotype outperforms others in every nutritional environment. These results provide insight into the potential roles of phenotypic plasticity and GEI in facilitating Tribolium adaptation to new environments in ecological and evolutionary time.     

Local adaptation and effect of host genotype on the rate of pathogen evolution: an experimental test in a plant pathosystem

Abstract Virulence is thought to be a driving force in host–pathogen coevolution. Theoretical models suggest that virulence is an unavoidable consequence of pathogens evolving towards a high rate of intrahost reproduction. These models predict a positive correlation between the reproductive fitness of a pathogen and its level of virulence. Theoretical models also suggest that the demography and genetic structure of a host population can influence the evolution of virulence. If evolution occurs faster in pathogen populations than in host populations, the predicted result is local adaptation of the pathogen population. In our studies, we used a combination of molecular and physiological markers to test these hypotheses in an agricultural system. We isolated five strains of the fungal pathogen Mycosphaerella graminicola from each of two wheat cultivars that differed in their level of resistance to this pathogen. Each of the 10 fungal strains had distinct genotypes as indicated by different DNA fingerprints. These fungal strains were re‐inoculated onto the same two host cultivars in a field experiment and their genotype frequencies were monitored over several generations of asexual reproduction. We also measured the virulence of these 10 fungal strains and correlated it to the reproductive fitness of each fungal strain. We found that host genotypes had a strong impact on the dynamics of the pathogen populations. The pathogen population collected from the moderately resistant cultivar Madsen showed greater stability, higher genotype diversity, and smaller selection coefficients than the pathogen populations collected from the susceptible cultivar Stephens or a mixture of the two host cultivars. The pathogen collection from the mixed host population was midway between the two pure lines for most parameters measured. Our results also revealed that the measures of reproductive fitness and virulence of a pathogen strain were not always correlated. The pathogen strains varied in their patterns of local adaptation, ranging from locally adapted to locally maladapted.