GENETIC VARIATION IN HIF SIGNALING UNDERLIES QUANTITATIVE VARIATION IN PHYSIOLOGICAL AND LIFE‐HISTORY TRAITS WITHIN LOWLAND BUTTERFLY POPULATIONS

Oxygen conductance to the tissues determines aerobic metabolic performance in most eukaryotes but has cost/benefit tradeoffs. Here we examine in lowland populations of a butterfly a genetic polymorphism affecting oxygen conductance via the hypoxia‐inducible factor (HIF) pathway, which senses intracellular oxygen and controls the development of oxygen delivery networks. Genetically distinct clades of Glanville fritillary (Melitaea cinxia) across a continental scale maintain, at intermediate frequencies, alleles in a metabolic enzyme (succinate dehydrogenase, SDH) that regulates HIF‐1α. One Sdhd allele was associated with reduced SDH activity rate, twofold greater cross‐sectional area of tracheoles in flight muscle, and better flight performance. Butterflies with less tracheal development had greater post‐flight hypoxia signaling, swollen & disrupted mitochondria, and accelerated aging of flight metabolic performance. Allelic associations with metabolic and aging phenotypes were replicated in samples from different clades. Experimentally elevated succinate in pupae increased the abundance of HIF‐1α and expression of genes responsive to HIF activation, including tracheal morphogenesis genes. These results indicate that the hypoxia inducible pathway, even in lowland populations, can be an important axis for genetic variation underlying intraspecific differences in oxygen delivery, physiological performance, and life history.     

Antagonistic pleiotropy can promote adaptation to patchy environments*

How do gene variants with opposing effects on fitness in juvenile and adult insects perform in different ecological settings? Marden et al. used alleles of two antagonistic genes involved in metabolism and oxygen sensing in the Glanville fritillary butterfly as a model to demonstrate how these genes can antagonistically affect larval development and the adaptation of adults to different landscapes. This paper provides a case study for understanding how antagonistic pleiotropy can contribute to species adaption in patchy environments.     

Host-associated variation in sexual size dimorphism and fitness effects of adult feeding in a bruchid beetle

We studied the effect of larval host (two Convolvulus L. species, Convolvulaceae) on sexual size dimorphism and on the fitness consequences of adult feeding in the bruchid beetle Megacerus eulophus (Erichson) (Coleoptera: Bruchidae). Whereas Convolvulus chilensis Pers. occurs in low density in semiarid habitats, Convolvulus bonariensis Cav. occurs in less stressful environments and exhibits higher population density. Host plants neither differ in seed mass nor in seed nitrogen content, and there were no consistent host‐associated differences in female fitness. Consequently, host quality was considered to be similar. Sexual size dimorphism, expressed as the female:male ratio of body size, was significantly greater in C. chilensis than in C. bonariensis. A greater female size in the C. chilensis population could be selectively advantageous due to higher survival during host‐plant search in stressful environments. Female longevity was affected by food availability (starvation vs. honey‐pollen solution) and by the interaction between food availability and larval host. Fed females lived longer than starved females. This trend was found in both hosts, but it was of greater magnitude in C. chilensis than in C. bonariensis. Fecundity was significantly affected by adult feeding and larval host, and marginally affected by their interaction. Fed females laid more eggs than starved females. However, the increase in fitness of fed females was greater in C. chilensis. Results might be explained by differential selection on plasticity in life‐history traits in contrasting environments. Females in stressful environments should take a greater advantage of the transient availability of resources.     

Experimentally induced host‐shift changes life‐history strategy in a seed beetle

Expansion of the host range in phytophagous insects depends on their ability to form an association with a novel plant through changes in host‐related traits. Phenotypic plasticity has important effects on initial survival of individuals faced with a new plant, as well as on the courses of evolutionary change during long‐term adaptation to novel conditions. Using experimental populations of the seed beetle that evolved on ancestral (common bean) or novel (chickpea) host and applying reciprocal transplant at both larval and adult stage on the alternative host plant, we studied the relationship between the initial (plastic) phases of host‐shift and the subsequent stages of evolutionary divergence in life‐history strategies between populations exposed to the host‐shift process. After 48 generations, populations became well adapted to chickpea by evolving the life‐history strategy with prolonged larval development, increased body mass, earlier reproduction, shorter lifespan and decreased plasticity of all traits compared with ancestral conditions. In chickpea‐adapted beetles, negative fitness consequences of low plasticity of pre‐adult development (revealed as severe decrease in egg‐to‐adult viability on beans) exhibited mismatch with positive effects of low plasticity (i.e. low host sensitivity) in oviposition and fecundity. In contrast, beetles adapted to the ancestral host showed high plasticity of developmental process, which enabled high larval survival on chickpea, whereas elevated plasticity in adult behaviour (i.e. high host sensitivity) resulted in delayed reproduction and decreased fecundity on chickpea. The analysis of population growth parameters revealed significant fluctuation during successive phases of the host‐shift process in A. obtectus.     

Life history of the Glanville fritillary butterfly in fragmented versus continuous landscapes

Habitat loss and fragmentation threaten the long‐term viability of innumerable species of plants and animals. At the same time, habitat fragmentation may impose strong natural selection and lead to evolution of life histories with possible consequences for demographic dynamics. The Baltic populations of the Glanville fritillary butterfly (Melitaea cinxia) inhabit regions with highly fragmented habitat (networks of small dry meadows) as well as regions with extensive continuous habitat (calcareous alvar grasslands). Here, we report the results of common garden studies on butterflies originating from two highly fragmented landscapes (FL) in Finland and Sweden and from two continuous landscapes (CL) in Sweden and Estonia, conducted in a large outdoor cage (32 by 26 m) and in the laboratory. We investigated a comprehensive set of 51 life‐history traits, including measures of larval growth and development, flight performance, and adult reproductive behavior. Seventeen of the 51 traits showed a significant difference between fragmented versus CL. Most notably, the growth rate of postdiapause larvae and several measures of flight capacity, including flight metabolic rate, were higher in butterflies from fragmented than CL. Females from CL had shorter intervals between consecutive egg clutches and somewhat higher life‐time egg production, but shorter longevity, than females from FL. These results are likely to reflect the constant opportunities for oviposition in females living in continuous habitats, while the more dispersive females from FL allocate more resources to dispersal capacity at the cost of egg maturation rate. This study supports theoretical predictions about small population sizes and high rate of population turnover in fragmented habitats selecting for increased rate of dispersal, but the results also indicate that many other life‐history traits apart from dispersal are affected by the degree of habitat fragmentation.     

Heritability of and strong single gene (Pgi) effects on life‐history traits in the Glanville fritillary butterfly

We estimated broad‐sense heritabilities (H²) of 13 female and seven male life‐history traits of the Glanville fritillary butterfly (Melitaea cinxia) under semi‐natural conditions in a large outdoor population cage. The analysis was based on full‐sib families collected as young larvae in the field and reared under common garden conditions. We found significant genetic variance in female lifespan, fecundity, number of matings and host‐plant preference as well as in male body mass and mobility. Apart from host‐plant preference, female traits that were more strongly correlated with lifetime reproductive success (LRS; measured as total number of eggs laid) had higher H². LRS itself exhibited significant heritability. Host‐plant preference had very high H², consistent with a previously reported genetically determined geographical cline in host‐plant preference in the study area. Lifespan and egg hatching rate were significantly associated with a SNP in the coding region of the Pgi gene, for which there is previous evidence for balancing selection. Selection on Pgi, which furthermore shows spatial and temporal variation, may maintain genetic variance in fitness‐related life‐history traits. In contrast, we found no strong evidence for life‐history trade‐offs.     

Divergent immune priming responses across flour beetle life stages and populations

Growing evidence shows that low doses of pathogens may prime the immune response in many insects, conferring subsequent protection against infection in the same developmental stage (within‐life stage priming), across life stages (ontogenic priming), or to offspring (transgenerational priming). Recent work also suggests that immune priming is a costly response. Thus, depending on host and pathogen ecology and evolutionary history, tradeoffs with other fitness components may constrain the evolution of priming. However, the relative impacts of priming at different life stages and across natural populations remain unknown. We quantified immune priming responses of 10 natural populations of the red flour beetle Tribolium castaneum, primed and infected with the natural insect pathogen Bacillus thuringiensis. We found that priming responses were highly variable both across life stages and populations, ranging from no detectable response to a 13‐fold survival benefit. Comparing across stages, we found that ontogenic immune priming at the larval stage conferred maximum protection against infection. Finally, we found that various forms of priming showed sex‐specific associations that may represent tradeoffs or shared mechanisms. These results indicate the importance of sex‐, life stage‐, and population‐specific selective pressures that can cause substantial divergence in priming responses even within a species. Our work highlights the necessity of further work to understand the mechanistic basis of this variability.     

Temperature‐ and sex‐related effects of serine protease alleles on larval development in the Glanville fritillary butterfly

The body reserves of adult Lepidoptera are accumulated during larval development. In the Glanville fritillary butterfly, larger body size increases female fecundity, but in males fast larval development and early eclosion, rather than large body size, increase mating success and hence fitness. Larval growth rate is highly heritable, but genetic variation associated with larval development is largely unknown. By comparing the Glanville fritillary population living in the Åland Islands in northern Europe with a population in Nantaizi in China, within the source of the post‐glacial range expansion, we identified candidate genes with reduced variation in Åland, potentially affected by selection under cooler climatic conditions than in Nantaizi. We conducted an association study of larval growth traits by genotyping the extremes of phenotypic trait distributions for 23 SNPs in 10 genes. Three genes in clip‐domain serine protease family were associated with larval growth rate, development time and pupal weight. Additive effects of two SNPs in the prophenoloxidase‐activating proteinase‐3 (ProPO3) gene, related to melanization, showed elevated growth rate in high temperature but reduced growth rate in moderate temperature. The allelic effects of the vitellin‐degrading protease precursor gene on development time were opposite in the two sexes, one genotype being associated with long development time and heavy larvae in females but short development time in males. Sexually antagonistic selection is here evident in spite of sexual size dimorphism.     

Environmental and genetic control of cold tolerance in the Glanville fritillary butterfly

Thermal tolerance has a major effect on individual fitness and species distributions and can be determined by genetic variation and phenotypic plasticity. We investigate the effects of developmental and adult thermal conditions on cold tolerance, measured as chill coma recovery (CCR) time, during the early and late adult stage in the Glanville fritillary butterfly. We also investigate the genetic basis of cold tolerance by associating CCR variation with polymorphisms in candidate genes that have a known role in insect physiology. Our results demonstrate that a cooler developmental temperature leads to reduced cold tolerance in the early adult stage, whereas cooler conditions during the adult stage lead to increased cold tolerance. This suggests that adult acclimation, but not developmental plasticity, of adult cold tolerance is adaptive. This could be explained by the ecological conditions the Glanville fritillary experiences in the field, where temperature during early summer, but not spring, is predictive of thermal conditions during the butterfly's flight season. In addition, an amino acid polymorphism (Ala‐Glu) in the gene flightin, which has a known function in insect flight and locomotion, was associated with CCR. These amino acids have distinct biochemical properties and may thus affect protein function and/or structure. To our knowledge, our study is the first to link genetic variation in flightin to cold tolerance, or thermal adaptation in general.     

The relationship between larval size and fitness in the tapeworm Schistocephalus solidus: bigger is better?

In organisms with complex life cycles, fitness often increases with body size at the transition from larva to adult. The translation of larval size into fitness, however, can depend on the source of size variation, with size, per se, not always increasing adult success. In parasitic worms, many factors influence larval growth, but little is known about the consequences of this growth variation. We examined how the size of the tapeworm Schistocephalus solidus in its copepod first intermediate host affects infection success and growth in the stickleback second host. Moreover, we assessed whether the conspicuous growth variation caused by copepod size is fitness‐relevant. Using larvae of the same age, we found that larger worms had a substantially higher infection probability and they tended to still be slightly larger after several months of growth in fish. However, big larvae from bigger copepods did not have higher fitness, suggesting that being large relative to the host, but not necessarily large in general, is important. These findings clarify some aspects of the life history strategy of S. solidus (e.g. why there is a flat ontogenetic reaction norm across copepod stages), but also raise questions (e.g. why growth costs have been hard to document). More generally, our results indicate that larval size can correlate with fitness in helminths, but that not all size variation is predictive of success in the next host.     

Temperature extremes and butterfly fitness: conflicting evidence from life history and immune function

Global warming and its associated increase in temperature extremes pose a substantial challenge on natural systems. Tropical ectotherms, living close to their (upper) critical thermal limits, may be particularly vulnerable to global warming, yet they are as a group understudied. Most studies assessing fitness effects under global warming focused on life‐history correlates such as body size and largely neglected immune function. Furthermore they did not consider to what extent temperature effects may be modulated under resource‐based trade‐offs. Against this background we here investigate effects of temperature extremes on fitness‐related adult traits (viz. body mass, fat content, and two key parameters of arthropod immune function: phenoloxidase (PO) activity and haemocyte numbers) at different levels of larval and adult food stress in the tropical butterfly Bicyclus anynana. Body mass and PO activity decreased after short‐term larval food stress, but not fat content and haemocyte numbers (probably owing to compensatory mechanisms during further development). Longer‐term food deprivation in the adult stage, in contrast, diminished performance throughout, confirming that the feeding treatments chosen imposed stress. Temperature manipulations yielded contrary responses between life‐history correlates and immune function: while body mass and fat content increased by increasing temperatures, PO activity and haemocyte numbers decreased. The latter was particularly pronounced under adult food stress, suggesting a resource‐allocation trade‐off. Our data suggest that global warming will not only reduce performance through direct effects of thermal stress, but also through secondary effects on adult immune function, which may be missed when exclusively focussing on other life‐history correlates.     

Integrating large‐scale geographic patterns in flight morphology,flight characteristics and sexual selection in a range‐expanding damselfly

While geographic trait variation along environmental clines is widespread, associated patterns in sexual selection remain largely unexplored. Geographic patterns in sexual selection may be expected if 1) phenotypes vary geographically and sexual selection is dependent on the local phenotypes in the population, and if 2) sexual selection is influenced by geographically structured environmental conditions. We quantified geographic variation in flight‐related traits and flight performance in mated and unmated males and tested for geographic variation in sexual selection on these traits in the poleward range‐expanding damselfly Coenagrion scitulum across a set of eleven core and edge populations ordered along thermal gradients in the larval and in the adult stage. We found little support for trait differentiation between core and edge populations, instead we found considerable geographic trait variation along the larval and adult thermal gradients. As expected under time constraints, body mass decreased with shorter larval growth seasons. Lower temperatures during the adult flight period were associated with a higher body mass, a higher flight speed and a higher fat content; these traits likely evolved to buffer flight ability at suboptimal temperatures and to optimize starvation resistance. Across the large geographic scale, we found a consistent higher flight duration in mated males. Instead, sexual selection for higher fat content was stronger in populations with lower adult flight temperatures and sexual selection for lower body mass acted only in edge populations. Our results indicate sexual selection on flight performance to be consistent over a large geographic scale and this despite the clear geographic patterns in sexual selection on the underlying morphological traits. Our results highlight that to fully understand the fitness implications of geographically changing trait patterns, researchers should consider the entire phenotype–performance–fitness axis and incorporate effects of geographically structured life‐stage specific environmental conditions on this axis.     

Effects of nutrition and density in Culex pipiens

Mosquito larvae face numerous biotic and abiotic challenges that affect their development and survivorship, as well as adult fitness. We conducted two experiments under semi‐natural conditions to evaluate the effects of intraspecific competition, nutrient limitation and sub‐lethal doses of malathion on individual life history traits in adult Culex pipiens (Diptera: Culicidae). In the first experiment, larvae of Cx. pipiens were reared at different intraspecific densities and exposed to sub‐lethal doses of malathion. In the second experiment, different intraspecific densities of Cx. pipiens larvae were reared under conditions of low or high larval nutrients, and subsequent adults were fed on either water or 10% sucrose solution. Malathion treatment had relatively minor effects compared with density, which had significant negative effects on development rate, survivorship to adulthood, body size (wing length) and longevity. As larval density increased, a sex ratio distortion in survivorship to adulthood emerged, in which a bias towards males was apparent. Nutrient‐rich larval environments alleviated, in part, the effects of increasing density and extended the lifespan of mosquitoes fed on water and 10% sucrose. Density‐dependent alterations in adult longevity attributable to the larval environment are complex and show contrasting results depending on interactions with other environmental factors. This study suggests that larval resource availability and competition influence Cx. pipiens population growth correlates and have lasting effects on traits that relate to a mosquito's ability to vector pathogens.     

Life history alterations upon oral and hemocoelic bacterial exposure in the butterfly Melitaea cinxia

Life history strategies often shape biological interactions by specifying the parameters for possible encounters, such as the timing, frequency, or way of exposure to parasites. Consequentially, alterations in life‐history strategies are closely intertwined with such interaction processes. Understanding the connection between life‐history alterations and host–parasite interactions can therefore be important to unveil potential links between adaptation to environmental change and changes in interaction processes. Here, we studied how two different host–parasite interaction processes, oral and hemocoelic exposure to bacteria, affect various life histories of the Glanville fritillary butterfly Melitaea cinxia. We either fed or injected adult butterflies with the bacterium Micrococcus luteus and observed for differences in immune defenses, reproductive life histories, and longevity, compared to control exposures. Our results indicate differences in how female butterflies adapt to the two exposure types. Orally infected females showed a reduction in clutch size and an earlier onset of reproduction, whereas a reduction in egg weight was observed for hemocoelically exposed females. Both exposure types also led to shorter intervals between clutches and a reduced life span. These results indicate a relationship between host–parasite interactions and changes in life‐history strategies. This relationship could cast restrictions on the ability to adapt to new environments and consequentially influence the population dynamics of a species in changing environmental conditions.     

Epistasis and maternal effects in experimental adaptation to chronic nutritional stress in Drosophila

Based on ecological and metabolic arguments, some authors predict that adaptation to novel, harsh environments should involve alleles showing negative (diminishing return) epistasis and/or that it should be mediated in part by evolution of maternal effects. Although the first prediction has been supported in microbes, there has been little experimental support for either prediction in multicellular eukaryotes. Here we use a line‐cross design to study the genetic architecture of adaptation to chronic larval malnutrition in a population of Drosophila melanogaster that evolved on an extremely nutrient‐poor larval food for 84 generations. We assayed three fitness‐related traits (developmental rate, adult female weight and egg‐to‐adult viability) under the malnutrition conditions in 14 crosses between this selected population and a nonadapted control population originally derived from the same base population. All traits showed a pattern of negative epistasis between alleles improving performance under malnutrition. Furthermore, evolutionary changes in maternal traits accounted for half of the 68% increase in viability and for the whole of 8% reduction in adult female body weight in the selected population (relative to unselected controls). These results thus support both of the above predictions and point to the importance of nonadditive effects in adaptive microevolution.     

Rapid and repeatable shifts in life‐history timing of Rhagoletis pomonella (Diptera: Tephritidae) following colonization of novel host plants in the Pacific Northwestern United States

Host shifts of phytophagous insect specialists to novel plants can result in divergent ecological adaptation, generating reproductive isolation and potentially new species. Rhagoletis pomonella fruit flies in eastern North America underwent a host shift ~160 ya from native downy hawthorn (Crataegus mollis) to introduced, domesticated apple (Malus domestica). Divergent selection on diapause phenology related to the earlier fruiting time of apples versus downy hawthorns resulted in partial allochronic reproductive isolation between the fly races. Here, we test for how rapid and repeatable shifts in life‐history timing are driving ecological divergence of R. pomonella in the Pacific Northwestern USA. The fly was introduced into the region via larval‐infested apples 40–65 ya and now attacks native black hawthorn (Crataegus douglasii) and introduced ornamental hawthorn (Crataegus monogyna), in addition to early‐ and late‐maturing apple varieties in the region. To investigate the life‐history timing hypothesis, we used a field‐based experiment to characterize the host‐associated eclosion and flight activity patterns of adults, and the feeding times of larvae at a field site in Vancouver, Washington. We also assessed the degree to which differences in host‐fruiting time generate allochronic isolation among apple‐, black hawthorn‐, and ornamental hawthorn‐associated fly populations. We conclude that host‐associated fly populations are temporally offset 24.4% to 92.6% in their seasonal distributions. Our results imply that R. pomonella possesses the capacity for rapid and repeatable shifts in diapause life history to match host‐fruiting phenology, which can generate ecologically based reproductive isolation, and potentially biodiversity in the process.     

Diet and a developmental time constraint alter life‐history trade‐offs in a caddis fly (Trichoptera: Limnephilidae)

Environmental factors influence variation in life histories by affecting growth, development, and reproduction. We conducted an experiment in outdoor mesocosms to examine how diet and a time constraint on juvenile development (pond‐drying) influence life‐history trade‐offs (growth, development, adult body mass) in the caddis fly Limnephilus externus (Trichoptera: Limnephilidae). We predicted that: (1) diet supplementation would accelerate larval growth and development, and enhance survival to adulthood; (2) pond‐drying would accelerate development and increase larval mortality; and (3) the relationship between adult mass and age at maturity would be negative. Diet supplementation did lead to larger adult mass under nondrying conditions, but did not significantly alter growth or development rates. Contrary to predictions, pond‐drying reduced growth rates and delayed development. The slope (positive or negative) of the female mass–age at maturity relationship depended on interactions with diet or pond‐drying, but the male mass–age relationship was negative and independent of treatment. Our results suggest that pond‐drying can have negative effects on the future fitness of individuals by increasing the risk of desiccation‐induced, pre‐reproductive mortality and decreasing adult body size at maturity. These negative effects on life history cannot be overcome with additional nutritional resources in this species. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 495–504.     

Lethal and sub‐lethal effects of spinosad on the life‐history traits of army worm,Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae), and its fitness cost of resistance

Spodoptera litura is one of the most destructive polyphagous insect pests, with more than 120 host‐plant species. In our present study, a field‐collected population of S. litura when selected with spinosad for 11 consecutive generations resulted in the development of 3921‐fold resistance to spinosad as compared to the susceptible strain. The spinosad‐resistant strain of S. litura had a relatively high fitness cost (0.17) as compared to the susceptible strain. Furthermore, the lethal and sub‐lethal effects of different concentrations of spinosad were checked on the susceptible strain at different levels; i.e., LC₄₀, LC₃₀, LC₂₀ and LC₁₀, which revealed that the impact of spinosad on the life‐history traits of S. litura increased with the increase in concentration of spinosad. A significant impact of spinosad was recorded on the larval duration, pre‐pupal weight, pupal duration, pupal weight, reproductive potential and adult emergence. The outcomes of the current research clearly indicate that fitness cost of spinosad and its sub‐lethal effects have a significant impact on population dynamics of S. litura, for which it can be incorporated in integrated pest management.