Does selection on increased cold tolerance in the adult stage confer resistance throughout development?

Genetic independence between traits separated by metamorphosis is widespread but varies with biological function

Why is metamorphosis so pervasive? Does it facilitate the independent (micro)evolution of quantitative traits in distinct life stages, similarly to how it enables some limbs and organs to develop at specific life stages? We tested this hypothesis by measuring the expression of 6400 genes in 41 Drosophila melanogaster inbred lines at larval and adult stages. Only 30% of the genes showed significant genetic correlations between larval and adult expression. By contrast, 46% of the traits showed some level of genetic independence between stages. Gene ontology terms enrichment revealed that across stages correlated traits were often involved in proteins synthesis, insecticide resistance and innate immunity, while a vast number of genes expression traits associated with energy metabolism were independent between life stages. We compared our results to a similar case: genetic constraints between males and females in gonochoric species (i.e. sexual antagonism). We expected selection for the separation between males and females to be higher than between juvenile and adult functions, as gonochorism is a more common strategy in the animal kingdom than metamorphosis. Surprisingly, we found that inter-stage constraints were lower than inter-sexual genetic constraints. Overall, our results show that metamorphosis enables a large part of the transcriptome to evolve independently at different life stages.

Transcriptomic and functional genetic evidence for distinct ecophysiological responses across complex life cycle stages

Organisms with complex life cycles demonstrate a remarkable ability to change their phenotypes across development, presumably as an evolutionary adaptation to developmentally variable environments. Developmental variation in environmentally sensitive performance, and thermal sensitivity in particular, has been well documented in holometabolous insects. For example, thermal performance in adults and juvenile stages exhibit little genetic correlation (genetic decoupling) and can evolve independently, resulting in divergent thermal responses. Yet, we understand very little about how this genetic decoupling occurs. We tested the hypothesis that genetic decoupling of thermal physiology is driven by fundamental differences in physiology between life stages, despite a potentially conserved Cellular Stress Response. We used RNAseq to compare transcript expression in response to a cold stressor in Drosophila melanogaster larvae and adults and used RNAi (RNA interference) to test whether knocking down nine target genes differentially affected larval and adult cold tolerance. Transcriptomic responses of whole larvae and adults during and following exposure to -5°C were largely unique both in identity of responding transcripts and in temporal dynamics. Further, we analyzed the tissue-specificity of differentially-expressed transcripts from FlyAtlas 2 data, and concluded that stage-specific differences in transcription were not simply driven by differences in tissue composition. In addition, RNAi of target genes resulted in largely stage-specific and sometimes sex-specific effects on cold tolerance. The combined evidence suggests that thermal physiology is largely stage-specific at the level of gene expression, and thus natural selection may be acting on different loci during the independent thermal adaptation of different life stages.

Direct and indirect effects of altered temperature regimes and phenological mismatches on insect populations

Climate change is transforming ecosystems by altering species ranges, the composition of communities, and trophic interactions. Here, we synthesize recent reviews and subsequent developments to provide an overview of insect ecological and evolutionary responses to altered temperature regimes. We discuss both direct responses to thermal stress and indirect responses arising from phenological mismatches, altered host quality, and changes in natural enemy activity. Altered temperature regimes can increase exposure to both cold and heat stress and result in phenological and morphological mismatches with adjacent trophic levels. Host plant quality varies in a heterogenous way in response to altered temperatures with both increases and decreases observed. Density-dependent effects, spatial heterogeneity, and rapid evolutionary change provide some resilience to these threats.

Elevational and Latitudinal Changes in Cold Tolerance of Nymph and Adult Mormon Crickets Anabrus simplex (Orthoptera: Tettigoniidae)

Insects that hatch in winter and early spring in desert and montane regions are likely to encounter extreme weather events, including precipitous drops in temperature. The susceptibility of insects to exposure to subzero temperatures is predicted to decrease with increasing latitude or elevation. Mormon crickets occur over a broad latitudinal range from southwestern United States to Canada and a broad elevational range from near sea-level to 3,000 m. Population declines have been attributed to late freezing events, but winter hatching suggests they may also be cold tolerant. Lower lethal temperature of high elevation populations in low latitude Arizona (AZ) and high latitude Wyoming (WY) was measured by exposing nymphs and adults to 6 h or 24 h of subzero temperature. From similar latitude, WY was compared with mid-elevation Idaho (ID) and low elevation Oregon (OR) populations. Contrary to the prediction, lethal temperature of third instar nymphs was lower in AZ than in the more northerly populations. Consistently, AZ was more tolerant of cold in early nymphal instars relative to populations from higher latitude. Early hatching at lower latitudes might increase the risk of early instars experiencing a severe cold snap relative to nymphs at high latitudes. Also, contrary to prediction, the lethal temperature of adults increased with elevation, whereas third instar nymphs from mid-elevation ID were the most susceptible to cold exposure. Cold tolerance in immature and mature stages is more likely to be uncoupled when life stages do not coincide, as with Mormon crickets.

Evolutionary impacts of winter climate change on insects

Overwintering is a serious challenge for insects, and winters are rapidly changing as climate shifts. The capacity for phenotypic plasticity and evolutionary adaptation will determine which species profit or suffer from these changes. Here we discuss current knowledge on the potential and evidence for evolution in winter-relevant traits among insect species and populations. We conclude that the best evidence for evolutionary shifts in response to changing winters remain those related to changes in phenology, but all evidence points to cold hardiness as also having the potential to evolve in response to climate change. Predicting future population sizes and ranges relies on understanding to what extent evolution in winter-related traits is possible, and remains a serious challenge.

Do traits separated by metamorphosis evolve independently? Concepts and methods

Despite the ubiquity of complex life cycles, we know little of the evolutionary constraints exerted by metamorphosis. Here, we present pitfalls and methods to answer whether animals with a complex life cycle can independently adapt to the environments encountered at each life stage, with a specific focus on the microevolution of quantitative characters. We first discuss challenges associated with study traits and populations. We further emphasize the benefits of using a combination of approaches. We then develop how multivariate methods can limit several issues by revealing genetic patterns that are invisible when only considering trait-by-trait genetic correlations. Finally, we detail how Lande's work on sexual dimorphism can be applied in measuring G matrices across life stages. The methods and tools described here will contribute towards building a predictive framework for trait evolution across life stages.

Stage-specific genotype-by-environment interactions for cold and heat hardiness in Drosophila melanogaster

Environments often vary across a life cycle, imposing fluctuating natural selection across development. Such fluctuating selection can favor different phenotypes in different life stages, but stage-specific evolutionary responses will depend on genetic variance, covariance, and their interaction across development and across environments. Thus, quantifying how genetic architecture varies with plastic responses to the environment and across development is vital to predict whether stage-specific adaptation will occur in nature. Additionally, the interaction of genetic variation and environmental plasticity (GxE) may be stage-specific, leading to a three-way interaction between genotype, environment, and development or GxDxE. To test for these patterns, we exposed larvae and adults of Drosophila melanogaster isogenic lines derived from a natural population to extreme heat and cold stress after developmental acclimation to cool (18 °C) and warm (25 °C) conditions and measured genetic variance for thermal hardiness. We detected significant GxE that was specific to larvae and adults for cold and heat hardiness (GxDxE), but no significant genetic correlation across development for either trait at either acclimation temperature. However, cross-development phenotypic correlations for acclimation responses suggest that plasticity itself may be developmentally constrained, though rigorously testing this hypothesis requires more experimentation. These results illustrate the potential for stage-specific adaptation within a complex life cycle and demonstrate the importance of measuring traits at appropriate developmental stages and environmental conditions when predicting evolutionary responses to changing climates.

Genetic Decoupling of Thermal Hardiness across Metamorphosis in Drosophila melanogaster

As organisms age the environment fluctuates, exerting differential selection across ontogeny. In particular, highly seasonal environments expose life stages to often drastically different thermal environments. This developmental variation is particularly striking in organisms with complex life cycles, wherein life history stages also exhibit distinct morphologies, physiologies, and behaviors. Genes acting pleiotropically on thermal responses may produce genetic correlations across ontogeny, constraining the independent evolution of each life stage to their respective thermal environments. To investigate whether developmental genetic correlations constrain the evolution thermal hardiness of the fly Drosophila melanogaster, we applied quantitative genetic analyses to cold hardiness measured in both larvae and adults from isogenic lines of the Drosophila Genetic Reference Panel (DGRP), using survival at stressful low temperatures as the phenotypic metric. Using full genome resequencing data for the DGRP, we also implemented genome-wide association (GWA) analysis using Bayesian Sparse Linear Mixed Models (BSLMMs) to estimate associations between naturally segregating variation and cold hardiness for both larvae and adults. Quantitative genetic analyses revealed no significant genetic correlation for cold hardiness between life stages, suggesting complete genetic decoupling of thermal hardiness across the metamorphic boundary. Both quantitative genetic and GWA analyses suggested that polygenic variation underlies cold hardiness in both stages, and that associated loci largely affected one stage or the other, but not both. However, reciprocal enrichment tests and correlations between BSLMM parameters for each life stage support some shared physiological mechanisms that may reflect common cellular thermal response pathways. Overall, these results suggest no developmental genetic constraints on cold hardiness across metamorphosis in D. melanogaster, an important consideration in evolutionary models of responses to changing climates. Genetic correlations for environmental sensitivity across ontogeny remains largely unexplored in other organisms, thus assessing the generality of genetic decoupling will require further quantitative or population genetic analysis in additional species.

No trade-off between growth rate and temperature stress resistance in four insect species

Although fast growth seems to be generally favored by natural selection, growth rates are rarely maximized in nature. Consequently, fast growth is predicted to carry costs resulting in intrinsic trade-offs. Disentangling such trade-offs is of great ecological importance in order to fully understand the prospects and limitations of growth rate variation. A recent study provided evidence for a hitherto unknown cost of fast growth, namely reduced cold stress resistance. Such relationships could be especially important under climate change. Against this background we here investigate the relationships between individual larval growth rate and adult heat as well as cold stress resistance, using eleven data sets from four different insect species (three butterfly species: Bicyclus anynana, Lycaena tityrus, Pieris napi; one Dipteran species: Protophormia terraenovae). Despite using different species (and partly different populations within species) and an array of experimental manipulations (e.g. different temperatures, photoperiods, feeding regimes, inbreeding levels), we were not able to provide any consistent evidence for trade-offs between fast growth and temperature stress resistance in these four insect species.

Directional selection on cold tolerance does not constrain plastic capacity in a butterfly

Background

Organisms may respond to environmental change by means of genetic adaptation, phenotypic plasticity or both, which may result in genotype-environment interactions (G x E) if genotypes differ in their phenotypic response. We here specifically target the latter source of variation (i.e. G x E) by comparing plastic responses among lines of the tropical butterfly Bicyclus anynana that had been selected for increased cold tolerance and according controls. Our main aim here was to test the hypothesis that directional selection on cold tolerance will interfere with plastic capacities.

Results

Plastic responses to temperature and feeding treatments were strong, with e.g. higher compared to lower temperatures reducing cold tolerance, longevity, pupal mass, and development time. We report a number of statistically significant genotype-environment interactions (i.e. interactions between selection regime and environmental variables), but most of these were not consistent across treatment groups. We found some evidence though for larger plastic responses to different rearing temperatures in the selection compared to the control lines, while plastic responses to different adult temperatures and feeding treatments were overall very similar across selection regimes.

Conclusion

Our results indicate that plastic capacities are not always constrained by directional selection (on cold tolerance) and therefore genetic changes in trait means, but may operate independently.

Does cold tolerance plasticity correlate with the thermal environment and metabolic profiles of a parasitoid wasp?

Tolerance of ectotherm species to cold stress is highly plastic according to thermal conditions experienced prior to cold stress. In this study, we investigated how cold tolerance varies with developmental temperature (at 17, 25 and 30°C) and whether developmental temperature induces different metabolic profiles. Experiments were conducted on the two populations of the parasitoid wasp, Venturia canescens, undergoing contrasting thermal regimes in their respective preferential habitat (thermally variable vs. buffered). We predicted the following: i) development at low temperatures improves the cold tolerance of parasitoid wasps, ii) the shape of the cold tolerance reaction norm differs between the two populations, and iii) these phenotypic variations are correlated with their metabolic profiles. Our results showed that habitat origin and developmental acclimation interact to determine cold tolerance and metabolic profiles of the parasitoid wasps. Cold tolerance was promoted when developmental temperatures declined and population originating from variable habitat presented a higher cold tolerance. Cold tolerance increases through the accumulation of metabolites with an assumed cryoprotective function and the depression of metabolites involved in energy metabolism. Our data provide an original example of how intraspecific cold acclimation variations correlate with metabolic response to developmental temperature.