Low genetic variation in cold tolerance linked to species distributions in butterflies

Species with restricted distributions make up the vast majority of biodiversity. Recent evidence from Drosophila suggests that species with restricted distributions may simply lack genetic variation in key traits, limiting their ability to adapt to conditions beyond their current range. Specifically, tropical species of Drosophila have been shown to have low means and low genetic variation for cold tolerance and desiccation tolerance. It has therefore been predicted that these species will be limited in their response to future climatic changes. However whether these results extend beyond Drosophila is not known. We assess levels of quantitative genetic variation for cold tolerance and body size in three species of butterfly from the genus Eurema that can be classified as tropically restricted (E. laeta), tropical/subtropical (E. hecabe) and widespread (E. smilax) in their distribution. Compared to the more widely distributed species, we show that the tropically restricted E. laeta has significantly lower mean cold tolerance and lacks genetic variation for this trait. Thus, we empirically confirm in non-model organisms that low levels of genetic variation in a key ecological trait may play a role in limiting the distribution of tropically restricted species.     

Evolutionary potential of thermal preference and heat tolerance in Drosophila subobscura

Evolutionary change of thermal traits (i.e., heat tolerance and behavioural thermoregulation) is one of the most important mechanisms exhibited by organisms to respond to global warming. However, the evolutionary potential of heat tolerance, estimated as narrow‐sense heritability, depends on the methodology employed. An alternative adaptive mechanism to buffer extreme temperatures is behavioural thermoregulation, although the association between heat tolerance and thermal preference is not clearly understood. We suspect that methodological effects associated with the duration of heat stress during thermal tolerance assays are responsible for missing this genetic association. To test this hypothesis, we estimated the heritabilities and genetic correlations for thermal traits in Drosophila subobscura, using high‐temperature static and slow ramping assays. We found that heritability for heat tolerance was higher in static assays (h² = 0.134) than in slow ramping assays (h² = 0.084), suggesting that fast assays may provide a more precise estimation of the genetic variation of heat tolerance. In addition, thermal preference exhibited a low heritability (h² = 0.066), suggesting a reduced evolutionary response for this trait. We also found that the different estimates of heat tolerance and thermal preference were not genetically correlated, regardless of how heat tolerance was estimated. In conclusion, our data suggest that these thermal traits can evolve independently in this species. In agreement with previous evidence, these results indicate that methodology may have an important impact on genetic estimates of heat tolerance and that fast assays are more likely to detect the genetic component of heat tolerance.     

Heritable variation in circulating glucocorticoids and endocrine flexibility in a free‐living songbird

Phenotypic flexibility is a central way that organisms cope with challenging and changing environments. As endocrine signals mediate many phenotypic traits, heritable variation in hormone levels, or their context‐dependent flexibility, could present an important target for selection. Several studies have estimated the heritability of circulating glucocorticoid levels under acute stress conditions, but little is known about the potential for either baseline hormone levels or rapid endocrine flexibility to evolve. Here, we assessed the potential for selection to operate on the elevation (circulating hormone levels) and flexibility of glucocorticoid reaction norms to acute restraint stress. Multivariate animal models revealed low but significant heritability in baseline (h² = 0.13–0.14) and stress‐induced glucocorticoids (h² = 0.18), and moderate heritability in glucocorticoid flexibility in response to acute stress (h² = 0.38) in free‐living juvenile tree swallows (Tachycineta bicolor; n = 408). Baseline glucocorticoids were not genetically correlated with either stress‐induced glucocorticoids or glucocorticoid flexibility. These findings indicate that baseline glucocorticoids and the acute stress response are distinct traits that can be independently shaped by selection. Microevolutionary changes that influence the expression or flexibility of these endocrine mediators of phenotype may be an important way that populations adapt to changing environments and novel threats.     

Heritability and Evolutionary Potential in Thermal Tolerance Traits in the Invasive Mediterranean Cryptic Species of Bemisia tabaci (Hemiptera: Aleyrodidae)

With advancing global climate change, the analysis of thermal tolerance and evolutionary potential is important in explaining the ecological adaptation and changes in the distribution of invasive species. To reveal the variation of heat resistance and evolutionary potential in the invasive Mediterranean cryptic species of Bemisia tabaci, we selected two Chinese populations—one from Harbin, N China, and one from Turpan, S China—that experience substantial heat and cold stress and conducted knockdown tests under static high- and low-temperature conditions. ANOVAs indicated significant effects of populations and sex on heat knockdown time and chill coma recovery time. The narrow-sense heritability (h ²) estimates of heat tolerance based on a parental half-sibling breeding design ranged from 0.47±0.03 to 0.51±0.06, and the estimates of cold tolerance varied from 0.33±0.07 to 0.36±0.06. Additive genetic variances were significantly different from zero for both heat and cold tolerance. These results suggest that invasive B. tabaci Mediterranean cryptic species possesses a strong ability to respond to thermal selection and develops rapid resistance to climate change.     

Scale‐dependent thermal tolerance variation in Australian mountain grasshoppers

Physiological variation among and within species is thought to play a key role in determining distribution patterns across environmental gradients. We tested inter‐ and intraspecific variation in cold and heat tolerances for three grasshopper species (genus Kosciuscola) with overlapping elevation distributions, across their respective ranges in the Australian mountains. Of the three cold tolerance traits measured, the critical thermal minimum was the only trait to vary among species, with greater cold tolerance associated with a distribution extending to a higher elevation. Cold tolerance limits were regularly exceeded in exposed microhabitats, suggesting a role for cold adaptation in structuring species distribution patterns. In contrast to cold tolerance, heat tolerance variation was primarily partitioned within species. For two species, populations from treeless alpine habitat were more heat tolerant than their lower‐elevation counterparts, supporting recent models that suggest greater exposure to temperature extremes at higher elevations. These contrasting patterns of physiological variation among and within species emphasise the importance of considering variation within species when attempting to understand how species distributions are affected by thermal extremes.     

Plasticity and local adaptation explain lizard cold tolerance

How does climate variation limit the range of species and what does it take for species to colonize new regions? In this issue of Molecular Ecology, Campbell‐Staton et al. ( 2018 ) address these broad questions by investigating cold tolerance adaptation in the green anole lizard (Anolis carolinensis) across a latitudinal transect. By integrating physiological data, gene expression data and acclimation experiments, the authors disentangle the mechanisms underlying cold adaptation. They first establish that cold tolerance adaptation in Anolis lizards follows the predictions of the oxygen‐ and capacity‐limited thermal tolerance hypothesis, which states that organisms are limited by temperature thresholds at which oxygen supply cannot meet demand. They then explore the drivers of cold tolerance at a finer scale, finding evidence that northern populations are adapted to cooler thermal regimes and that both phenotypic plasticity and heritable genetic variation contribute to cold tolerance. The integration of physiological and gene expression data further highlights the varied mechanisms that drive cold tolerance adaptation in Anolis lizards, including both supply‐side and demand‐side adaptations that improve oxygen economy. Altogether, their work provides new insight into the physiological and genetic mechanisms underlying adaptation to new climatic niches and demonstrates that cold tolerance in northern lizard populations is achieved through the synergy of physiological plasticity and local genetic adaptation for thermal performance.     

Heritable variation in the survival of seedlings from Patagonian cypress marginal xeric populations coping with drought and extreme cold

The rear edges of tree species have begun to be perceived as highly valuable for genetic resources conservation and management. In view of expected climatic changes, the responses of trees at their xeric limits may largely be determined by their capacity to cope with augmented environmental variance. We assess the heritability of early survival of Patagonian cypress in two common-garden field tests with contrasting summer water deficits, comprising 140 and 163 open-pollinated families from 10 marginal xeric populations. The first experiment underwent less rigorous conditions than the average mesic, Mediterranean climatic conditions, which were sufficient to reveal additive genetic effects of summer drought on seedling survival. The second trial suffered strong summer water-deficit stress and a winter extreme cold event. In this harsher environment, the heritabilities of survival under summer water-deficit stress were high in all the populations (h ²?=?0.84 on average), while the heritabilities of seasonal, extreme cold survival were moderate or even nil (h ²?=?0.28 on average). We did not find evidence of genetic differentiation among populations in their capabilities to survive droughts and cold extremes. Our results indicate that even when climatic changes were strong enough to cause the extinction of the most threatened populations, heritable variation for traits underlying drought and cold tolerances may allow the marginal xeric edge of cypress to persist under augmented environmental variance, without losing overall genetic diversity.     

Thermal tolerance and geographical range size in the Agabus brunneus group of European diving beetles (Coleoptera: Dytiscidae)

Aim Within clades, most taxa are rare, whilst few are common, a general pattern for which the causes remain poorly understood. Here we investigate the relationship between thermal performance (tolerance and acclimation ability) and the size of a species’ geographical range for an assemblage of four ecologically similar European diving beetles (the Agabus brunneus group) to examine whether thermal physiology relates to latitudinal range extent, and whether Brown’s hypothesis and the environmental variability hypothesis apply to these taxa. Location Europe. Methods In order to determine the species tolerances to either low or high temperatures we measured the lethal thermal limits of adults, previously acclimated at one of two temperatures, by means of thermal ramping experiments (± 1°C min^?1). These measures of upper and lower thermal tolerances (UTT and LTT respectively) were then used to estimate each species’ thermal tolerance range, as total thermal tolerance polygons and marginal UTT and LTT thermal polygons. Results Overall, widespread species have higher UTTs and lower LTTs than restricted ones. Mean upper lethal limits of the Agabus brunneus group (43 to 46°C), are similar to those of insects living at similar latitudes, whilst mean lower lethal limits (?6 to ?9°C) are relatively high, suggesting that this group is not particularly cold‐hardy compared with other mid‐temperate‐latitude insects. Widespread species possess the largest thermal tolerance ranges and have a relatively symmetrical tolerance to both high and low temperatures, when compared with range‐restricted relatives. Over the temperature range employed, adults did not acclimate to either high or low temperatures, contrasting with many insect groups, and suggesting that physiological plasticity has a limited role in shaping distribution. Main conclusions Absolute thermal niche appears to be a good predictor of latitudinal range, supporting both Brown’s hypothesis and the environmental variability hypothesis. Restricted‐range species may be more susceptible to the direct effect of climate change than widespread species, notwithstanding the possibility that even ‘thermally‐hardy’, widespread species may be influenced by the indirect effects of climate change such as reduction in habitat availability in Mediterranean areas.     

Thermal stress exposure of pupal oriental fruit fly has strong and trait-specific consequences in adult flies

Global climate change is projected to increase the incidence of heat waves, their magnitude and duration resulting in insects experiencing increasing environmental stress in both natural and managed ecosystems. While studies on insect thermal tolerance are rapidly increasing, variation across developmental or juvenile stress cross-stage effects within and across generations remain largely unexplored. Yet in holometabolous insects, heat stress at an early developmental stage may influence performance and survival during later stages. Here, we investigated the effects of pupal mild heat stress on the performance of laboratory-reared adult Bactrocera dorsalis (Hendel) (Diptera: Tephritidae) measured as longevity, critical thermal maximum (CT_max), critical thermal minima (CT_min), heat knockdown time (HKDT) and chill coma recovery time (CCRT). Pupal heat stress significantly influenced performance of B. dorsalis adults resulting in impaired longevity and heat tolerance (CT_max and HKDT) in both sexes with improved and compromised cold tolerance (CT_min and CCRT) in females and males, respectively. These findings highlight the role of juvenile stages in mediating stress responses at adult stages. For B. dorsalis, pupal heat stress largely compromised thermal tolerance implying that the species has limited potential to shift its geographic range in heat prone areas. Significant benefits in cold tolerance in females following heat stress may help in improving survival in the cold in the short-term despite restricted activity to the same traits in males. This study suggests that basal heat tolerance and not short-term compensatory thermal plasticity following heat stress may have aided the recent invasion of B. dorsalis in African landscapes.     

Cold tolerance, overwintering and establishment potential of Thrips palmi

The cold tolerance and overwintering survival of the quarantine regulated pest and virus vector, Thrips palmi (Karny) (Thysanoptera: Thripidae), is examined and discussed in terms of its establishment potential in the U.K. Thrips palmi adults and first‐instar larvae have a wide distribution of supercooling points (SCPs) but show ‘pre‐freeze’ mortality as a result of both acute and chronic exposures to temperatures above the SCP range. Thrips palmi did not develop enhanced cold tolerance in response to cues previously shown to cold‐acclimate other thrips species. The acute cold tolerance of T. palmi is higher than that of the recently established and biologically similar species, Frankliniella occidentalis, which is thought to be capable only of very limited winter survival outdoors in the U.K. However, the more ecologically meaningful chronic assays reveal the opposite pattern. If introduced to the U.K., overwintering of T. palmi would thus be largely restricted to protected environments, as its cold tolerance is insufficient to permit outdoor survival for a complete winter. This assertion was demonstrated by caged populations that died out after as little as 25 days in outdoor winter conditions in Yorkshire, U.K. The reversal of relative tolerance of the two species when considering chronic and acute assays suggests that these forms of low temperature mortality have different physiological bases, and emphasizes the need to use both forms of assay in assessments of overwintering capacity.     

Thermal tolerance in widespread and tropical Drosophila species: does phenotypic plasticity increase with latitude?

The distribution of insects can often be related to variation in their response to thermal extremes, which in turn may reflect differences in plastic responses or innate variation in resistance. Species with widespread distributions are expected to have evolved higher levels of plasticity than those from restricted tropical areas. This study compares adult thermal limits across five widespread species and five restricted tropical species of Drosophila from eastern Australia and investigates how these limits are affected by developmental acclimation and hardening after controlling for environmental variation and phylogeny. Irrespective of acclimation, cold resistance was higher in the widespread species. Developmental cold acclimation simulating temperate conditions extended cold limits by 2°-4°C, whereas developmental heat acclimation under simulated tropical conditions increased upper thermal limits by <1°C. The response to adult heat-hardening was weak, whereas widespread species tended to have a larger cold-hardening response that increased cold tolerance by 2°-5°C. These patterns persisted after phylogenetic correction and when flies were reared under high and low constant temperatures. The results do not support the hypothesis that widely distributed species have larger phenotypic plasticity for thermal tolerance limits, and Drosophila species distributions are therefore more closely linked to differences in innate thermal tolerance limits.     

Life history adaptations and stress tolerance of four domestic species of Drosophila

Life history traits and stress tolerance were studied in four domestic species of Drosophila–D. melanogaster, D. simulans, D. auraria and D. immigrans– to understand how they adapt to their environments. In all species, larval weight approximately doubled in 1 day. The relative egg weight (egg weight : pupal weight) was smaller and the larval period was longer in D. immigrans than in the other three species. The pupal period was the longest in D. auraria. However, the adaptive significance of these differences in larval and pupal periods was not clear. The pupal case was generally thicker in the larger species, probably to support the larger pupal body. The start of oviposition was earliest and reproductive effort was greatest in female D. simulans, followed by female D. melanogaster. In contrast, starvation tolerance and the increase in bodyweight after eclosion was greater in D. immigrans and D. auraria than in the other two species. Pupal desiccation tolerance was greatest in D. melanogaster and lowest in D. auraria, and the less tolerant species seemed to select more humid sites for pupation. Adult tolerance to desiccation was greatest in D. melanogaster and lowest in D. simulans. In contrast, adult cold tolerance was greater in D. auraria and adult heat tolerance was lower in D. immigrans than in the other species. These differences in life history traits and stress tolerance represent the Drosophila species differential adaptations, and are assumed to allow coexistence of the species.     

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.     

Different combinations of morpho‐physiological traits are responsible for tolerance to drought in wild tomatoes Solanum chilense and Solanum peruvianum

Herbaceous species can modify leaf structure during the growing season in response to drought stress and water loss. Evolution can select combinations of traits in plants for efficient water use in restricted environments. We investigated plant traits that mediate adaptation and acclimation to water stress in two herbaceous drought‐tolerant species. Anatomical, morphological and physiological traits related to stems and leaves were examined under optimal watering (OW) and a long period of restricted watering (RW) in 11 accessions from three Solanaceae species (Solanum chilense, S. peruvianum and S. lycopersicum). The relationships between these traits were tested using linear regression and PCA. There were significant differences in anatomical traits between the species under both OW and RW, where leaf area correlated with stem diameter. Proline and total carbohydrates accumulated highly in S. chilense and S. peruvianum, respectively, and these osmolytes were strongly correlated with increased osmotic potential. Stomatal density varied between species but not between acclimation treatments, while stomatal rate was significantly higher in wild tomatoes. There was a strong positive relationship between stem growth rate and a group of traits together expressed as total stomatal number. Total stomata is described by integration of leaf area, stomatal density, height and internode length. It is proposed that constitutive adaptations and modifications through acclimation that mediate RW play an important role in tolerance to drought stress in herbaceous plants. The capacity for growth under drought stress was not associated with any single combination of traits in wild tomatoes, since the two species differed in relative levels of expression of various phenotypic traits.     

Heritability of sperm competition success in the bulb mite,Rhizoglyphus robini

Multiple mating by females has been proposed to function as a form of mate-choice, which implies that males should show heritable variation in sperm-competitive abilities. In this study, repeatability and heritability of sperm competition success was estimated in the bulb mite, Rhizoglyphus robini. Fertilization success of males was estimated in competition with sperm of two other males. Males differed consistently in their sperm competition success, with repeatability estimated at 0.22. The heritability of sperm competition success was estimated using parent-offspring regression, with the mean fertilization success from two matings used as a measure of each male's competitive ability. There was a significant association between the sperm competition success of fathers and sons. Narrow sense heritability (h²) was 0.284. This result supports the hypotheses proposing the multiple mating is selectively maintained in females by enhancing the reproductive success of their progeny.     

Distinct cold tolerance traits independently vary across genotypes in Drosophila melanogaster

Cold tolerance, the ability to cope with low temperature stress, is a critical adaptation in thermally variable environments. An individual's cold tolerance comprises several traits including minimum temperatures for growth and activity, ability to survive severe cold, and ability to resume normal function after cold subsides. Across species, these traits are correlated, suggesting they were shaped by shared evolutionary processes or possibly share physiological mechanisms. However, the extent to which cold tolerance traits and their associated mechanisms covary within populations has not been assessed. We measured five cold tolerance traits—critical thermal minimum, chill coma recovery, short- and long-term cold tolerance, and cold-induced changes in locomotor behavior—along with cold-induced expression of two genes with possible roles in cold tolerance (heat shock protein 70 and frost)—across 12 lines of Drosophila melanogaster derived from a single population. We observed significant genetic variation in all traits, but few were correlated across genotypes, and these correlations were sex-specific. Further, cold-induced gene expression varied by genotype, but there was no evidence supporting our hypothesis that cold-hardy lines would have either higher baseline expression or induction of stress genes. These results suggest cold tolerance traits possess unique mechanisms and have the capacity to evolve independently.     

Overexpression of ethylene response factor <Emphasis Type="Italic">TERF2</Emphasis> confers cold tolerance in rice seedlings

Rice (Oryza sativa L.) is a warm-season plant exposed to various stresses. Low temperature is an important factor limiting extension of rice cultivation areas and productivity. Previously, we have demonstrated that tomato ERF protein TERF2 enhances freezing tolerance of transgenic tobacco and tomato plants. Herein, we report that overexpression of TERF2 enhances transgenic rice tolerance to cold without affecting growth or agronomic traits. Physiological assays revealed that TERF2 could not only increase accumulation of osmotic substances and chlorophyll, but also reduce reactive oxygen species (ROS) and malondialdehyde (MDA) content and decrease electrolyte leakage in rice under cold stress. Further analysis of gene expression showed that TERF2 could activate expression of cold-related genes, including OsMyb, OsICE1, OsCDPK7, OsSODB, OsFer1, OsTrx23, and OsLti6, in transgenic rice plants under natural condition or cold stress. Thus, our findings demonstrated that TERF2 modulated expression of stress-related genes and a series of physiological adjustments under cold stress, indicating that TERF2 might have important regulatory roles in response to abiotic stress in rice and possess potential utility in improving crop cold tolerance.     

The relationship between freezing tolerance and thermotropic leaf movement in five Rhododendron species

Summary Leaf movement kinetics in five species of Rhododendron were studied in response to leaf temperature, leaf freezing point, and leaf water deficit. There was a gradient in the degree of leaf curling among species in the following order from the greatest curling to the least curling: Rhododendron catawbiense, R. maximum, R. minus, R. macrophyllum, R. ponticum. Those species found to be tolerant of winter conditions had the most intense leaf movements (both curling and angle) while those species with minimal cold tolerance had limited or no leaf movements. Leaf curling occurred at leaf temperatures above the tissue freezing points in all species. Athough leaf angle was influenced by leaf turgor, general tissue desiccation was not the ultimate cause for thermotropic leaf curling in any species tested. Those species with the greatest leaf curling and angle movements had the highest osmotic potential, the lowest water deficit at the turgor loss point, and the lowest symplastic water fraction. These data suggest that there is a trade off in Rhododendron leaf physiology between cold tolerance (due to leaf movements) and water stress tolerance (due to turgor maintenance mechanisms).