A rapidly spreading deleterious aphid endosymbiont that uses horizontal as well as vertical transmission.

Endosymbiotic bacteria that live inside the cells of insects are typically only transmitted maternally and can spread by increasing host fitness and/or modifying reproduction in sexual hosts. Transinfections of Wolbachia endosymbionts are now being used to introduce useful phenotypes into sexual host populations, but there has been limited progress on applications using other endosymbionts and in asexual populations. Here, we develop a unique pathway to application in aphids by transferring the endosymbiont Rickettsiella viridis to the major crop pest Myzus persicae. Rickettsiella infection greatly reduced aphid fecundity, decreased heat tolerance, and modified aphid body color, from light to dark green. Despite inducing host fitness costs, Rickettsiella spread rapidly through caged aphid populations via plant-mediated horizontal transmission. The phenotypic effects of Rickettsiella were sensitive to temperature, with spread only occurring at 19 °C and not 25 °C. Body color modification was also lost at high temperatures despite Rickettsiella maintaining a high density. Rickettsiella shows the potential to spread through natural M. persicae populations by horizontal transmission and subsequent vertical transmission. Establishment of Rickettsiella in natural populations could reduce crop damage by modifying population age structure, reducing population growth and providing context-dependent effects on host fitness. Our results highlight the importance of plant-mediated horizontal transmission and interactions with temperature as drivers of endosymbiont spread in asexual insect populations.

Experimental evolution in a warming world - the omics era.

A comprehensive understanding of the genetic mechanisms that shape species responses to thermal variation is essential for more accurate predictions of the impacts of climate change on biodiversity. Experimental evolution with high-throughput resequencing approaches (E&R) is a highly effective tool that has been increasingly employed to elucidate the genetic basis of adaptation. The number of thermal E&R studies is rising, yet there is a dearth of efforts to integrate this new wealth of knowledge. Here we review this literature showing how these studies have contributed to increase our understanding on the genetic basis of thermal adaptation. We identify two major trends: highly polygenic basis of thermal adaptation and general lack of consistency in candidate targets of selection between studies. These findings indicate that the adaptive responses to specific environments are rather independent. A review of the literature reveals several gaps in the existing research. Firstly, there is a paucity of studies done with organisms of diverse taxa. Secondly, there is a need to apply more dynamic and ecologically relevant thermal environments. Thirdly, there is a lack of studies that integrate genomic changes with changes in life-history and behavioural traits. Addressing these issues would allow a more in-depth understanding of the relationship between genotype and phenotype. We highlight key methodological aspects that can address some of the limitations and omissions identified. These include the need for greater standardisation of methodologies and the utilisation of new technologies focusing on the integration of genomic and phenotypic variation in the context of thermal adaptation.

Conservation genetics as a management tool: The five best-supported paradigms to assist the management of threatened species.

About 50 y ago, Crow and Kimura [An Introduction to Population Genetics Theory (1970)] and Ohta and Kimura [Genet. Res. 22, 201-204 (1973)] laid the foundations of conservation genetics by predicting the relationship between population size and genetic marker diversity. This work sparked an enormous research effort investigating the importance of population dynamics, in particular small population size, for population mean performance, population viability, and evolutionary potential. In light of a recent perspective [J. C. Teixeira, C. D. Huber, Proc. Natl. Acad. Sci. U.S.A. 118, 10 (2021)] that challenges some fundamental assumptions in conservation genetics, it is timely to summarize what the field has achieved, what robust patterns have emerged, and worthwhile future research directions. We consider theory and methodological breakthroughs that have helped management, and we outline some fundamental and applied challenges for conservation genetics.

Responses of terrestrial polar arthropods to high and increasing temperatures.

Terrestrial arthropods in the Arctic and Antarctic are exposed to extreme and variable temperatures, and climate change is predicted to be especially pronounced in these regions. Available ecophysiological studies on terrestrial ectotherms from the Arctic and Antarctic typically focus on the ability of species to tolerate the extreme low temperatures that can occur in these regions, whereas studies investigating species plasticity and the importance of evolutionary adaptation to periodically high and increasing temperatures are limited. Here, we provide an overview of current knowledge on thermal adaptation to high temperatures of terrestrial arthropods in Arctic and Antarctic regions. Firstly, we summarize the literature on heat tolerance for terrestrial arthropods in these regions, and discuss variation in heat tolerance across species, habitats and polar regions. Secondly, we discuss the potential for species to cope with increasing and more variable temperatures through thermal plasticity and evolutionary adaptation. Thirdly, we summarize our current knowledge of the underlying physiological adjustments to heat stress in arthropods from polar regions. It is clear that very little data are available on the heat tolerance of arthropods in polar regions, but that large variation in arthropod thermal tolerance exists across polar regions, habitats and species. Further, the species investigated show unique physiological adjustments to heat stress, such as their ability to respond quickly to increasing or extreme temperatures. To understand the consequences of climate change on terrestrial arthropods in polar regions, we suggest that more studies on the ability of species to cope with stressful high and variable temperatures are needed.

Responses of terrestrial polar arthropods to high and increasing temperatures.

Terrestrial arthropods in the Arctic and Antarctic are exposed to extreme and variable temperatures, and climate change is predicted to be especially pronounced in these regions. Available ecophysiological studies on terrestrial ectotherms from the Arctic and Antarctic typically focus on the ability of species to tolerate the extreme low temperatures that can occur in these regions, whereas studies investigating species plasticity and the importance of evolutionary adaptation to periodically high and increasing temperatures are limited. Here, we provide an overview of current knowledge on thermal adaptation to high temperatures of terrestrial arthropods in Arctic and Antarctic regions. Firstly, we summarize the literature on heat tolerance for terrestrial arthropods in these regions, and discuss variation in heat tolerance across species, habitats and polar regions. Secondly, we discuss the potential for species to cope with increasing and more variable temperatures through thermal plasticity and evolutionary adaptation. Thirdly, we summarize our current knowledge of the underlying physiological adjustments to heat stress in arthropods from polar regions. It is clear that very little data are available on the heat tolerance of arthropods in polar regions, but that large variation in arthropod thermal tolerance exists across polar regions, habitats and species. Further, the species investigated show unique physiological adjustments to heat stress, such as their ability to respond quickly to increasing or extreme temperatures. To understand the consequences of climate change on terrestrial arthropods in polar regions, we suggest that more studies on the ability of species to cope with stressful high and variable temperatures are needed.

Heat hardening capacity in Drosophila melanogaster is life stage-specific and juveniles show the highest plasticity.

Variations in stress resistance and adaptive plastic responses during ontogeny have rarely been addressed, despite the possibility that differences between life stages can affect species' range margins and thermal tolerance. Here, we assessed the thermal sensitivity and hardening capacity of Drosophila melanogaster across developmental stages from larval to the adult stage. We observed strong differences between life stages in heat resistance, with adults being most heat resistant followed by puparia, pupae and larvae. The impact of heat hardening (1 h at 35°C) on heat resistance changed during ontogeny, with the highest positive effect of hardening observed in puparia and pupae and the lowest in adults. These results suggest that immobile life stages ( puparia and pupae) have evolved high plasticity in upper thermal limits whereas adults and larvae rely more on behavioural responses to heat stress allowing them to escape from extreme high temperatures. While most studies on the plasticity of heat resistance in ectotherms have focused on the adult life stage, our findings emphasize the crucial importance of juvenile life stages of arthropods in understanding the thermal biology and life stage-specific physiological responses to variable and stressful high temperatures. Failure to acknowledge this complication might lead to biased estimates of species' ability to cope with environmental changes, such as climate change.

Temperature preference across life stages and acclimation temperatures investigated in four species of Drosophila.

Ectotherms can use microclimatic variation and behavioral thermoregulation to cope with unfavorable environmental temperatures. However, relatively little is known about how and if thermoregulatory behavior is used across life stages in small ectothermic insects. Here we investigate differences between three specialized Drosophila species from temperate, tropical or desert habitats and one cosmopolitan species by estimating the preferred temperature (Tpref) and the breadth (Tbreadth) of the distribution of adults, adult egg-laying, and larvae in thermal gradients. We also assess the plasticity of thermal preference following developmental acclimation to three constant temperatures. For egg-laying and larvae, we observe significant species differences in preferred temperature but this is not predicted by thermal ecology of the species. We corroborated this with previous studies of other Drosophila species and found that Tpref for egg laying and larvae have no relationship with annual mean temperature of the species' natural habitat. While adults have the greatest mobility, they show the greater variation in preference compared to juveniles contradicting common assumptions. We found evidence of developmental thermal acclimation in adult egg-laying preferred temperature, Tpref increasing with acclimation temperature, and in the breadth of the temperature preference distributions, Tbreadth decreasing with increasing acclimation temperature. Together, these data provide a high resolution and comprehensive look at temperature preferences across life stages and in response to acclimation. Results suggest that thermal preference, particularly in the early life stages, is relatively conserved among species and unrelated to temperature at species origin. Measuring thermal preference, in addition to thermal performance, is essential for understanding how species have adapted/will adapt to their thermal environment.

Linking developmental diet to adult foraging choice in Drosophila melanogaster.

Rather than maximizing intake of available macronutrients, insects increase intake of some nutrients and restrict intake of others. This selective consumption influences, and potentially optimizes, developmental time, reproduction and lifespan of the organism. Studies so far have focused on discriminating between protein and carbohydrate uptake and the consequences on fitness components at different life stages. However, it is largely unknown whether and how the developmental diets, which may entail habitat-specific nutrient restrictions, affect selective consumption in adults. We show that adult female D. melanogaster opt for the same protein to carbohydrate (P:C) ratio regardless of their developmental diet (P:C ratio of 1:1, 1:4 or 1:8). In contrast, males choose a diet that makes up for deficiencies; when protein is low during development, males increase protein consumption despite this being detrimental to starvation resistance. The sexual dimorphism in foraging choice could be due to the different energetic requirements of males and females. To investigate the effect of developmental diet on lifespan once an adult nutritional environment has been established, we also conducted a no-choice experiment. Here, adult lifespan increased as P:C ratio decreased, irrespective of developmental diet, thus demonstrating a 'cancelling out' effect of the nutritional environment experienced during early life stages. Our study provides novel insights into how developmental diet is linked to adult diet by presenting evidence for sexual dimorphism in foraging choice as well as life-stage dependency of diet on lifespan.

Increased lipid accumulation but not reduced metabolism explains improved starvation tolerance in cold-acclimated arthropod predators.

Predatory arthropods are used for biological control in greenhouses, but there is increasing interest to extend their use to the outdoor environment where temperatures are typically lower. Acclimation at low temperature increases the ability of ectotherms to cope with subsequent more extreme cold, but may involve costs or benefits to other performance traits. A recent study in mesostigmatid mites (Gaeolaelaps aculeifer) showed that starvation tolerance was improved following a period of cold exposure. However, the physiological mechanisms that underlie improved starvation tolerance following cold exposure were not investigated. To examine whether cold acclimation would also improve starvation tolerance in an insect, we repeated the starvation study in another arthropod predator, the pirate bug Orius majusculus, as well as in G. aculeifer. Before tests, the two species were acclimated at 10, 15, or 20 °C for 7 (G. aculeifer) or 16 (O. majusculus) days. We then analyzed the effects of thermal exposure on body composition, consumption, and basal metabolic rate in both species. Our results confirmed that exposure to low temperature improves starvation tolerance in these arthropod predators. Body composition analyses revealed that both species had accumulated larger lipid stores during exposure to colder temperature, which at least in part can explain the larger starvation tolerance following cold exposure. In contrast, consumption and basal metabolic rate were not changed by thermal acclimation. Our study indicates that predatory arthropods exposed to cold increase their physiological robustness and ability to endure environmental challenges, including low temperature and low prey availability.

Metabolic and functional characterization of effects of developmental temperature in Drosophila melanogaster.

The ability of ectotherms to respond to changes in their thermal environment through plastic mechanisms is central to their adaptive capability. However, we still lack knowledge on the physiological and functional responses by which ectotherms acclimate to temperatures during development, and in particular, how physiological stress at extreme temperatures may counteract beneficial acclimation responses at benign temperatures. We exposed Drosophila melanogaster to 10 developmental temperatures covering their entire permissible temperature range. We obtained metabolic profiles and reaction norms for several functional traits: egg-to-adult viability, developmental time, and heat and cold tolerance. Females were more heat tolerant than males, whereas no sexual dimorphism was found in cold tolerance. A group of metabolites, mainly free amino acids, had linear reaction norms. Several energy-carrying molecules, as well as some sugars, showed distinct inverted U-shaped norms of reaction across the thermal range, resulting in a positive correlation between metabolite intensities and egg-to-adult viability. At extreme temperatures, low levels of these metabolites were interpreted as a response characteristic of costs of homeostatic perturbations. Our results provide novel insights into a range of metabolites reported to be central for the acclimation response and suggest several new candidate metabolites. Low and high temperatures result in different adaptive physiological responses, but they also have commonalities likely to be a result of the failure to compensate for the physiological stress. We suggest that the regulation of metabolites that are tightly connected to the performance curve is important for the ability of ectotherms to cope with variation in temperature.

Unexpected high genetic diversity in small populations suggests maintenance by associative overdominance.

The effective population size (Ne ) is a central factor in determining maintenance of genetic variation. The neutral theory predicts that loss of variation depends on Ne , with less genetic drift in larger populations. We monitored genetic drift in 42 Drosophila melanogaster populations of different adult census population sizes (10, 50 or 500) using pooled RAD sequencing. In small populations, variation was lost at a substantially lower rate than expected. This observation was consistent across two ecological relevant thermal regimes, one stable and one with a stressful increase in temperature across generations. Estimated ratios between Ne and adult census size were consistently higher in small than in larger populations. The finding provides evidence for a slower than expected loss of genetic diversity and consequently a higher than expected long-term evolutionary potential in small fragmented populations. More genetic diversity was retained in areas of low recombination, suggesting that associative overdominance, driven by disfavoured homozygosity of recessive deleterious alleles, is responsible for the maintenance of genetic diversity in smaller populations. Consistent with this hypothesis, the X-chromosome, which is largely free of recessive deleterious alleles due to hemizygosity in males, fits neutral expectations even in small populations. Our experiments provide experimental answers to a range of unexpected patterns in natural populations, ranging from variable diversity on X-chromosomes and autosomes to surprisingly high levels of nucleotide diversity in small populations.

Reversibility of developmental heat and cold plasticity is asymmetric and has long-lasting consequences for adult thermal tolerance.

The ability of insects to cope with stressful temperatures through adaptive plasticity has allowed them to thrive under a wide range of thermal conditions. Developmental plasticity is generally considered to be a non-reversible phenotypic change, e.g. in morphological traits, while adult acclimation responses are often considered to be reversible physiological responses. However, physiologically mediated thermal acclimation might not follow this general prediction. We investigated the magnitude and rate of reversibility of developmental thermal plasticity responses in heat and cold tolerance of adult flies, using a full factorial design with two developmental and two adult temperatures (15 and 25°C). We show that cold tolerance attained during development is readily adjusted to the prevailing conditions during adult acclimation, with a symmetric rate of decrease or increase. In contrast, heat tolerance is only partly reversible during acclimation and is thus constrained by the temperature during development. The effect of adult acclimation on heat tolerance was asymmetrical, with a general loss of heat tolerance with age. Surprisingly, the decline in adult heat tolerance at 25°C was decelerated in flies developed at low temperatures. This result was supported by correlated responses in two senescence-associated traits and in accordance with a lower rate of ageing after low temperature development, suggesting that physiological age is not reset at eclosion. The results have profound ecological consequences for populations, as optimal developmental temperatures will be dependent on the thermal conditions faced in the adult stage and the age at which they occur.

Proteomic data reveal a physiological basis for costs and benefits associated with thermal acclimation.

Physiological adaptation through acclimation is one way to cope with temperature changes. Biochemical studies on acclimation responses in ectotherms have so far mainly investigated consequences of short-term acclimation at the adult stage and focussed on adaptive responses. Here, we assessed the consequences of rearing Drosophila melanogasterat low (12°C), benign (25°C) and high (31°C) temperatures. We assessed cold and heat tolerance and obtained detailed proteomic profiles of flies from the three temperatures. The proteomic profiles provided a holistic understanding of the underlying biology associated with both adaptive and non-adaptive temperature responses. Results show strong benefits and costs across tolerances: rearing at low temperature increased adult cold tolerance and decreased adult heat tolerance and vice versa with development at high temperatures. In the proteomic analysis, we were able to identify and quantify a large number of proteins compared with previous studies on ectotherms (1440 proteins across all replicates and rearing regimes), enabling us to extend the proteomic approach using enrichment analyses. This gave us detailed information on individual proteins, as well as pathways affected by rearing temperature, pinpointing potential mechanisms responsible for the strong costs and benefits of rearing temperature on functional phenotypes. Several well-known heat shock proteins, as well as proteins not previously associated with thermal stress, were among the differentially expressed proteins. Upregulation of proteasome proteins was found to be an important adaptive process at high-stress rearing temperatures, and occurs at the expense of downregulation of basal metabolic functions.

Preservation of potassium balance is strongly associated with insect cold tolerance in the field: a seasonal study of Drosophila subobscura.

There is interest in pinpointing genes and physiological mechanisms explaining intra- and interspecific variations in cold tolerance, because thermal tolerance phenotypes strongly impact the distribution and abundance of wild animals. Laboratory studies have highlighted that the capacity to preserve water and ion homeostasis is linked to low temperature survival in insects. It remains unknown, however, whether adaptive seasonal acclimatization in free-ranging insects is governed by the same physiological mechanisms. Here, we test whether cold tolerance in field-caught Drosophila subobscura is high in early spring and lower during summer and whether this transition is associated with seasonal changes in the capacity of flies to preserve water and ion balance during cold stress. Indeed, flies caught during summer were less cold tolerant, and exposure of these flies to sub-zero temperatures caused a loss of haemolymph water and increased the concentration of K(+) in the haemolymph (as in laboratory-reared insects). This pattern of ion and water balance disruption was not observed in more cold-tolerant flies caught in early spring. Thus, we here provide a field verification of hypotheses based on laboratory studies and conclude that the ability to maintain ion homeostasis is important for the ability of free-ranging insects to cope with chilling.

Cellular damage as induced by high temperature is dependent on rate of temperature change - investigating consequences of ramping rates on molecular and organismal phenotypes in Drosophila melanogaster.

Ecological relevance and repeatability of results obtained in different laboratories are key issues when assessing thermal tolerance of ectotherms. Traditionally, assays have used acute exposures to extreme temperatures. The outcomes of ecologically more relevant ramping experiments, however, are dependent on the rate of temperature change leading to uncertainty of the causal factor for loss of function. Here, we test the physiological consequences of exposing female Drosophila melanogaster to gradually increasing temperatures in so-called ramping assays. We exposed flies to ramping at rates of 0.06 and 0.1°C min(-1), respectively. Flies were sampled from the two treatments at 28, 30, 32, 34, 36 and 38°C and tested for heat tolerance and expression levels of the heat shock genes hsp23 and hsp70, as well as Hsp70 protein. Heat shock genes were upregulated more with a slow compared with a faster ramping rate, and heat knock-down tolerance was higher in flies exposed to the faster rate. The fact that slow ramping induces a stronger stress response (Hsp expression) compared with faster ramping suggests that slow ramping induces more heat damage at the cellular level due to longer exposure time. This is supported by the observation that fast ramped flies have higher heat knock-down tolerance. Thus we observed both accumulation of thermal damage at the molecular level and heat hardening at the phenotypic level as a consequence of heat exposure. The balance between these processes is dependent on ramping rate leading to the observed variation in thermal tolerance when using different rates.

Stress-induced plastic responses in Drosophila simulans following exposure to combinations of temperature and humidity levels.

Plastic responses to heat and desiccation stress in insects have been studied in many laboratory experiments on Drosophila. However, in these studies the possible interaction between the corresponding stress factors in natural environments has not been taken into consideration. We investigated changes in heat and desiccation resistance of adult Drosophila simulans after short-term exposures to different temperatures (35, 31 and 18°C) in combination with high and low relative humidity (ca. 90 and 20%, respectively). Hardening under extreme conditions (35 or 31°C and low relative humidity) commonly resulted in higher resistance to heat and desiccation as compared with other less stressful combinations of temperature and humidity levels. The concentration of the heat-shock protein Hsp70 in the experimental flies increased following almost all applied treatments. Life span of the hardened flies under non-stressful conditions was reduced irrespective of the stress dose, indicating a fitness cost for the plastic responses. The results of the study show that hardening using combined heat and desiccation stress can be very efficient with regard to induction of plastic responses improving tolerance to both types of stress. This may favour adaptation to hot and dry climatic conditions, though the negative effects on fitness are likely to constrain evolution of such plastic responses.

Into the wild-a field study on the evolutionary and ecological importance of thermal plasticity in ectotherms across temperate and tropical regions.

Understanding how environmental factors affect the thermal tolerance of species is crucial for predicting the impact of thermal stress on species abundance and distribution. To date, species' responses to thermal stress are typically assessed on laboratory-reared individuals and using coarse, low-resolution, climate data that may not reflect microhabitat dynamics at a relevant scale. Here, we examine the daily temporal variation in heat tolerance in a range of species in their natural environments across temperate and tropical Australia. Individuals were collected in their habitats throughout the day and tested for heat tolerance immediately thereafter, while local microclimates were recorded at the collection sites. We found high levels of plasticity in heat tolerance across all the tested species. Both short- and long-term variability of temperature and humidity affected plastic adjustments of heat tolerance within and across days, but with species differences. Our results reveal that plastic changes in heat tolerance occur rapidly at a daily scale and that environmental factors on a relatively short timescale are important drivers of the observed variation in thermal tolerance. Ignoring such fine-scale physiological processes in distribution models might obscure conclusions about species' range shifts with global climate change. This article is part of the theme issue 'Species' ranges in the face of changing environments (part 1)'.

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.

Dietary protein content affects evolution for body size, body fat and viability in Drosophila melanogaster.

The ability to use different food sources is likely to be under strong selection if organisms are faced with natural variation in macro-nutrient (protein, carbohydrate and lipid) availabilities. Here, we use experimental evolution to study how variable dietary protein content affects adult body composition and developmental success in Drosophila melanogaster. We reared flies on either a standard diet or a protein-enriched diet for 17 generations before testing them on both diet types. Flies from lines selected on protein-rich diet produced phenotypes with higher total body mass and relative lipid content when compared with those selected on a standard diet, irrespective of which of the two diets they were tested on. However, selection on protein-rich diet incurred a cost as flies reared on this diet had markedly lower developmental success in terms of egg-to-adult viability on both medium types, suggesting a possible trade-off between the traits investigated.

Costs and benefits of cold acclimation in field-released Drosophila.

One way animals can counter the effects of climatic extremes is via physiological acclimation, but acclimating to one extreme might decrease performance under different conditions. Here, we use field releases of Drosophila melanogaster on two continents across a range of temperatures to test for costs and benefits of developmental or adult cold acclimation. Both types of cold acclimation had enormous benefits at low temperatures in the field; in the coldest releases only cold-acclimated flies were able to find a resource. However, this advantage came at a huge cost; flies that had not been cold-acclimated were up to 36 times more likely to find food than the cold-acclimated flies when temperatures were warm. Such costs and strong benefits were not evident in laboratory tests where we found no reduction in heat survival of the cold-acclimated flies. Field release studies, therefore, reveal costs of cold acclimation that standard laboratory assays do not detect. Thus, although physiological acclimation may dramatically improve fitness over a narrow set of thermal conditions, it may have the opposite effect once conditions extend outside this range, an increasingly likely scenario as temperature variability increases under global climate change.