Adult heat tolerance variation in Drosophila melanogaster is not related to Hsp70 expression

Naturally segregating genetic variants contribute to thermal tolerance in a Drosophila melanogaster model system

Thermal tolerance is a fundamental physiological complex trait for survival in many species. For example, everyday tasks such as foraging, finding a mate, and avoiding predation are highly dependent on how well an organism can tolerate extreme temperatures. Understanding the general architecture of the natural variants within the genes that control this trait is of high importance if we want to better comprehend thermal physiology. Here, we take a multipronged approach to further dissect the genetic architecture that controls thermal tolerance in natural populations using the Drosophila Synthetic Population Resource as a model system. First, we used quantitative genetics and Quantitative Trait Loci mapping to identify major effect regions within the genome that influences thermal tolerance, then integrated RNA-sequencing to identify differences in gene expression, and lastly, we used the RNAi system to (1) alter tissue-specific gene expression and (2) functionally validate our findings. This powerful integration of approaches not only allows for the identification of the genetic basis of thermal tolerance but also the physiology of thermal tolerance in a natural population, which ultimately elucidates thermal tolerance through a fitness-associated lens.

Heat hardening of a larval amphibian is dependent on acclimation period and temperature

Plasticity in heat tolerance provides ectotherms the ability to reduce overheating risk during thermal extremes. However, the tolerance-plasticity trade-off hypothesis states that individuals acclimated to warmer environments have a reduced plastic response, including hardening, limiting their ability to further adjust their thermal tolerance. Heat hardening describes the short-term increase in heat tolerance following a heat shock that remains understudied in larval amphibians. We sought to examine the potential trade-off between basal heat tolerance and hardening plasticity of a larval amphibian, Lithobates sylvaticus, in response to differing acclimation temperatures and periods. Lab-reared larvae were exposed to one of two acclimation temperatures (15°C and 25°C) for either 3 or 7 days, at which time heat tolerance was measured as critical thermal maximum (CT_max ). A hardening treatment (sub-critical temperature exposure) was applied 2 h before the CT_max assay for comparison to control groups. We found that heat-hardening effects were most pronounced in 15°C acclimated larvae, particularly after 7 days of acclimation. By contrast, larvae acclimated to 25°C exhibited only minor hardening responses, while basal heat tolerance was significantly increased as shown by elevated CT_max temperatures. These results are in line with the tolerance-plasticity trade-off hypothesis. Specifically, while exposure to elevated temperatures induces acclimation in basal heat tolerance, shifts towards upper thermal tolerance limits constrain the capacity for ectotherms to further respond to acute thermal stress.

Hsian-Tsao (Mesona chinensis Benth.) Extract Improves the Thermal Tolerance of Drosophila melanogaster

Global warming has prompted scientific communities to consider how to alleviate thermal stress in humans and animals. The present study assessed the supplementation of hsian-tsao extract (HTE) on thermal stress in Drosophila melanogaster and preliminarily explicated its possible physiological and molecular mechanisms. Our results indicated that the lethal time for 50% of female flies fed on HTE was significantly longer than that of male flies at the same heat stress temperature. Under thermal stress, the survival time of females was remarkably increased in the HTE addition groups compared to the non-addition group. Thermal hardening by acute exposure to 36°C for 30 min (9:00 to 9:30 a.m.) every day could significantly prolong the longevity of females. Without thermal hardening, HTE increased the antioxidant capacity of females under heat stress, accompanied by an increment of catalase (CAT) activity, and the inhibition for hydroxyl radicals (OH⋅) and superoxide anions (⋅O2 -). Superoxide dismutase (SOD) activity and the inhibition for ⋅O2 - was significantly affected by thermal hardening in the non-HTE addition groups, and significant differences were shown in CAT and SOD activities, and the inhibition for ⋅O2 - among groups with thermal hardening. After heat exposure, heat shock protein 70 (Hsp70) was only up-regulated in the group with high levels of added HTE compared with the group without and this was similar in the thermal hardening group. It was concluded that the heat stress-relieving ability of HTE might be partly due to the enhancement of enzymatic activities of SOD and CAT, and the inhibition for OH⋅ and ⋅O2 -. However, the expression levels of Hsp70 were not well related to thermal tolerance or heat survival.

Temporal specific coevolution of Hsp70 and co-chaperone stv expression in Drosophila melanogaster under selection for heat tolerance

Heat shock proteins (Hsps) have long been candidates for ecological adaptation given their unequivocal role in mitigating cell damage from heat stress, but linking Hsps to heat tolerance has proven difficult given the complexity of thermal adaptation. Experimental evolution has been utilized to examine direct and correlated responses to selection for increased heat tolerance in Drosophila, often focusing on the major Hsp family Hsp70 and/or the master regulator HSF as a selection response, but rarely on other aspects of the heat shock complex. We examined Hsp70 and co-chaperone stv isoform transcript expression in Australian D. melanogaster lines selected for static heat tolerance, and observed a temporal and stv isoform specific, coordinated transcriptional selection response with Hsp70, suggesting that increased chaperone output accompanied increased heat tolerance. We hypothesize that the coordinated evolutionary response of Hsp70 and stv may have arisen as a correlated response resulting from a shared regulatory hierarchy. Our work highlights the complexity and specificity of the heat shock response in D. melanogaster. The selected lines examined also showed correlated responses for other measures of heat tolerance, and the coevolution of Hsp70 and stv provide new avenues to examine the common mechanisms underpinning direct and correlated phenotypic responses to selection for heat tolerance.

Membrane lipid metabolism, heat shock response and energy costs mediate the interaction between acclimatization and heat-hardening response in the razor clam Sinonovacula constricta

Thermal plasticity on different time scales, including acclimation/acclimatization and heat-hardening response - a rapid adjustment for thermal tolerance after non-lethal thermal stress, can interact to improve the resilience of organisms to thermal stress. However, little is known about physiological mechanisms mediating this interaction. To investigate the underpinnings of heat-hardening responses after acclimatization in warm seasons, we measured thermal tolerance plasticity, and compared transcriptomic and metabolomic changes after heat hardening at 33 or 37°C followed by recovery of 3 or 24 h in an intertidal bivalve Sinonovacula constricta. Clams showed explicit heat-hardening responses after acclimatization in a warm season. The higher inducing temperature (37°C) caused less effective heat-hardening effects than the inducing temperature that was closer to the seasonal maximum temperature (33°C). Metabolomic analysis highlighted the elevated content of glycerophospholipids in all heat-hardened clams, which may help to maintain the structure and function of the membrane. Heat shock proteins (HSPs) tended to be upregulated after heat hardening at 37°C but not at 33°C, indicating that there was no complete dependency of heat-hardening effects on upregulated HSPs. Enhanced energy metabolism and decreased energy reserves were observed after heat hardening at 37°C, suggesting more energy costs during exposure to a higher inducing temperature, which may restrict heat-hardening effects. These results highlight the mediating role of membrane lipid metabolism, heat shock responses and energy costs in the interaction between heat-hardening response and seasonal acclimatization, and contribute to the mechanistic understanding of evolutionary change and thermal plasticity during global climate change.

Expression of Heat Shock Protein 70 Is Insufficient To Extend Drosophila melanogaster Longevity

It has been known for over 20 years that Drosophila melanogaster flies with twelve additional copies of the hsp70 gene encoding the 70 kD heat shock protein lives longer after a non-lethal heat treatment. Since the heat treatment also induces the expression of additional heat shock proteins, the biological effect can be due either to HSP70 acting alone or in combination. This study used the UAS/GAL4 system to determine whether hsp70 is sufficient to affect the longevity and the resistance to thermal, oxidative or desiccation stresses of the whole organism. We observed that HSP70 expression in the nervous system or muscles has no effect on longevity or stress resistance but ubiquitous expression reduces the life span of males. We also observed that the down-regulation of hsp70 using RNAi did not affect longevity.

Heat shock protein 70 from a thermotolerant Diptera species provides higher thermoresistance to Drosophila larvae than correspondent endogenous gene

Heat shock proteins (Hsp70s) from two Diptera species that drastically differ in their heat shock response and longevity were investigated. Drosophila melanogaster is characterized by the absence of Hsp70 and other hsps under normal conditions and the dramatic induction of hsp synthesis after temperature elevation. The other Diptera species examined belongs to the Stratiomyidae family (Stratiomys singularior) and exhibits high levels of inducible Hsp70 under normal conditions coupled with a thermotolerant phenotype and much longer lifespan. To evaluate the impact of hsp70 genes on thermotolerance and longevity, we made use of a D. melanogaster strain that lacks all hsp70 genes. We introduced single copies of either S. singularior or D. melanogaster hsp70 into this strain and monitored the obtained transgenic flies in terms of thermotolerance and longevity. We developed transgenic strains containing the S. singularior hsp70 gene under control of a D. melanogaster hsp70 promoter. Although these adult flies did synthesize the corresponding mRNA after heat shock, they were not superior to the flies containing a single copy of D. melanogaster hsp70 in thermotolerance and longevity. By contrast, Stratiomyidae Hsp70 provided significantly higher thermotolerance at the larval stage in comparison with endogenous Hsp70.

The expression pattern of hsp70 plays a critical role in thermal tolerance of marine demersal fish: Multilevel responses of Paralichthys olivaceus and its hybrids (P. olivaceus ♀ × P. dentatus ♂) to chronic and acute heat stress

Ocean warming has multifaceted impacts on marine organisms. This study investigated the different responses of Paralichthys olivaceus and the hybrids (P. olivaceus ♀ × P. dentatus ♂) to chronic and acute heat stress. By comparing their survival, behavioural and histological changes, we found that the hybrids possess a better thermal tolerance with a higher cumulative survival rate (CSR), relatively fewer behavioural changes and less gill damage. Moreover, we analysed the relationship between thermal tolerance and the hsp70 expression pattern and found that thermal tolerant species (the hybrids) exhibited higher threshold induction temperature, shorter durations, stronger magnitudes and a delay in hsp70 expression. We speculated that the expression mode of hsp70, rather than itself, plays a critical role in thermal tolerance. These findings would improve the understanding of hsp70 in future marine climate research and help clarify the profound effects of rising temperature on marine demersal fishes.

Evolutionary potential of upper thermal tolerance: biogeographic patterns and expectations under climate change

How will organisms respond to climate change? The rapid changes in global climate are expected to impose strong directional selection on fitness-related traits. A major open question then is the potential for adaptive evolutionary change under these shifting climates. At the most basic level, evolutionary change requires the presence of heritable variation and natural selection. Because organismal tolerances of high temperature place an upper bound on responding to temperature change, there has been a surge of research effort on the evolutionary potential of upper thermal tolerance traits. Here, I review the available evidence on heritable variation in upper thermal tolerance traits, adopting a biogeographic perspective to understand how heritability of tolerance varies across space. Specifically, I use meta-analytical models to explore the relationship between upper thermal tolerance heritability and environmental variability in temperature. I also explore how variation in the methods used to obtain these thermal tolerance heritabilities influences the estimation of heritable variation in tolerance. I conclude by discussing the implications of a positive relationship between thermal tolerance heritability and environmental variability in temperature and how this might influence responses to future changes in climate.

Expression profiling of heat shock genes in a scuttle fly Megaselia scalaris (Diptera, Phoridae)

The Phoridae are a family of small, hump-backed flies, dominating in post-fire areas. Some of these flies are probably able to survive a fire as an egg, larva, or pupa, and may be adapted to the fire-altered environment at the genomic level. In this study, we describe the influence of short-term temperature treatment on the expression of seven heat shock protein genes in the third-instar larvae and imagoes of a scuttle fly Megaselia scalaris-one of the cosmopolitan and polyphagous phorids. In terms of the response to temperature treatment, these genes tested against tubulin as a reference split into three classes. The first class consists of hsp22 (larvae), hsp23 (larvae), and hsp26 (both larvae and imagoes), and is upregulated at the lowest temperature (33°C). The second class consists of hsp22 (imagoes), hsp23 (imagoes), hsp40 (larvae and imagoes), and hsp70 (larvae and imagoes), and is upregulated or induced at 37°C. Expression of genes of the third class (hsp27 and hsp83-larvae and imagoes) increased after treatment at 41°C temperature. Expression of the first two classes of genes occurred at a temperature lower than the LT₅₀ of larvae and imagoes. The fact that there is a gap between the temperature upregulating hsp genes and the temperature leading to the loss of viability suggests that not only the level of hsp gene expression but also the temperature at which gene expression increased is important in an adaptation of M. scalaris to harsh environment. J. Exp. Zool. 323A: 704-713, 2015. © 2015 Wiley Periodicals, Inc.

Heat shock protein expression enhances heat tolerance of reptile embryos

The role of heat shock proteins (HSPs) in heat tolerance has been demonstrated in cultured cells and animal tissues, but rarely in whole organisms because of methodological difficulties associated with gene manipulation. By comparing HSP70 expression patterns among representative species of reptiles and birds, and by determining the effect of HSP70 overexpression on embryonic development and hatchling traits, we have identified the role of HSP70 in the heat tolerance of amniote embryos. Consistent with their thermal environment, and high incubation temperatures and heat tolerance, the embryos of birds have higher onset and maximum temperatures for induced HSP70 than do reptiles, and turtles have higher onset and maximum temperatures than do lizards. Interestingly, the trade-off between benefits and costs of HSP70 overexpression occurred between life-history stages: when turtle embryos developed at extreme high temperatures, HSP70 overexpression generated benefits by enhancing embryo heat tolerance and hatching success, but subsequently imposed costs by decreasing heat tolerance of surviving hatchlings. Taken together, the correlative and causal links between HSP70 and heat tolerance provide, to our knowledge, the first unequivocal evidence that HSP70 promotes thermal tolerance of embryos in oviparous amniotes.

Natural populations of Arabidopsis thaliana differ in seedling responses to high-temperature stress

Little is known about adaptive within-species variation in thermotolerance in wild plants despite its likely role in both functional adaptation at range limits and in predicting response to climate change. Heat shock protein Hsp101, rapidly heat induced in Arabidopsis thaliana, plays a central role in thermotolerance in laboratory studies, yet little is known about variation in its expression in natural populations. We explored variation in thermotolerance and Hsp101 expression in seedlings from 16 natural populations of A. thaliana sampled along an elevation and climate gradient. We tested both naive controls (maintained at 22 °C until heat stress) and thermally pre-acclimated plants (exposed to a 38 °C 3-h acclimation treatment). After acclimation, seedlings were exposed to one of two heat stresses: 42 or 45 °C. Thermotolerance was measured as post-stress seedling survival and root growth. When stressed at 45 °C, both thermotolerance and Hsp101 expression were significantly increased by pre-acclimation. However, thermotolerance did not differ between pre-acclimation and control when followed by a 42 °C stress. Immediately after heat stress, pre-acclimated seedlings contained significantly more Hsp101 than control seedlings. At 45 °C, Hsp101 expression was positively associated with survival (r(2) = 0.37) and post-stress root growth (r(2) = 0.15). Importantly, seedling survival, post-stress root growth at 45 °C and Hsp101 expression at 42 °C were significantly correlated with the home sites' first principal component of climate variation. This climate gradient mainly reflects a temperature and precipitation gradient. Thus, the extent of Hsp101 expression modulation and thermotolerance appear to be interrelated and to evolve adaptively in natural populations of A. thaliana.

Spatial analysis of gene regulation reveals new insights into the molecular basis of upper thermal limits

The cellular stress response has long been the primary model for studying the molecular basis of thermal adaptation, yet the link between gene expression, RNA metabolism and physiological responses to thermal stress remains largely unexplored. We address this by comparing the transcriptional and physiological responses of three geographically distinct populations of Drosophila melanogaster from eastern Australia in response to, and recovery from, a severe heat stress with and without a prestress hardening treatment. We focus on starvin (stv), recently identified as an important thermally responsive gene. Intriguingly, stv encodes seven transcripts from alternative transcription sites and alternative splicing, yet appears to be rapidly heat inducible. First, we show genetic differences in upper thermal limits of the populations tested. We then demonstrate that the stv locus does not ubiquitously respond to thermal stress but is expressed as three distinct thermal and temporal RNA phenotypes (isoforms). The shorter transcript isoforms are rapidly upregulated under stress in all populations and show similar molecular signatures to heat-shock proteins. Multiple stress exposures seem to generate a reserve of pre-mRNAs, effectively 'priming' the cells for subsequent stress. Remarkably, we demonstrate a bypass in the splicing blockade in these isoforms, suggesting an essential role for these transcripts under heat stress. Temporal profiles for the weakly heat responsive stv isoform subset show opposing patterns in the two most divergent populations. Innate and induced transcriptome responses to hyperthermia are complex, and warrant moving beyond gene-level analyses.

Heat-dependent fecundity enhancement observed in Nilaparvata lugens (Hemiptera: Delphacidae) after treatment with triazophos

The brown planthopper Nilaparvata lugens (Stål) (Hemiptera: Delphacidae) is a serious pest of rice crops in the temperate and tropical regions of Asia and Australia, and it is also a classic example of an insecticide-induced resurgent pest. Brown planthopper outbreaks have been reported to be closely associated with pesticide application. Previous studies have shown that the insecticide triazophos enhances thermal tolerance and fecundity in brown planthopper. However, the effects of triazophos and high temperature on reproductive capacity have not been studied in high temperature-conditioned reciprocal pairs of adult males and females. The present experiments showed that triazophos enhanced the reproductive capacity of brown planthopper under high temperature (34°C). The number of eggs laid by females treated with 40 ppm triazophos at 34°C approximately doubled compared with untreated insects. Furthermore, the triazophos-induced fecundity enhancement at 34°C was significantly greater than that at 26°C, and the number of eggs laid for mating pair of adult males at 34°C with adult females under 34°C (♂(34) × ♀(34)) were significantly greater than that of adult males at 26°C (♂(26) × ♀3(4)), suggesting that this insecticide enhances the resistance of brown planthopper to high-temperature stress. Insecticide-induced enhancement of reproductive capacity in brown planthopper under high temperatures should be of great concern, and it has important implications for forecasting future brown planthopper outbreaks as well as the pesticide-selection process.

Intraspecific variation in cellular and biochemical heat response strategies of Mediterranean Xeropicta derbentina [Pulmonata, Hygromiidae]

Dry and hot environments challenge the survival of terrestrial snails. To minimize overheating and desiccation, physiological and biochemical adaptations are of high importance for these animals. In the present study, seven populations of the Mediterranean land snail species Xeropicta derbentina were sampled from their natural habitat in order to investigate the intraspecific variation of cellular and biochemical mechanisms, which are assigned to contribute to heat resistance. Furthermore, we tested whether genetic parameters are correlated with these physiological heat stress response patterns. Specimens of each population were individually exposed to elevated temperatures (25 to 52°C) for 8 h in the laboratory. After exposure, the health condition of the snails' hepatopancreas was examined by means of qualitative description and semi-quantitative assessment of histopathological effects. In addition, the heat-shock protein 70 level (Hsp70) was determined. Generally, calcium cells of the hepatopancreas were more heat resistant than digestive cells - this phenomenon was associated with elevated Hsp70 levels at 40°C.We observed considerable variation in the snails' heat response strategy: Individuals from three populations invested much energy in producing a highly elevated Hsp70 level, whereas three other populations invested energy in moderate stress protein levels - both strategies were in association with cellular functionality. Furthermore, one population kept cellular condition stable despite a low Hsp70 level until 40°C exposure, whereas prominent cellular reactions were observed above this thermal limit. Genetic diversity (mitochondrial cytochrome c oxidase subunit I gene) within populations was low. Nevertheless, when using genetic indices as explanatory variables in a multivariate regression tree (MRT) analysis, population structure explained mean differences in cellular and biochemical heat stress responses, especially in the group exposed to 40°C. Our study showed that, even in similar habitats within a close range, populations of the same species use different stress response strategies that all rendered survival possible.

Genetics of climate change adaptation

The rapid rate of current global climate change is having strong effects on many species and, at least in some cases, is driving evolution, particularly when changes in conditions alter patterns of selection. Climate change thus provides an opportunity for the study of the genetic basis of adaptation. Such studies include a variety of observational and experimental approaches, such as sampling across clines, artificial evolution experiments, and resurrection studies. These approaches can be combined with a number of techniques in genetics and genomics, including association and mapping analyses, genome scans, and transcription profiling. Recent research has revealed a number of candidate genes potentially involved in climate change adaptation and has also illustrated that genetic regulatory networks and epigenetic effects may be particularly relevant for evolution driven by climate change. Although genetic and genomic data are rapidly accumulating, we still have much to learn about the genetic architecture of climate change adaptation.

Hsp70 protein levels and thermotolerance in Drosophila subobscura: a reassessment of the thermal co-adaptation hypothesis

Theory predicts that geographic variation in traits and genes associated with climatic adaptation may be initially driven by the correlated evolution of thermal preference and thermal sensitivity. This assumes that an organism's preferred body temperature corresponds with the thermal optimum in which performance is maximized; hence, shifts in thermal preferences affect the subsequent evolution of thermal-related traits. Drosophila subobscura evolved worldwide latitudinal clines in several traits including chromosome inversion frequencies, with some polymorphic inversions being apparently associated with thermal preference and thermal tolerance. Here we show that flies carrying the warm-climate chromosome arrangement O(3+4) have higher basal protein levels of Hsp70 than their cold-climate O(st) counterparts, but this difference disappears after heat hardening. O(3+4) carriers are also more heat tolerant, although it is difficult to conclude from our results that this is causally linked to their higher basal levels of Hsp70. The observed patterns are consistent with the thermal co-adaptation hypothesis and suggest that the interplay between behaviour and physiology underlies latitudinal and seasonal shifts in inversion frequencies.

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 Meigen 1830

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 per minute, 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 up-regulated more with a slow as compared to 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 to 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 on the molecular level and heat hardening on 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.

Basal cold but not heat tolerance constrains plasticity among Drosophila species (Diptera: Drosophilidae)

Thermal tolerance and its plasticity are important for understanding ectotherm responses to climate change. However, it is unclear whether plasticity is traded-off at the expense of basal thermal tolerance and whether plasticity is subject to phylogenetic constraints. Here, we investigated associations between basal thermal tolerance and acute plasticity thereof in laboratory-reared adult males of eighteen Drosophila species at low and high temperatures. We determined the high and low temperatures where 90% of flies are killed (ULT(90) and LLT(90) , respectively) and also the magnitude of plasticity of acute thermal pretreatments (i.e. rapid cold- and heat-hardening) using a standardized, species-specific approach for the induction of hardening responses. Regression analyses of survival variation were conducted in ordinary and phylogenetically informed approaches. Low-temperature pretreatments significantly improved LLT(90) in all species tested except for D. pseudoobscura, D. mojavensis and D. borealis. High-temperature pretreatment only significantly increased ULT(90) in D. melanogaster, D. simulans, D. pseudoobscura and D. persimilis. LLT(90) was negatively correlated with low-temperature plasticity even after phylogeny was accounted for. No correlations were found between ULT(90) and LLT(90) or between ULT(90) and rapid heat-hardening (RHH) in ordinary regression approaches. However, after phylogenetic adjustment, there was a positive correlation between ULT(90) and RHH. These results suggest a trade-off between basal low-temperature tolerance and acute low-temperature plasticity, but at high temperatures, increased basal tolerance was accompanied by increased plasticity. Furthermore, high- and low-temperature tolerances and their plasticity are clearly decoupled. These results are of broad significance to understanding how organisms respond to changes in habitat temperature and the degree to which they can adjust thermal sensitivity.

Survival rate and expression of Heat-shock protein 70 and Frost genes after temperature stress in Drosophila melanogaster lines that are selected for recovery time from temperature coma

In this study, we investigated the physiological mechanisms underlying temperature tolerance using Drosophila melanogaster lines with rapid, intermediate, or slow recovery from heat or chill coma that were established by artificial selection or by free recombination without selection. Specifically, we focused on the relationships among their recovery from heat or chill coma, survival after severe heat or cold, and survival enhanced by rapid cold hardening (RCH) or heat hardening. The recovery time from heat coma was not related to the survival rate after severe heat. The line with rapid recovery from chill coma showed a higher survival rate after severe cold exposure, and therefore the same mechanisms are likely to underlie these phenotypes. The recovery time from chill coma and survival rate after severe cold were unrelated to RCH-enhanced survival. We also examined the expression of two genes, Heat-shock protein 70 (Hsp70) and Frost, in these lines to understand the contribution of these stress-inducible genes to intraspecific variation in recovery from temperature coma. The line showing rapid recovery from heat coma did not exhibit higher expression of Hsp70 and Frost. In addition, Hsp70 and Frost transcription levels were not correlated with the recovery time from chill coma. Thus, Hsp70 and Frost transcriptional regulation was not involved in the intraspecific variation in recovery from temperature coma.

Genetic constraints for thermal coadaptation in Drosophila subobscura

Background

Behaviour has been traditionally viewed as a driver of subsequent evolution because behavioural adjustments expose organisms to novel environments, which may result in a correlated evolution on other traits. In Drosophila subobscura, thermal preference and heat tolerance are linked to chromosomal inversion polymorphisms that show parallel latitudinal clines worldwide, such that "cold-climate" ("warm-climate") chromosome arrangements collectively favour a coherent response to colder (warmer) settings as flies carrying them prefer colder (warmer) conditions and have lower (higher) knock out temperatures. Yet, it is not clear whether a genetic correlation between thermal preference and heat tolerance can partially underlie such response.

Results

We have analyzed the genetic basis of thermal preference and heat tolerance using isochromosomal lines in D. subobscura. Chromosome arrangements on the O chromosome were known to have a biometrical effect on thermal preference in a laboratory temperature gradient, and also harbour several genes involved in the heat shock response; in particular, the genes Hsp68 and Hsp70. Our results corroborate that arrangements on chromosome O affect adult thermal preference in a laboratory temperature gradient, with cold-climate O_st carriers displaying a lower thermal preference than their warm-climate O₃₊₄ and O₃₊₄₊₈ counterparts. However, these chromosome arrangements did not have any effect on adult heat tolerance and, hence, we putatively discard a genetic covariance between both traits arising from linkage disequilibrium between genes affecting thermal preference and candidate genes for heat shock resistance. Nonetheless, a possible association of juvenile thermal preference and heat resistance warrants further analysis.

Conclusions

Thermal preference and heat tolerance in the isochromosomal lines of D. subobscura appear to be genetically independent, which might potentially prevent a coherent response of behaviour and physiology (i.e., coadaptation) to thermal selection. If this pattern is general to all chromosomes, then any correlation between thermal preference and heat resistance across latitudinal gradients would likely reflect a pattern of correlated selection rather than genetic correlation.

A comprehensive assessment of geographic variation in heat tolerance and hardening capacity in populations of Drosophila melanogaster from eastern Australia

We examined latitudinal variation in adult and larval heat tolerance in Drosophila melanogaster from eastern Australia. Adults were assessed using static and ramping assays. Basal and hardened static heat knockdown time showed significant linear clines; heat tolerance increased towards the tropics, particularly for hardened flies, suggesting that tropical populations have a greater hardening response. A similar pattern was evident for ramping heat knockdown time at 0.06°C min(-1) increase. There was no cline for ramping heat knockdown temperature (CT(max) ) at 0.1°C min(-1) increase. Acute (static) heat knockdown temperature increased towards temperate latitudes, probably reflecting a greater capacity of temperate flies to withstand sudden temperature increases during summer in temperate Australia. Larval viability showed a quadratic association with latitude under heat stress. Thus, patterns of heat resistance depend on assay methods. Genetic correlations in thermotolerance across life stages and evolutionary potential for critical thermal limits should be the focus of future studies.