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 (evolve and resequence) is a highly effective tool that has been increasingly employed to elucidate the genetic basis of adaptation. The number of thermal evolve and resequence 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 behavioral 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 standardization of methodologies and the utilization of new technologies focusing on the integration of genomic and phenotypic variation in the context of thermal adaptation.

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.

Fahrenheit 101

Is there any convincing explanation for why mammals and birds maintain their body temperature close to 40°C?

CHROMOSOMAL ANALYSIS OF HEAT-SHOCK TOLERANCE IN DROSOPHILA MELANOGASTER EVOLVING AT DIFFERENT TEMPERATURES IN THE LABORATORY

We investigated the heat tolerance of adults of three replicated lines of Drosophila melanogaster that have been evolving independently by laboratory natural selection for 15 yr at "nonextreme" temperatures (18°C, 25°C, or 28°C). These lines are known to have diverged in body size and in the thermal dependence of several life-history traits. Here we show that they differ also in tolerance of extreme high temperature as well as in induced thermotolerance ("heat hardening"). For example, the 28°C flies had the highest probability of surviving a heat shock, whereas the 18°C flies generally had the lowest probability. A short heat pretreatment increased the heat tolerance of the 18°C and 25°C lines, and the threshold temperature necessary to induce thermotolerance was lower for the 18°C line than for the 25°C line. However, neither heat pretreatment nor acclimation to different temperatures influenced heat tolerance of the 28°C line, suggesting the loss of capacity for induced thermotolerance and for acclimation. Thus, patterns of tolerance of extreme heat, of acclimation, and of induced thermotolerance have evolved as correlated responses to natural selection at nonextreme temperatures. A genetic analysis of heat tolerance of a representative replicate population each from the 18°C and 28°C lines indicates that chromosomes 1, 2, and 3 have significant effects on heat tolerance. However, the cytoplasm has little influence, contrary to findings in an earlier study of other stocks that had been evolving for 7 yr at 14°C versus 25°C. Because genes for heat stress proteins (hsps) are concentrated on chromosome 3, the potential role of hsps in the heat tolerance and of induced thermotolerance in these naturally selected lines is currently unclear. In any case, species of Drosophila possess considerable genetic variation in thermal sensitivity and thus have the potential to evolve rapidly in response to climate change; but predicting that response may be difficult.

Protein synthesis rates in Drosophila associate with levels of the hsr-omega nuclear transcript

Transcripts of the Drosophila hsr-omega gene are known to interact with RNA processing factors and ribosomes and are postulated to aid in co-ordinating nuclear and cytoplasmic activities particularly in stressed cells. However, the significance of these interactions for physiological processes and in turn for whole-organism fitness remains an open question. Because hsr-omega's cellular expression characteristics suggest it may influence protein synthesis, and because both genotypic and expression variation of hsr-omega have been associated with thermotolerance, we characterised 30 lines for variation in the rates of protein synthesis, measured in ovarian tissues, both before and after a mild heat shock, and for basal levels of the two main hsr-omega transcripts, omega-n and omega-c. As expected, the mild heat shock reduced protein synthesis rates. Large variation occurred among lines in levels of omega-n which was negatively associated with rates of basal protein synthesis--a result that supports the model for the cellular function of omega-n. Furthermore, omega-n levels were associated with hsr-omega genotype of the line parents. Little variation occurred among lines for omega-c levels and no associations were detected with protein synthesis or genotype. Since protein synthesis is a fundamental process for growth and development, we characterised the lines for several life-history traits; however, no associations with protein synthesis, omega-n or omega-c levels were detected. Our results are consistent with the idea that natural variation in hsr-omega expression influence rates of protein synthesis in this species.

Effect of exposure to short-term heat stress on survival and fecundity in Drosophila ananassae

In ectothermic organisms, temperature plays a vital role in survival and reproductive success. Founding isofemale lines from wild-collected females is a basic tool for characterizing quantitative traits in a natural population. The effects of heat shock, i.e., short exposure to heat stress, on survival and reproductive success in 10 recently collected isofemale lines of Drosophila ananassae Doleschall, 1858 were compared. Flies were treated as follows: (i) unstressed control; (ii) placed at 37 °C for 90 min (pre-treatment); (iii) placed at 40 °C for 60 min 24 h before the fecundity test; and (iv) placed at 40 °C for 60 min with pre-treatment 16 h before they were exposed to 40 °C. The heat stress strongly affected survival. However, there was no significant difference between the survival of males and females. Furthermore, the female fecundity, measured as F1 offspring produced over the next 12 days, was also reduced. Heat pre-treatment improved survival of both male and female and also improved female productivity. We found significant variation in female fecundity among isofemale lines, and intraclass correlations increased for stress treatments. The results suggest that a small increase in environmental stress may affect fitness traits, and there is enough genetic variation for thermal adaptation in D. ananassae.

Functional and physiological consequences of genetic variation at phosphoglucose isomerase: heat shock protein expression is related to enzyme genotype in a montane beetle

Allele frequency variation at the phosphoglucose isomerase (PGI) locus in Californian populations of the beetle Chrysomela aeneicollis suggests that PGI may be undergoing natural selection. We quantified (i) apparent Michaelis-Menten constant (K(m)) of fructose 6-phosphate at different temperatures and (ii) thermal stability for three common PGI genotypes (1-1, 1-4, and 4-4). We also measured air temperature (T(a)) and beetle body temperature (T(b)) in three montane drainages in the Sierra Nevada, California. Finally, we measured 70-kDa heat shock protein (Hsp70) expression in field-collected and laboratory-acclimated beetles. We found that PGI allele 1 predominated in the northernmost drainage, Rock Creek (RC), which was also significantly cooler than the southernmost drainage, Big Pine Creek (BPC), where PGI allele 4 predominated. Allele frequencies and air temperatures were intermediate in the middle drainage, Bishop Creek (BC). Differences among genotypes in K(m) (1-1 > 1-4 > 4-4) and thermal stability (4-4 > 1-4 > 1-1) followed a pattern consistent with temperature adaptation. In nature, T(b) was closely related to T(a). Hsp70 expression in adult beetles decreased with elevation and differed among drainages (BPC > BC > RC). After laboratory acclimation (8 days, 20 degrees C day, 4 degrees C night) and heat shock (4 h, 28-36 degrees C), Hsp70 expression was greater for RC than BPC beetles. In RC, field-collected beetles homozygous for PGI 1-1 had higher Hsp70 levels than heterozygotes or a 4-4 homozygote. These results reveal functional and physiological differences among PGI genotypes, which suggest that montane populations of this beetle are locally adapted to temperature.

Heat-shock proteins, molecular chaperones, and the stress response: evolutionary and ecological physiology

Molecular chaperones, including the heat-shock proteins (Hsps), are a ubiquitous feature of cells in which these proteins cope with stress-induced denaturation of other proteins. Hsps have received the most attention in model organisms undergoing experimental stress in the laboratory, and the function of Hsps at the molecular and cellular level is becoming well understood in this context. A complementary focus is now emerging on the Hsps of both model and nonmodel organisms undergoing stress in nature, on the roles of Hsps in the stress physiology of whole multicellular eukaryotes and the tissues and organs they comprise, and on the ecological and evolutionary correlates of variation in Hsps and the genes that encode them. This focus discloses that (a) expression of Hsps can occur in nature, (b) all species have hsp genes but they vary in the patterns of their expression, (c) Hsp expression can be correlated with resistance to stress, and (d) species' thresholds for Hsp expression are correlated with levels of stress that they naturally undergo. These conclusions are now well established and may require little additional confirmation; many significant questions remain unanswered concerning both the mechanisms of Hsp-mediated stress tolerance at the organismal level and the evolutionary mechanisms that have diversified the hsp genes.

Hsp70 and larval thermotolerance in Drosophila melanogaster: how much is enough and when is more too much?

Heat shock proteins (Hsps) and other molecular chaperones perform diverse cellular roles (e.g., inducible thermotolerance) whose functional consequences are concentration dependent. We manipulated Hsp70 concentration quantitatively in intact larvae of Drosophila melanogaster to examine its effect on survival, developmental time and tissue damage after heat shock. Larvae of an extra-copy strain, which has 22 hsp70 copies, produced Hsp70 more rapidly and to higher concentrations than larvae of a control strain, which has the wild-type 10 copies of the gene. Increasing the magnitude and duration of pretreatment increased Hsp70 concentrations, improved tolerance of more severe stress, and reduced delays in development. Pretreatment, however, did not protect against acute tissue damage. For larvae provided a brief or mild intensity pretreatment, faster expression of Hsp70 in the extra-copy strain improved survival to adult and reduced tissue damage 21h after heat shock. Negative effects on survival ensued in extra-copy larvae pretreated most intensely, but their overexpression of Hsp70 did not increase tissue damage. Because rapid expression to yield a low Hsp70 concentration benefits larvae but overexpression harms them, natural selection may balance benefits and costs of high and low expression levels in natural populations.

High-temperature stress and the evolution of thermal resistance in Drosophila

The evolution of thermal resistance and acclimation is reviewed at the population level using populations and isofemale lines of Drosophila buzzatii and D. melanogaster originating from different climatic regions. In general, ample genetic variation for thermal resistance was found within and among populations. A rough correlation between the climate of origin and thermal resistance was apparent. Acclimation at a non-lethal temperature led to a significant increase in survival after heat shock, and recurrent acclimation events generally increased survival even further. Acclimation effects lasted over several days, but this effect decreased gradually with time since acclimation. Protein studies showed that the concentration of Hsp70 in adult flies is greatly increased by acclimation and thereafter gradually decreases with time. For populations with relatively high survival at one life stage, survival often was low at other life stages. Furthermore, selection on different life stages showed that a selection response in one life stage did not necessarily result in a correlated response in another. These observations indicate that different mechanisms or genes at least in part are responsible for or are expressed at different developmental stages. Selection for increased resistance was successful despite low heritabilities for the trait. Survival and fertility were compared between acclimated and non-acclimated flies, and a cost of expressing the "heat shock response" was identified in that increased survival after acclimation was accompanied by reduced fertility. The relative costs increased under nutritional stress. Metabolic rate was genetically variable but did not correlate with temperature resistance. The more resistant lines, however, often had shorter developmental time. Inbreeding reduced thermal stress tolerance of adult flies, but it did not reduce tolerance of embryos that possibly are exposed to strong natural selection for thermal stress resistance. In general, inbreeding may reduce stress resistance, and thus multiple stressful events may account for increased inbreeding depression in harsh environments.

Genetic and environmental factors in the resistance of Drosophila subobscura adults to high temperature shock. III. Chromosomal-inversion and enzymatic polymorphism variation in lines selected for heat shock resistance

Two replicate selection experiments to increase and decrease heat shock resistance of Drosophila subobscura adults were carried out maintaining control lines. In the present paper, the chromosomal-inversion and enzymatic polymorphism variation with selection is analyzed. The results indicate an erratic variation of chromosomal arrangement frequencies for practically all the chromosomes in the selected lines, showing a loss of the less frequent arrangements especially in sensitive lines. Only the A chromosome and the O + 4 arrangement show a behaviour that may not be due to random effects, which points to the possible existence of heat shock factor(s) in these chromosomes. Similarly, an erratic variation of allele frequencies is observed for all the enzymes studied (Aph, Pept-1) except for the Hk-1 enzyme. We cannot establish the possible participation of this locus in heat shock resistance from the results obtained up to now. A significant decrease in heterozygosity is detected in sensitive lines from chromosomal-inversion polymorphism.

Heat shock phenomena in Aspergillus nidulans. I. The effect of heat on mycelial protein synthesis

Heat shock was found to induce characteristic changes in the pattern of protein synthesis in Aspergillus nidulans as analysed by SDS-polyacrylamide gel electrophoresis. Six to seven new bands were found to show increased incorporation to 35S-methionine at 43 degrees C compared to 37 degrees C, the standard temperature for this organism. The heat shock response of five different strains of A. nidulans was examined. This comparative study showed that these strains (haploids and diploids) show exactly the same set of heat shock proteins.

Adaptation of Drosophila enzymes to temperature--V. Heat shock effect on the malate dehydrogenase of Drosophila melanogaster

Drosophila melanogaster cMdh allozymic variants (MdhF MdhF/S, MdhS) were subjected to heat shock (33 degrees C/30 min----40 degrees C/30 min). This stress increases differentially MDH specific activity, with the cMdhF strain showing greater response as compared with the cMdhS one; heterozygotes exhibited generally intermediate values. Correlative differences were also revealed for some catalytic properties (Vmax, Vmax/Km ratio, thermostability) of the cMDH; this is not true for the mMDH. The catalytic behavior of the enzyme is correlated with the differential survival of the cMdh variants, with the cMdhF showing again higher survival than the cMdhS one, a fact which seems to contribute to temperature adaptation of D. melanogaster.

Indirect thermal selection in Drosophila melanogaster and adaptive consequences

Short-term indirect selection in Drosophila melanogaster for heat-sensitivity and heat resistance resulted in two strains, one heat sensitive and another heat resistant, and correlated responses were found for the rate of heat shock protein synthesis, behavioral patterns (asymmetrical sexual isolation) and fitness components (fecundity, fertility, viability, developmental time), as well as for several enzyme activities (MDH, G-6-PDH, ADH, ACHE). These responses associated with temperature selection may reflect the effects of differential inbreeding depression caused by homozygosity of temperature sensitive mutations with different pleiotropic effects. Selection even of a very short duration can induce significant adaptive and evolutionary changes.