Torsten Nygård Kristensen

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.

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 (CTmax) 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.

Effects of inbreeding and rate of inbreeding in Drosophila melanogaster– Hsp70 expression and fitness

Induction of heat shock proteins (Hsp) is a well‐known mechanism through which cells cope with stressful conditions. Hsp are induced by a variety of extrinsic stressors. However, recently intrinsic stressors (aging and inbreeding) have been shown to affect expression of Hsp. Increased homozygosity due to inbreeding may disrupt cellular homeostasis by causing increased expression of recessive deleterious mutations and breakdown of epistatic interactions. We investigated the effect of inbreeding and the rate of inbreeding on the expression of Hsp70, larval heat resistance and fecundity. In Drosophila melanogaster we found that inbred lines (F ≈ 0.67) had significantly up‐regulated expression of Hsp70, and reduced heat resistance and fecundity as compared with outbred control lines. A significant negative correlation was observed between Hsp70 expression and resistance to an extreme heat stress in inbred lines. We interpreted this as an increased requirement for Hsp70 in the lines suffering most from inbreeding depression. Inbreeding depression for fecundity was reduced with a slower rate of inbreeding compared with a fast rate of inbreeding. Thus, the effectiveness of purging seems to be improved with a slower rate of inbreeding.

Sex specific effects of heat induced hormesis in Hsf-deficient Drosophila melanogaster

In insects mild heat stress early in life has been reported to increase life span and heat resistance later in life, a phenomenon termed hormesis. Here, we test if the induction of the heat shock response by mild heat stress is mediating hormesis in longevity and heat resistance at older age. To test this hypothesis we used two heat shock transcription factor (Hsf) mutant stocks. One stock harbours a mutation giving rise to a heat sensitive Hsf which inactivates the heat shock response at high temperature and the other is a rescued mutant giving rise to a wild-type phenotype. We measured longevity, heat resistance and expression level of a heat shock protein, Hsp70, in controls and mildly heat treated flies. We found a marked difference between males and females with males showing a beneficial effect of the early heat treatment on longevity and heat resistance later in life in the rescued line, seemingly mediated by the production of heat shock proteins (Hsps). The results indicate that heat inducible Hsps are important for heat induced hormesis in longevity and heat stress resistance. However, the results also suggest that other processes are involved and that different mechanisms might have marked sex specific impact.

Extreme temperatures increase the deleterious consequences of inbreeding under laboratory and semi-natural conditions.

The majority of experimental studies of the effects of population bottlenecks on fitness are performed under laboratory conditions, which do not account for the environmental complexity that populations face in nature. In this study, we test inbreeding depression in multiple replicates of inbred when compared with non-inbred lines of Drosophila melanogaster under different temperature conditions. Egg-to-adult viability, developmental time and sex ratio of emerging adults are studied under low, intermediate and high temperatures under laboratory as well as semi-natural conditions. The results show inbreeding depression for egg-to-adult viability. The level of inbreeding depression is highly dependent on test temperature and is observed only at low and high temperatures. Inbreeding did not affect the developmental time or the sex ratio of emerging adults. However, temperature affected the sex ratio with more females relative to males emerging at low temperatures, suggesting that selection against males in pre-adult life stages is stronger at low temperatures. The coefficient of variation (CV) of egg-to-adult viability within and among lines is higher for inbred flies and generally increases at stressful temperatures. Our results contribute to knowledge on the environmental dependency of inbreeding under different environmental conditions and emphasize that climate change may impact negatively on fitness through synergistic interactions with the genotype.

Candidate Genes Detected in Transcriptome Studies Are Strongly Dependent on Genetic Background

Whole genome transcriptomic studies can point to potential candidate genes for organismal traits. However, the importance of potential candidates is rarely followed up through functional studies and/or by comparing results across independent studies. We have analysed the overlap of candidate genes identified from studies of gene expression in Drosophila melanogaster using similar technical platforms. We found little overlap across studies between putative candidate genes for the same traits in the same sex. Instead there was a high degree of overlap between different traits and sexes within the same genetic backgrounds. Putative candidates found using transcriptomics therefore appear very sensitive to genetic background and this can mask or override effects of treatments. The functional importance of putative candidate genes emerging from transcriptome studies needs to be validated through additional experiments and in future studies we suggest a focus on the genes, networks and pathways affecting traits in a consistent manner across backgrounds.

Linking inbreeding effects in captive populations with fitness in the wild: release of replicated Drosophila melanogaster lines under different temperatures.

Inbreeding effects have been detected in captive populations of threatened species, but the extent to which these effects translate into fitness under field conditions is mostly unknown. We address this issue by comparing the performance of replicated noninbred and inbred Drosophila lines under field and laboratory conditions. We asked whether environment-dependent effects of inbreeding can be demonstrated for a field-fitness component in Drosophila, the ability of flies to locate resources, and associated the results with results on effects of inbreeding investigated in the laboratory. Inbreeding effects were evident when releases were undertaken under warm conditions, but not under cold conditions, which illustrates the environment-dependent nature of inbreeding depression. Inbreeding effects were much stronger in the field at warm temperatures than in laboratory stress tests, particularly for females. Effects of inbreeding based on performance in traditional inbreeding assays (viability, productivity) or from laboratory stress tests poorly predicted performance in the field. Inbreeding effects on resource location in the field can be strongly deleterious under some thermal conditions and involve traits not easily measured under laboratory conditions. More generally, inbreeding effects measured in captive populations may not necessarily predict their field performance, and programs to purge captive populations of deleterious alleles may not necessarily lead to fitness benefits in the wild.

Humidity affects genetic architecture of heat resistance in Drosophila melanogaster

Laboratory experiments on Drosophila have often demonstrated increased heritability for morphological and life‐history traits under environmental stress. We used parent–offspring comparisons to examine the impact of humidity levels on the heritability of a physiological trait, resistance to heat, measured as knockdown time at constant temperature. Drosophila melanogaster were reared under standard nonstressful conditions and heat‐shocked as adults at extreme high or low humidity. Mean knockdown time was decreased in the stressful dry environment, but there was a significant sex‐by‐treatment interaction: at low humidity, females were more heat resistant than males, whereas at high humidity, the situation was reversed. Phenotypic variability of knockdown time was also lower in the dry environment. The magnitude of genetic correlation between the sexes at high humidity indicated genetic variation for sexual dimorphism in heat resistance. Heritability estimates based on one‐parent–offspring regressions tended to be higher under desiccation stress, and this could be explained by decreased environmental variance of heat resistance at low humidity. There was no indication that the additive genetic variance and evolvability of heat resistance differed between the environments. The pattern of heritability estimates suggests that populations of D. melanogaster may have a greater potential for evolving higher thermal tolerance under arid conditions.

Constant, cycling, hot and cold thermal environments: strong effects on mean viability but not on genetic estimates

It has frequently been suggested that trait heritabilities are environmentally sensitive, and there are genetic trade‐offs between tolerating different environments such as hot and cold or constant and fluctuating temperatures. Future climate predictions suggest an increase in both temperatures and their fluctuations. How species will respond to these changes is uncertain, particularly as there is a lack of studies which compare genetic performances in constant vs. fluctuating environments. In this study, we used a nested full‐sib/half‐sib breeding design to examine how the genetic variances and heritabilities of egg‐to‐adult viability differ at high and low temperatures with and without daily fluctuations in temperatures using Drosophila melanogaster as a model organism. Although egg‐to‐adult viability was clearly sensitive to developmental temperatures, heritabilities were not particularly sensitive to developmental temperatures. Moreover, we found that egg‐to‐adult viabilities at different developmental temperatures were positively correlated, suggesting a common genetic background for egg‐to‐adult viability at different temperatures. Finding both a uniform genetic background coupled with rather low heritabilities insensitive to temperatures, our results suggest evolutionary responses are unlikely to be limited by temperature effects on genetic parameters or negative genetic correlations, but by the direct effects of stressful temperatures on egg‐to‐adult viability accompanied with low heritabilities.

Temperature-Induced Hormesis in Drosophila

The phenomenon that a mild exposure to an otherwise detrimental stress factor can be beneficial, termed hormesis, is well known for many organisms and life-history traits (Khazaeli et al. 1997; Le Bourg and Minois 1997; Bubliy et al. 1998; Minois 2000; Parsons 2000). Mild stress treatments have been shown to induce hormesis in mammals and insects (Rattan 1998; Minois 2000; Le Bourg et al. 2001; Hercus et al. 2003) and increased performance has been reported with respect to, e.g., delayed aging, increased longevity and (heat) resistance to severe stress long after the hormesis inducing stress was applied (Le Bourg and Minois 1999; Hercus et al. 2003, as exemplified in Fig. 1). Thus, mild stress exposure may have long-lasting effects, much longer than the vast majority of the stress-induced changes in metabolites, proteins and gene expression (Dahlgaard et al. 1998; Sorensen et al. 2005; Malmendal et al. 2006). High temperature is one of the stress factors that has been shown to induce hormesis (Rattan 1998; Le Bourg et al. 2001; Hercus et al. 2003; Kristensen et al. 2003; Scannapieco et al. 2007). The reason for choosing high temperature as a model stress is that it is easy to expose experimental organisms to well-defined thermal regimes and because it is a natural occurring stress for many plant and animal species that often are not able to avoid high temperatures in their environment. Thus, it can be expected that adaptations to temperature/heat stress are frequent in nature. Furthermore heat stress shares characteristics with other stress factors, e.g., the type of cellular damage induced, and induces a suit of relatively well-studied molecular chaperones through the heat shock response, which are among the prime candidates conferring hormetic effects. Effects of exposure to stressful low temperatures show similarities to exposure to high temperatures as both induce cellular