Evolvability of Hsp70 expression under artificial election for inducible thermotolerance in independent populations of Drosophila melanogaster

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.

LPS-Induced Garcia Effect and Its Pharmacological Regulation Mediated by Acetylsalicylic Acid: Behavioral and Transcriptional Evidence

Lymnaea stagnalis learns and remembers to avoid certain foods when their ingestion is followed by sickness. This rapid, taste-specific, and long-lasting aversion-known as the Garcia effect-can be formed by exposing snails to a novel taste and 1 h later injecting them with lipopolysaccharide (LPS). However, the exposure of snails to acetylsalicylic acid (ASA) for 1 h before the LPS injection, prevents both the LPS-induced sickness state and the Garcia effect. Here, we investigated novel aspects of this unique form of conditioned taste aversion and its pharmacological regulation. We first explored the transcriptional effects in the snails' central nervous system induced by the injection with LPS (25 mg), the exposure to ASA (900 nM), as well as their combined presentation in untrained snails. Then, we investigated the behavioral and molecular mechanisms underlying the LPS-induced Garcia effect and its pharmacological regulation by ASA. LPS injection, both alone and during the Garcia effect procedure, upregulated the expression levels of immune- and stress-related targets. This upregulation was prevented by pre-exposure to ASA. While LPS alone did not affect the expression levels of neuroplasticity genes, its combination with the conditioning procedure resulted in their significant upregulation and memory formation for the Garcia effect.

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.

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.

Heat shock proteins and resistance to desiccation in congeneric land snails

Land snails are subject to daily and seasonal variations in temperature and in water availability and depend on a range of behavioral and physiological adaptations for coping with problems of maintaining water, ionic, and thermal balance. Heat shock proteins (HSPs) are a multigene family of proteins whose expression is induced by a variety of stress agents. We used experimental desiccation to test whether adaptation to different habitats affects HSP expression in two closely related Sphincterochila snail species, a desiccation-resistant, desert species Sphincterochila zonata, and a Mediterranean-type, desiccation-sensitive species Sphincterochila cariosa. We examined the HSP response in the foot, hepatopancreas, and kidney tissues of snails exposed to normothermic desiccation. Our findings show variations in the HSP response in both timing and magnitude between the two species. The levels of endogenous Hsp72 in S. cariosa were higher in all the examined tissues, and the induction of Hsp72, Hsp74, and Hsp90 developed earlier than in S. zonata. In contrary, the induction of sHSPs (Hsp25 and Hsp30) was more pronounced in S. zonata compared to S. cariosa. Our results suggest that land snails use HSPs as part of their survival strategy during desiccation and as important components of the aestivation mechanism in the transition from activity to dormancy. Our study underscores the distinct strategy of HSP expression in response to desiccation, namely the delayed induction of Hsp70 and Hsp90 together with enhanced induction of sHSPs in the desert-dwelling species, and suggests that evolution in harsh environments will result in selection for reduced Hsp70 expression.

Molecular and cellular studies in evolutionary physiology of natural vertebrate populations: influences of individual variation and genetic components on sampling and measurements

Studies combining ecological, genetic and physiological approaches are needed in evolutionary biology. Although the combination of approaches has been emphasized, such studies have been rare with regard to molecular and cellular studies on natural vertebrate populations. The major reasons for this are that the generation time of vertebrates is long and it is difficult to find a molecular or cell physiological measurement that is both relevant for the fitness of the population and can be repeated an adequate number of times to enable estimations of individual variability. The paucity of suitable physiological parameters is partly due to the fact that most physiological studies have not been directed towards understanding the behaviour of populations but towards understanding the basic mechanisms of the function of individuals. Also, physiological measurements that appear most relevant from the point of view of evolutionary studies are often integrative functions,composed of the function of many genes. When dissecting the integrative functions into components, it is often observed that the same integrative response can be achieved via different routes, i.e. changes in the responses of different genes. To enable cellular and molecular physiological studies to be increasingly combined with ecological and genetic studies, it is important that such studies include and report individual variability and that the sample size is increased. In addition, more sophisticated statistical methods should be used than is traditionally done, and when the function of most genes in the integrative response are not known, techniques such as QTL mapping should be used. Hitherto in vertebrates, the methodology has mainly been used in production biology (e.g. meat or milk production). With regard to combining genomic and physiological studies, one must bear in mind that the massive datasets associated with genomic studies need to be further enlarged to enable estimates of individual variation. It is also important to remember that microarray and proteomic data give the levels of mRNA and proteins,respectively. Since the function of the protein can be regulated independently of its transcription or its level in the cell, direct physiological measurements are also needed if estimations of protein activity in the individuals of a population are wanted.

The role of stress proteins in responses of a montane willow leaf beetle to environmental temperature variation

The heat shock response is a critical mechanism by which organisms buffer effects of variable and unpredictable environmental temperatures. Upregulation of heat shock proteins (Hsps) increases survival after exposure to stressful conditions in nature, although benefits of Hsp expression are often balanced by costs to growth and reproductive success. Hsp-assisted folding of variant polypeptides may prevent development of unfit phenotypes; thus, some differences in Hsp expression among natural populations of ectotherms may be due to interactions between enzyme variants (allozymes) and Hsps. In the Sierra willow leaf beetle Chrysomela aeneicollis, which lives in highly variable thermal habitats at the southern edge of their range in the Eastern Sierra Nevada, California, allele frequencies at the enzyme locus phosphoglucose isomerase (PGI) vary across a climatic latitudinal gradient. PGI allozymes differ in kinetic properties,and expression of a 70 kDa Hsp differs between populations, along elevation gradients,and among PGI genotypes. Differences in Hsp70 expression among PGI genotypes correspond to differences in thermal tolerance and traits important for reproductive success, such as running speed, survival and fecundity. Thus, differential Hsp expression among genotypes may allow functionally important genetic variation to persist, allowing populations to respond effectively to environmental change.

Direct and correlated effects of selection on flight after exposure to thermal stress in Drosophila melanogaster

To demonstrate how insects may adapt to ecologically relevant levels of heat stress, we performed artificial selection on the ability of Drosophila melanogaster to fly after an exposure to a high but non-lethal thermal stress. Both tolerance and intolerance to heat stress arose very quickly, as only a few generations of selection were necessary to cause significant separation between high and low lines for heat tolerance. Estimates of heritability based on the lines artificially selected for increased flight ability ranged from 0.024 to 0.052, while estimates of heritability based on the lines selected for the inability to fly after heat stress varied between 0.035 and 0.091. Reciprocal F1 crosses among these lines revealed strong additive effects of one or more autosomes and a weaker X-chromosome effect. This variation apparently affected flight specifically; neither survival to a more extreme stress nor knockdown by high temperature changed between lines selected for high and low heat tolerance as measured by flight ability. As the well-studied heat-shock response is associated with heat tolerance as measured by survival and knockdown, the aspects of the stress physiology that actually affect flight ability remains unknown.

Naturally occurring transposable elements disrupt hsp70 promoter function in Drosophila melanogaster

Naturally occurring transposable element (TE) insertions that disrupt Drosophila promoters are correlated with modified promoter function and are posited to play a significant role in regulatory evolution, but their phenotypes have not been established directly. To establish the functional consequences of these TE insertions, we created constructs with either TE-bearing or TE-lacking hsp70 promoters fused to a luciferase reporter gene and assayed luciferase luminescence in transiently transfected Drosophila cells. Each of the four TEs reduces luciferase signal after heat shock and heat inducibility of the hsp70 promoter. To test if the differences in hsp70 promoter activity are TE-sequence dependent, we replaced each of the TEs with multiple intergenic sequences of equal length. These replacement insertions similarly reduced luciferase signal, suggesting that the TEs affect hsp70 promoter function by altering promoter architecture. These results are consistent with differences in Hsp70 expression levels, inducible thermotolerance, and fecundity previously associated with the TEs. That two different varieties of TEs in two different hsp70 genes have common effects suggests that TE insertion represents a general mechanism through which selection manipulates hsp70 gene expression.

Modification of heat-shock gene expression in Drosophila melanogaster populations via transposable elements

We report multiple cases in which disruption of hsp70 regulatory regions by transposable element (TE) insertions underlies natural variation in expression of the stress-inducible molecular chaperone Hsp70 in Drosophila melanogaster. Three D. melanogaster populations from different continents are polymorphic for jockey or P element insertions in the promoter of the hsp70Ba gene. All three TE insertions are within the same 87-bp region of hsp70Ba promoter, and we suggest that the distinctive promoter architecture of hsp genes may make them vulnerable to TE insertions. Each of the TE insertions reduces Hsp70 levels, and RNase protection assays demonstrate that such insertions can reduce transcription of the hsp70Ba gene. In addition, the TEs alter two measures of organismal fitness, inducible thermotolerance and female reproductive success. Thus, transposition can create quantitative genetic variation in gene expression within populations, on which natural selection can act.