Three selections are better than one: clinal variation of thermal QTL from independent selection experiments in Drosophila.

We report the results of two independent selection experiments that have exposed distinct populations of Drosophila melanogaster to different forms of thermal selection. A recombinant population derived from Arvin California and Zimbabwe isofemale lines was exposed to laboratory natural selection at two temperatures (T(AZ): 18°C and 28°C). Microsatellite mapping identified quantitative trait loci (QTL) on the X-chromosome between the replicate "Hot" and "Cold" populations. In a separate experiment, disruptive selection was imposed on an outbred California population for the "knockdown" temperature (T(KD)) in a thermal column. Microsatellite mapping of the "High" and "Low" populations also uncovered primarily X-linked QTL. Notably, a marker in the shaggy locus at band 3A was significantly differentiated in both experiments. Finer scale mapping of the 3A region has narrowed the QTL to the shaggy gene region, which contains several candidate genes that function in circadian rhythms. The same allele that was increased in frequency in the High T(KD) populations is significantly clinal in North America and is more common at the warm end of the cline (Florida vs. Maine; however, the cline was not apparent in Australia). Together, these studies show that independent selection experiments can uncover the same target of selection and that evolution in the laboratory can recapitulate putatively adaptive clinal variation in nature.

Clinal patterns of desiccation and starvation resistance in ancestral and invading populations of Drosophila subobscura.

As invading species expand, they eventually encounter physical and biotic stressors that limit their spread. We examine latitudinal and climatic variation in physiological tolerance in one native and two invading populations of Drosophila subobscura. These flies are native to the Palearctic region, but invaded both South and North America around 1980 and spread rapidly across 15° of latitude on each continent. Invading flies rapidly evolved latitudinal clines in chromosome inversion frequencies and in wing size that parallel those of native populations in the Old World. Here we investigate whether flies on all three continents have evolved parallel clines in desiccation and starvation tolerance, such that flies in low-latitude regions (hot, dry) might have increased stress resistance. Starvation tolerance does not vary with latitude or climate on any continent. In contrast, desiccation tolerance varies clinally with latitude on all three continents, although not in parallel. In North American and Europe, desiccation tolerance is inversely related to latitude, as expected. But in South America, desiccation tolerance increases with latitude and is greatest in relatively cool and wet areas. Differences among continents in latitudinal patterns of interspecific-competition potentially influence clinal selection for physiological resistance, but no simple pattern is evident on these continents.

Transcriptional profiling of MnSOD-mediated lifespan extension in Drosophila reveals a species-general network of aging and metabolic genes.

BACKGROUND: Several interventions increase lifespan in model organisms, including reduced insulin/insulin-like growth factor-like signaling (IIS), FOXO transcription factor activation, dietary restriction, and superoxide dismutase (SOD) over-expression. One question is whether these manipulations function through different mechanisms, or whether they intersect on common processes affecting aging. RESULTS: A doxycycline-regulated system was used to over-express manganese-SOD (MnSOD) in adult Drosophila, yielding increases in mean and maximal lifespan of 20%. Increased lifespan resulted from lowered initial mortality rate and required MnSOD over-expression in the adult. Transcriptional profiling indicated that the expression of specific genes was altered by MnSOD in a manner opposite to their pattern during normal aging, revealing a set of candidate biomarkers of aging enriched for carbohydrate metabolism and electron transport genes and suggesting a true delay in physiological aging, rather than a novel phenotype. Strikingly, cross-dataset comparisons indicated that the pattern of gene expression caused by MnSOD was similar to that observed in long-lived Caenorhabditis elegans insulin-like signaling mutants and to the xenobiotic stress response, thus exposing potential conserved longevity promoting genes and implicating detoxification in Drosophila longevity. CONCLUSION: The data suggest that MnSOD up-regulation and a retrograde signal of reactive oxygen species from the mitochondria normally function as an intermediate step in the extension of lifespan caused by reduced insulin-like signaling in various species. The results implicate a species-conserved net of coordinated genes that affect the rate of senescence by modulating energetic efficiency, purine biosynthesis, apoptotic pathways, endocrine signals, and the detoxification and excretion of metabolites.

Critical thermal maxima in knockdown-selected Drosophila: are thermal endpoints correlated?

To explore the correlation of traits linked to thermotolerance, we compared three thermal endpoints (knockdown temperature and two critical thermal maxima) among replicate populations of Drosophila melanogaster selected for high, or low, knockdown temperature. The high knockdown flies maintain normal posture and locomotor ability within a knockdown column at temperatures >or=40 degrees C, whereas the low knockdown flies fall out of the column at much cooler temperatures (approximately 35 degrees C, on average). The critical thermal maximum (CT(max)) for respiratory control in the selected knockdown populations was determined by analyzing CO(2) output of individuals during exposure to a temperature ramp (from 30 degrees C to >45 degrees C) and was indicated by an abrupt alteration in the pattern of CO(2) release. The CT(max) for locomotor function was determined by monitoring activity (concurrent with CO(2) analysis) during the temperature ramp and was marked by the abrupt cessation of activity. We hypothesized that selection for high knockdown temperature may cause an upward shift in CT(max), whereas selection for low knockdown may lower CT(max). Correlations among the three thermal endpoints varied between the high and low knockdown flies. Finally, we compared metabolic profiles, as well as Q(10) values, among the high and low knockdown males and females during the temperature ramp.

Selection on knockdown performance in Drosophila melanogaster impacts thermotolerance and heat-shock response differently in females and males.

We studied adaptive thermotolerance in replicate populations of Drosophila melanogaster artificially selected for high and low knockdown temperature (T(KD)), the upper temperature at which flies can no longer remain upright or locomote effectively. Responses to selection have generated High T(KD) populations capable of maintaining locomotor function at approximately 40 degrees C, and Low T(KD) populations with T(KD) of approximately 35 degrees C. We examined inducible knockdown thermotolerance, as well as inducible thermal survivorship, following a pretreatment heat-shock (known to induce heat-shock proteins) for males and females from the T(KD) selected lines. Both selection for knockdown and sex influenced inducible knockdown thermotolerance, whereas inducible thermal survivorship was influenced only by sex, and not by selection. Overall, our findings suggest that the relationships between basal and inducible thermotolerance are contingent upon the methods used to gauge thermotolerance, as well as the sex of the flies. Finally, we compared temporal profiles of the combined expression of two major heat-shock proteins, HSC70 and HSP70, during heat stress among the females and males from the selected T(KD) lines. The temporal profiles of the proteins differed between High and Low T(KD) females, suggesting divergence of the heat-shock response. We discuss a possible mechanism that may lead to the heat-shock protein patterns observed in the selected females.

Adaptive evolution in the lab: unique phenotypes in fruit flies comprise a fertile field of study.

Laboratory selection for desiccation resistance, which has been imposed on five replicate populations of Drosophila melanogaster for >200 generations, has resulted in enhanced survivability during periods of extreme water stress. The ability of these populations to persistently resist the fatal effects of desiccation is correlated with evolved physiological traits, namely preferential storage of carbohydrates (associated with reduced lipid reserves) and a dramatic increase in blood volume, which has led to a significant increase in extracellular sodium and chloride content, as well as body mass. When compared to other populations of this drosophilid species, these adaptive traits are unique. While some may argue against the value of evolved traits that have not been found in natural populations, we counter that such traits are of considerable value to the analyses of physiological functions, as well as the underlying mechanisms and evolutionary trajectories of these functions. We propose that multiple physiological consequences almost certainly derive from the evolution of these singular traits; and, furthermore, we discuss future directions for the elucidation of such consequences.

The evolution of recovery from desiccation stress in laboratory-selected populations of Drosophila melanogaster.

We examined the capacity for physiological recovery from the effects of desiccation in five replicate populations of Drosophila melanogaster that have been selected for enhanced desiccation resistance (D populations) and in five replicate control populations (C populations). The capacity to recover was signified by the ability to restore three somatic components, namely whole-body water, dry mass and sodium content, all of which are reduced during desiccation. Throughout a period of recovery following a bout of desiccation, the flies were offered one of three fluids: distilled water, saline solution, or saline+sucrose solution. Our findings indicate that, when allowed to recover on saline+sucrose solution, D populations have the capacity to restore water at a greater rate than C populations and are able to fully restore dry mass and sodium content to the levels observed in non-desiccated, hydrated D flies. When provided with this same solution during recovery, C flies are unable to restore dry mass and are faced with an elevated sodium load. Desiccation resistance of the flies subsequent to recovery was also examined. We provide evidence that the greatest desiccation resistance in the D populations is associated with the restoration of all three somatic components, suggesting that not only water content, but also dry mass and sodium, may contribute to the enhanced desiccation resistance that has evolved in these populations.

Analyses of physiological evolutionary response.

Selection studies are useful if they can provide us with insights into the patterns and processes of evolution in populations under controlled conditions. In this context it is particularly valuable to be able to analyze the limitations of and constraints on evolutionary responses to allow predictions concerning evolutionary change. The concept of a selection pathway is presented as a means of visualizing this predictive process and the constraints that help define the population's response to selection. As pointed out by Gould and Lewontin, history and chance are confounding forces that can mask or distort the adaptive response. Students of the evolutionary responses of organisms are very interested in the effects of these confounding forces, since they play a critical role not only in the laboratory but also in natural selection in the field. In this article, we describe some methods that are a bit different from those used in most studies for examining data from laboratory selection studies. These analytical methods are intended to provide insights into the physiological mechanisms by which evolutionary responses to the environment proceed. Interestingly, selection studies often exhibit disparate responses in replicate populations. We offer methods for analyzing these disparate responses in replicate populations to better understand this very important source of variability in the evolutionary response. We review the techniques of Travisano et al. and show that these approaches can be used to investigate the relative roles of adaptation, history, and chance in the evolutionary responses of populations of Drosophila melanogaster to selection for enhanced desiccation resistance. We anticipate that a wider application of these techniques will provide valuable insights into the organismal, genetic, and molecular nature of the constraints, as well as the factors that serve to enhance or, conversely, to mask the effects of chance. Such studies should help to provide a more detailed understanding of the processes producing evolutionary change in populations.

Evolved patterns and rates of water loss and ion regulation in laboratory-selected populations of Drosophila melanogaster.

We have investigated water loss from, and ion regulation within, the hemolymph and tissues of five replicate populations of Drosophila melanogaster that have undergone laboratory selection for enhanced desiccation resistance (i.e. the D populations). We compared the patterns and rates of water loss and the ion content of the D populations prior to and during desiccation with those of five replicate control (C) populations. The net rate of water loss in the C flies was approximately 3-fold greater than that of the D flies during the initial hours of desiccation. After 8 h, both C and D flies had considerable reductions in water loss rate. During 24 h of desiccation, the tissue water content of the D flies was conserved, while the C flies were faced with significant loss of tissue water during the initial 8 h of desiccation. We propose that the increased hemolymph volume of the D flies plays a role in buffering water loss from the tissues. One consequence of this large hemolymph pool is that the hydrated D flies contained approximately seven times more sodium within the hemolymph than did the hydrated C flies. Despite a continual loss of hemolymph volume in the D flies during lengthy periods of desiccation, the sodium content of the hemolymph was significantly reduced only during a single event. We provide evidence that the regulation of extracellular sodium, as well as chloride, occurred via excretory processes during desiccation. In addition, whole-body potassium was not significantly decreased in the D flies during desiccation but was reduced (i.e. excreted) in the C flies; hence, we suggest that the potassium content paralleled tissue water level.

Induced overexpression of mitochondrial Mn-superoxide dismutase extends the life span of adult Drosophila melanogaster.

A transgenic system ("FLP-out") based on yeast FLP recombinase allowed induced overexpression of MnSOD enzyme in adult Drosophila melanogaster. With FLP-out a brief heat pulse (HP) of young, adult flies triggered the rearrangement and subsequent expression of a MnSOD transgene throughout the adult life span. Control (no HP) and overexpressing (HP) flies had identical genetic backgrounds. The amount of MnSOD enzyme overexpression achieved varied among six independent transgenic lines, with increases up to 75%. Life span was increased in proportion to the increase in enzyme. Mean life span was increased by an average of 16%, with some lines showing 30-33% increases. Maximum life span was increased by an average of 15%, with one line showing as much as 37% increase. Simultaneous overexpression of catalase with MnSOD had no added benefit, consistent with previous observations that catalase is present in excess in the adult fly with regard to life span. Cu/ZnSOD overexpression also increases mean and maximum life span. For both MnSOD and Cu/ZnSOD lines, increased life span was not associated with decreased metabolic activity, as measured by O2 consumption.