Pernille Sarup

Flies selected for longevity retain a young gene expression profile.

We investigated correlated responses in the transcriptomes of longevity-selected lines of Drosophila melanogaster to identify pathways that affect life span in metazoan systems. We evaluated the gene expression profile in young, middle-aged, and old male flies, finding that 530 genes were differentially expressed between selected and control flies when measured at the same chronological age. The longevity-selected flies consistently showed expression profiles more similar to control flies one age class younger than control flies of the same age. This finding is in accordance with a younger gene expression profile in longevity-selected lines. Among the genes down-regulated in longevity-selected lines, we found a clear over-representation of genes involved in immune functions, supporting the hypothesis of a life-shortening effect of an overactive immune system, known as inflammaging. We judged the physiological age as the level of cumulative mortality. Eighty-four genes were differentially expressed between the control and longevity-selected lines at the same physiological age, and the overlap between the same chronological and physiological age gene lists included 40 candidate genes for increased longevity. Among these candidates were genes with roles in starvation resistance, immune response regulation, and several that have not yet been linked to longevity. Investigating these genes would provide new knowledge of the pathways that affect life span in invertebrates and, potentially, mammals.

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.

Longevity for free? Increased reproduction with limited trade-offs in Drosophila melanogaster selected for increased life span

Selection for increased life span in Drosophila melanogaster has been shown to correlate with decreased early fecundity and increased fecundity later in life. This phenomenon has been ascribed to the existence of trade-offs in which limited resources can be invested in either somatic maintenance or reproduction. In our longevity selection lines, we did not find such a trade-off. Rather, we find that females have similar or higher fecundity throughout life compared to non-selected controls. To determine whether increased longevity affects responses in other traits, we looked at several stress resistance traits (chill coma recovery, heat knockdown, desiccation and starvation), geotactic behaviour, egg-to-adult viability, body size, developmental time as well as metabolic rate. Longevity selected flies were more starvation resistant. However, in females longevity and fecundity were not negatively correlated with the other traits assayed. Males from longevity selected lines were slower at recovering from a chill induced coma and resting metabolic rate increased with age, but did not correlate with life span.

Life extension and the position of the hormetic zone depends on sex and genetic background in Drosophila melanogaster

Hormesis, the beneficial effect of a mild stress, has been proposed as a means to prolong the period of healthy ageing as it can increase the average lifespan of a cohort. However, if we want to use hormesis therapeutically it is important that the treatment is beneficial on the individual level and not just on average at the population level. Long lived lines have been shown not to benefit from a, in other lines, hormesis inducing heat treatment in Drosophila melanogaster, D. buzzatii and mice. Also in many experiments hormesis has been reported to occur in one sex only, usually males but not in females. Here we investigated the interaction between the hormetic response and genetic background, sex and duration of a mild heat stress in D. melanogaster, using three replicate lines that have been selected for increased longevity and their respective control lines. We found that genetic background influences the position of the hormetic zone. The implication of this result could be that in a genetically diverse populations a treatment that is life prolonging in one individual could be life shortening in other individuals. However, we did find a hormetic response in all combinations of line and sex in at least one of the experiments which suggests that if it is possible to identify the optimal hormetic dose individually hormesis might become a therapeutic treatment.

Local adaptation of stress related traits in Drosophila buzzatii and Drosophila simulans in spite of high gene flow

We addressed the question if local adaptation to a thermal gradient is possible in spite of a high gene flow among closely spaced populations of two species of Drosophila from the island of La Gomera (Canary Islands). Variation in multiple traits related to stress resistance in different life stages was measured in both species in flies collected from five localities at different altitudes and thereby with different climatic conditions. Based on microsatellite loci, the populations were not genetically differentiated. However, 18 of the 24 independent traits measured showed significant differentiation among populations of Drosophila buzzatii, but only nine of 25 for Drosophila simulans. This difference in the number of traits might reflect higher habitat specificity and thus higher potential for local adaptation of D. buzzatii than D. simulans. We found clinal variation, as some traits showed significant linear regressions on altitude, but more on altitude cubed.

Genomic Prediction for Quantitative Traits Is Improved by Mapping Variants to Gene Ontology Categories in Drosophila melanogaster

Predicting individual quantitative trait phenotypes from high-resolution genomic polymorphism data is important for personalized medicine in humans, plant and animal breeding, and adaptive evolution. However, this is difficult for populations of unrelated individuals when the number of causal variants is low relative to the total number of polymorphisms and causal variants individually have small effects on the traits. We hypothesized that mapping molecular polymorphisms to genomic features such as genes and their gene ontology categories could increase the accuracy of genomic prediction models. We developed a genomic feature best linear unbiased prediction (GFBLUP) model that implements this strategy and applied it to three quantitative traits (startle response, starvation resistance, and chill coma recovery) in the unrelated, sequenced inbred lines of the Drosophila melanogaster Genetic Reference Panel. Our results indicate that subsetting markers based on genomic features increases the predictive ability relative to the standard genomic best linear unbiased prediction (GBLUP) model. Both models use all markers, but GFBLUP allows differential weighting of the individual genetic marker relationships, whereas GBLUP weighs the genetic marker relationships equally. Simulation studies show that it is possible to further increase the accuracy of genomic prediction for complex traits using this model, provided the genomic features are enriched for causal variants. Our GFBLUP model using prior information on genomic features enriched for causal variants can increase the accuracy of genomic predictions in populations of unrelated individuals and provides a formal statistical framework for leveraging and evaluating information across multiple experimental studies to provide novel insights into the genetic architecture of complex traits.

The metabolic profile of long-lived Drosophila melanogaster.

We investigated the age-related changes in the metabolic profile of male Drosophila melanogaster and compared the metabolic profile of flies selected for increased longevity to that of control flies of equal age. We found clear differences in metabolite composition between selection regimes and among age groups. Contrary to results found in a previous study of the transcriptome of these lines the metabolic profile did not show a younger pattern in longevity-selected (LS) flies than in same aged control (C) flies. Rather, many of the metabolites affected by age had levels common to older control individuals in the young LS flies. Furthermore, ageing affected the metabolome in a different LS specific direction. The selection induced difference increased with age. Some metabolites involved in oxidative phosphorylation changed with age highlighting the importance of mitochondrial function in the ageing process. However, these metabolites were not affected by selection for increased longevity, indicating that improvements of mitochondrial function were not involved in the increased lifespan of LS lines. Of the eight metabolites identified as having a significant difference in relative abundance between selection regimes in our study choline, lysine and glucose also show difference among lifespan phenotypes in C. elegans indicating that the correlation between the concentration of these metabolites and longevity was evolutionary conserved. Links between longevity and choline concentration is also found in mice making this metabolite an obvious target for further study.

The long-term effects of a life-prolonging heat treatment on the Drosophila melanogaster transcriptome suggest that heat shock proteins extend lifespan

Heat-induced hormesis, i.e. the beneficial effect of mild heat-induced stress, increases the average lifespan of many organisms. This effect, which depends on the heat shock factor, decreases the log mortality rate weeks after the stress has ceased. To identify candidate genes that mediate this lifespan-prolonging effect late in life, we treated flies with mild heat stress (34 °C for 2 h) 3 times early in life and compared the transcriptomic response in these flies versus non-heat-treated controls 10-51 days after the last heat treatment. We found significant transcriptomic changes in the heat-treated flies. Several hsp70 probe sets were up-regulated 1.7-2-fold in the mildly stressed flies weeks after the last heat treatment (P<0.01). This result was unexpected as the major Drosophila heat shock protein, Hsp70, is reported to return to normal levels of expression shortly after heat stress. We conclude that the heat shock response, and Hsp70 in particular, may be central to the heat-induced increase in the average lifespan in flies that are exposed to mild heat stress early in life.

Developmental acclimation affects clinal variation in stress resistance traits in Drosophila buzzatii

Patterns of clinal genetic variation in Drosophila are often characterized after rearing at constant temperatures. However, clinal patterns might change after acclimation if populations differ in their plastic response to fluctuating environments. We studied longevity, starvation and heat knock‐down resistance after development at either constant or fluctuating temperatures in nine Drosophila buzzatii populations collected along an altitudinal gradient in Tenerife, Spain. Flies that developed at fluctuating temperatures had higher stress resistance despite experiencing a slightly lower average temperature than those at constant temperatures. Genetic variation along the gradient was found in both stress‐resistance traits. Because QST values greatly exceeded FST values, genetic drift could not explain this diversification. In general, differences among populations were larger after rearing at fluctuating temperatures, especially in heat knock‐down, for which clinal patterns disappeared when flies were reared at constant temperatures. This result emphasizes the importance of determining whether populations originating from different environments differ in their plastic responses to stress.

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