Heat-induced hormesis in longevity is linked to heat-stress sensitivity across laboratory populations from diverse altitude of origin in Drosophila buzzatii.

Heat-induced hormesis in longevity is the increase in life span resulting from the previous exposure to a mild heat stress early in life. Here we examined heat-induced hormesis of Drosophila buzzatii in five mass-mating populations, which were derived from five wild populations along an elevation gradient from 202 to 1855 m above sea level in North-Western Argentina. Five day old flies were exposed to 37.5 °C for 90 min to induce hormesis and its possible variation across altitudinal populations. This heat treatment strongly extended longevity in lowland-derived flies from the most heat-resistant population only. Both heat-induced effects on longevity and heat-knockdown time (heat-stress sensitivity) were negatively correlated to altitude of population of origin. Hormesis was positively correlated to heat-knockdown time across populations. These results indicate that variation in heat-induced hormesis can not be considered as independent of heat-stress sensitivity (or heat-knockdown time) in populations of insects.

Genetic variation in the heat-stress survival of embryos is largely decoupled from adult thermotolerance in an intercontinental set of recombinant lines of Drosophila melanogaster.

In insects, thermal adaptation works on the genetic variation for thermotolerance of not only larvae and adults but also of the immobile stages of the life cycle including eggs. In contrast to adults and larvae, the genetic basis for thermal adaptation in embryos (eggs) remains to be tested in the model insect Drosophila melanogaster. Quantitative-trait loci (QTL) for heat-stress resistance in embryos could largely differ from previously identified QTL for larvae and adults. Here we used an intercontinental set of recombinant inbred lines (RIL), which were previously used to identify thermotolerance-QTLs in adults and larvae because of their high variation segregating for adult thermotolerance. Eggs appeared to be more heat resistant than larvae and adults from previous studies on these RIL, though different heat-shock assays were used in previous studies. We found that variation in thermotolerance in embryos can be, at least partially, genetically decoupled from thermotolerance in the adult insect. Some RIL that are heat resistant in the adult and larvae can be heat susceptible in embryos. Only one small-effect QTL out of five autosomal QTL co-localized between embryo and other ontogenetic stages. These results suggest that selection for thermal adaptation in adult flies and larvae is predicted to have only a small impact on embryo thermotolerance. In addition, heat-stress tolerance of insects can be measured across ontogenetic stages including embryos in order to better predict thermal adaptive limits of populations and species.

Heat knockdown resistance and chill-coma recovery as correlated responses to selection on mating success at high temperature in Drosophila buzzatii.

Reproduction and related traits such as mating success are strongly affected by thermal stress. We tested direct and correlated responses to artificial selection in replicated lines of Drosophila buzzatii that were selected for mating success at high temperature. Knockdown resistance at high temperature (KRHT) and chill-coma recovery (CCR) were tested as correlated selection responses. Virgin flies were allowed to mate for four hours at 33°C in three replicated lines (S lines) to obtain the selected flies and then returned at 25°C to lay eggs. Other three replicated lines were maintained at 25°C without any selection as control (C lines). After 15 selection generations, KRHT and CCR were measured. Both traits were assessed in flies that did not receive any hardening pretreatments as well as in flies that were either heat or cold hardened. Thermotolerance traits showed significant correlated responses with higher KRHT in S than in C lines, both with a heat-hardening pretreatment and without a heat-hardening pretreatment. CCR time was longer in S than in C lines both with a cold-hardening pretreatment and without a cold-hardening pretreatment. Hardening treatments improved both KRHT and CCR in all cases excepting KRHT in C lines. Overall, KRHT and CCR showed an antagonistic pattern of correlated responses to our selection regime, suggesting either pleiotropy or tightly linked trait-specific genes partially affecting KRHT and CCR.

Negative genetic correlation between longevity and its hormetic extension by dietary restriction in Drosophila melanogaster.

Longevity is a highly malleable trait which is influenced by many genetic and environmental factors including nutrition. Mild stress of dietary restriction (DR) is often beneficial by extending longevity in many organisms. Here, DR-induced effects on longevity were tested for genetic variation in a set of recombinant inbred lines (RIL) in D. melanogaster. Genetic variability was significant in the longevity response following a DR-treatment across RIL, with detrimental effects in several RIL but beneficial effects in other RIL. One quantitative trait locus (QTL) was consistently significant in the middle of chromosome 2 for DR-induced changes in longevity, including hormesis (an increase in longevity by DR). Another QTL co-localized with a previously found QTL for starvation resistance in females. Several other QTL were also significant on most chromosomal arms. Longevity in controls was negatively correlated to DR effects across RIL for longevity in females, the sex showing higher DR-induced hormesis. This negative genetic correlation highlights the importance to further investigate the effects of genetic variation in the strength of DR-induced hormesis in longevity and its sex-specificity.

Quantitative Trait Loci and Antagonistic Associations for Two Developmentally Related Traits in the Drosophila Head.

In insects, some developmentally related traits are negatively correlated. Here, we mapped Quantitative Trait Loci (QTL) for traits of eye size and head capsule, in an intercontinental set of recombinant inbred lines (RILs) of Drosophila melanogaster Composite interval mapping identified QTL on all major chromosomes. Two negatively correlated traits (size of eyes and between-eyes distance) were influenced by one QTL that appeared to be antagonistic between the traits (QTL cytological range is 25F5-30A6), consistent with a negative genetic correlation between these traits of the head capsule. Comparisons of QTL across traits indicated a nonrandom distribution over the genome, with a considerable overlap between some QTL across traits. Developmentally-related traits were influenced by QTL in a pattern that is consistent both with 1) the sign of the genetic correlation between the traits and 2) a constraint in the micro-evolutionary differentiation in the traits.

Elevated extension of longevity by cyclically heat stressing a set of recombinant inbred lines of Drosophila melanogaster throughout their adult life.

An extremely high (about 100 %) increase in longevity is reported for a subset of recombinant inbred lines (RILs) of Drosophila melanogaster subjected to a cyclic heat stress throughout the adult life. Previous work showed that both longevity and heat sensitivity highly differed among RILs. The novel heat stress treatment used in this study consisted of 5 min at 38 °C applicated approximately every 125 min throughout the adult life starting at the age of 2 days. In spite of the exceptionally high increase in longevity in a set of RILs, the same heat stress treatment reduced rather than increased longevity in other RILs, suggesting that heat-induced hormesis is dependent on the genotype and/or the genetic background. Further, one quantitative trait locus (QTL) was identified for heat-induced hormesis on chromosome 2 (bands 28A1-34D2) in one RIL panel (RIL-D48) but it was not significant in its reciprocal panel (RIL-SH2). The level of heat-induced hormesis showed a sexual dimorphism, with a higher number of lines exhibiting higher hormesis effects in males than in females. The new heat stress treatment in this study suggests that longevity can be further extended than previously suggested by applying a cyclic and mild stress throughout the life, depending on the genotype.

Effects of dietary composition on life span of Drosophila buzzatii and its short-lived sibling species D. koepferae.

Two sibling Drosophila species dramatically divergent in longevity, Drosophila buzzatii and D. koepferae, were examined for possible effects of both developmental culture medium and dietary composition (DC) on longevity. Longevity was greatly increased in the longer lived D. buzzatii when flies were reared and fed on a rich-in-nutrient and cactus-based culture (R-CBC) as compared to longevity in a poor nutrient culture (PNC). In D. buzzatii, life span was further increased by exposing flies to short periods of a poor-in-nutrient and cactus-based culture (P-CBC). In contrast, variation in the here used nutrient composition did not change life span in the shorter lived D. koepferae, as longevity in this species did not differ among R-CBC, P-CBC and PNC cultures. Hormesis is a plausible explanation for the beneficial biological effects against aging arising from brief exposure to a lowed calorie food source in D. buzzatii. This study shows that genetic variation between closely related species is substantial for dietary effects on longevity.

QTL for survival to UV-C radiation in Drosophila melanogaster.

PURPOSE: The aim of this study was to investigate tolerance to UV-C (ultraviolet C, 280-100 nm) radiation in Drosophila melanogaster, implementing a quantitative trait locus (QTL) mapping approach. This is of interest to test for genetic variation in survival to UV (ultraviolet) radiation. MATERIALS AND METHODS: We performed a QTL scan in D. melanogaster recombinant inbred lines (RIL) constructed from parental stocks derived from a crossing between northern and southern hemisphere populations that segregated substantial genetic variation in thermal resistance in a previous study. Here, two experimental treatments were implemented: Continuous and cyclic UV-C radiation. RESULTS: Significant QTL were detected on all three major chromosomes. Among these, multiple trait composite interval mapping revealed a significant QTL in the pericentromeric region of chromosome 2, a genome region consistently implicated in thermotolerance in previous studies. CONCLUSIONS: This study shows substantial genetic variation for UV-C radiation resistance in D. melanogaster, with QTL for survival to UV-C radiation generally overlapping with major thermotolerance QTL. The genetic architecture of UV-C radiation resistance appears to be more complex in continuously irradiated individuals.

Quantitative trait loci for longevity in heat-stressed Drosophila melanogaster.

Longevity is a typical quantitative trait which is influenced by multiple genes. Here we explore the genetic variation in longevity of Drosophila melanogaster in both mildly heat-stressed and control flies. Quantitative trait loci (QTL) analysis for longevity was performed in a single-sex environment at 25°C with and without a mild heat-stress pre-treatment, using a previously reported set of recombinant inbred lines (RIL). QTL regions for longevity in heat-stressed flies overlapped with QTL for longevity in control flies. All longevity QTL co-localized with QTL for longevity identified in previous studies using very different sets of RIL in mixed sex environments, though the genome is nearly saturated with QTL for longevity when considering all previous studies. Heat stress decreased the number of significant QTL for longevity if compared to the control environment. Our mild heat-stress pre-treatment had a beneficial effect (hormesis) more often in shorter-lived than in longer-lived RIL.

Quantitative trait locus for starvation resistance in an intercontinental set of mapping populations of Drosophila melanogaster.

Starvation resistance (SR) is an important trait for survival of insects in the wild. We used recombinant inbred lines (RIL) to search for quantitative trait loci (QTL) in crosses between intercontinental inbred lines that were originally selected for heat-knockdown resistance. SR was measured as the time of survival under repeated events of starvation. SR was consistently higher in females than in males. Composite interval mapping identified one QTL region (cytological range 64D-66E2) on the left arm of chromosome 3 in males, and no QTL was found in females. Many candidate genes that were identified in previous studies of QTL for stress resistance are included within this QTL region. The QTL-allele that decreased SR was found in the line originating from the colder population (Denmark). We discuss our results with regard to multiple candidate genes, noncolocalization with thermotolerance QTL, and possible geographical variation.

Knockdown resistance to heat stress and slow recovery from chill coma are genetically associated in a quantitative trait locus region of chromosome 2 in Drosophila melanogaster.

In insects, two ecologically relevant traits of thermal adaptation are knockdown resistance to high temperature (KRHT) and chill-coma recovery (CCR). Chromosome 2 of Drosophila melanogaster was tested for quantitative trait loci (QTL) affecting both CCR and KRHT in backcrosses between homosequential lines that are fixed for the standard (noninverted) sequence of this autosome. These lines were obtained by artificial selection on KRHT and subsequent inbreeding from a stock that was derived from a single wild population. Heat-induced expression of the 70KD heat-shock protein (Hsp70) was also examined for variation between the lines. Composite interval mapping was performed for each trait on each reciprocal backcross, identifying one QTL region in the middle of chromosome 2 for both KRHT and CCR. The largest estimates of additive effects were found in pericentromeric regions of chromosome 2, accounting for 10-14% (CCR) and 10-17% (KRHT) of the phenotypic variance in BC populations. No QTL was found in the region of the heat-shock factor (hsf) gene. However, the two parental lines have diverged in the heat-induced Hsp70 expression. Distribution of KRHT QTL on chromosome 2 was similar between this study based on crosses between lines selected from a single wild population and previous work based on crosses between selection lines from different continents. Colocalized QTL showed a trade-off association between CCR and KRHT, which should be the result of either multiple, tightly linked trait-specific genes or a single gene with pleiotropic effects on the traits. We discuss candidate loci contained within the QTL regions.