Genomics of Parallel Experimental Evolution in Drosophila

What are the genomic foundations of adaptation in sexual populations? We address this question using fitness–character and whole-genome sequence data from 30 Drosophila laboratory populations. These 30 populations are part of a nearly 40-year laboratory radiation featuring 3 selection regimes, each shared by 10 populations for up to 837 generations, with moderately large effective population sizes. Each of 3 sets of the 10 populations that shared a selection regime consists of 5 populations that have long been maintained under that selection regime, paired with 5 populations that had only recently been subjected to that selection regime. We find a high degree of evolutionary parallelism in fitness phenotypes when most-recent selection regimes are shared, as in previous studies from our laboratory. We also find genomic parallelism with respect to the frequencies of single-nucleotide polymorphisms, transposable elements, insertions, and structural variants, which was expected. Entirely unexpected was a high degree of parallelism for linkage disequilibrium. The evolutionary genetic changes among these sexual populations are rapid and genomically extensive. This pattern may be due to segregating functional genetic variation that is abundantly maintained genome-wide by selection, variation that responds immediately to changes of selection regime.

Skin temperature reveals the intensity of acute stress

Acute stress triggers peripheral vasoconstriction, causing a rapid, short-term drop in skin temperature in homeotherms. We tested, for the first time, whether this response has the potential to quantify stress, by exhibiting proportionality with stressor intensity. We used established behavioural and hormonal markers: activity level and corticosterone level, to validate a mild and more severe form of an acute restraint stressor in hens (Gallus gallus domesticus). We then used infrared thermography (IRT) to non-invasively collect continuous temperature measurements following exposure to these two intensities of acute handling stress. In the comb and wattle, two skin regions with a known thermoregulatory role, stressor intensity predicted the extent of initial skin cooling, and also the occurrence of a more delayed skin warming, providing two opportunities to quantify stress. With the present, cost-effective availability of IRT technology, this non-invasive and continuous method of stress assessment in unrestrained animals has the potential to become common practice in pure and applied research.

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We measured skin temperature in hens following a mild or more severe acute stressor.

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The temperature of thermoregulatory tissues temporarily dropped under acute stress.

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The magnitude of this skin temperature change reflected acute stressor intensity.

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Infrared thermography offers a non-invasive method of stress assessment.

Population density effects on longevity

Population density, or the number of adults in an environment relative to the limiting resources, may have important long and short term consequences for the longevity of organisms. In this paper we summarize the way in which crowding may have an immediate impact on longevity, either through the phenomenon known as dietary restriction or through alterations in the quality of the environment brought on by the presence of large numbers of individuals. We also consider the possible long term consequences of population density on longevity by the process of natural selection. There has been much theoretical speculation about the possible impact of population density on the evolution of longevity but little experimental evidence has been gathered to test these ideas. We discuss some of the theory and empirical evidence that exists and show that population density is an important factor in determining both the immediate chances of survival and the course of natural selection.

The effect of superoxide dismutase alleles on aging in Drosophila

The effects of superoxide dismutase on aging were tested using two different experimental approaches. In the first, replicated populations with postponed aging were compared with their controls for frequencies of electrophoretic alleles at the SOD locus. Populations with postponed aging had consistently greater frequencies of the allele coding for more active SOD protein. This allele was not part of a segregating inversion polymorphism. The second experimental approach was the extraction of SOD alleles from different natural populations followed by the construction of different SOD genotypes on hybrid genetic backgrounds. This procedure did not uncover any statistical effect of SOD genotype on longevity or fecundity. There were large effects on longevity and fecundity due to the family from which a particular SOD genotype was derived. To detect the effects of SOD genotypes on longevity with high probability would require a ten-fold increase in the number of families used.

Selection on stress resistance increases longevity in Drosophila melanogaster

Tests for the causal involvement of specific physiological mechanisms in the control of aging require evidence that these mechanisms can be used to increase longevity or reproductive lifespan. Selection for later reproduction in Drosophila has been shown to lead to increased longevity, as well as increased resistance to starvation and desiccation stresses. Selection for increased resistance to starvation and desiccation in Drosophila melanogaster is here shown to lead to increased longevity, indicating that alleles that increase stress resistance also may increase longevity. The responses of desiccation and starvation resistance to selection are partly independent of each other, indicating a multiplicity of physiological mechanisms involved in selectively postponed aging, and thus aging in general.

What evolutionary biology can do for gerontology

Evolutionary biologists have shown mathematically that aging is an inevitable consequence of age-specific natural selection acting on species with somata separate from germ lines. Two specific genetic mechanisms are known which could underlie the evolution of aging under these conditions: age-specificity of gene effects and antagonistic pleiotropy between early and late ages. Comparative evidence indicates that senescence occurs only when the stipulations of the evolutionary theory are met. Laboratory experiments with Drosophila indicate that prolonging the action of natural selection leads to the evolution of postponed senescence. The genetic variation involved in such postponed senescence exhibits both age-specificity and antagonistic pleiotropy. These theories and empirical findings together suggest that the best general theory of aging now available is the evolutionary theory. In addition, this work has yielded Drosophila stocks with postponed senescence that are being used to unravel physiological mechanisms of senescence.

Localizing genes that defer senescence in Drosophila melanogaster

Selection for age-specific reproduction has produced replicate stocks in which life span exceeds that in short-lived controls by about 30 per cent, in unpaired individuals. Crosses between a selected long-lived (L) stock, short-lived (S) stock and a strain with balancer chromosomes were used to create all possible combinations of their chromosomes. The longest and shortest-lived genotypes are found to be (LSL) and (SLS), with other combinations distributed between them approximately according to their first and third chromosomes. Longevity appears to be under polygenic control with contributing elements on all chromosomes. The third chromosome is by far the most influential, accounting for 66 to 72 per cent of the observed variation in females. The first chromosome is less effective. Epistatic interactions are more important in males than females, but are significant only in measurements of single individuals. Some controlling elements for longevity appear to differ in males and females. Crosses of selected stocks with known P and M-cytotype strains show no effect on either sterility or longevity.