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Despite tremendous progress in finding genes that, when manipulated, affects lifespan, little is known about the genetics underlying natural variation in lifespan. While segregating genetic variants for lifespan has been notoriously difficult to find in genome-wide association studies (GWAS), a complementary approach is to manipulate key genetic pathways in lines that differ in lifespan. If these candidate pathways are down regulated in long-lived lines, these lines can be predicted to respond less to pharmaceutical down-regulation of these pathways than short-lived lines. Experimental studies have identified the nutrient-sensing pathway TOR as a key regulator of lifespan in model organisms, and this pathway can effectively be down regulated using the drug rapamycin, which extends lifespan in all tested species. We expose short- and long-lived lines of the nematode Caenorhabditis remanei to rapamycin, and investigate if long-lived lines, which are hypothesized to already have down-regulated TOR signaling, respond less to rapamycin. We found no interaction between line and rapamycin treatment, since rapamycin extended lifespan independent of the intrinsic lifespan of the lines. This shows that rapamycin is equally effective on long and short-lived lines, and suggests that the evolution of long life may involve more factors that down-regulation of TOR.

Classic theories of ageing evolution predict that increased extrinsic mortality due to an environmental hazard selects for increased early reproduction, rapid ageing and short intrinsic lifespan. Conversely, emerging theory maintains that when ageing increases susceptibility to an environmental hazard, increased mortality due to this hazard can select against ageing in physiological condition and prolong intrinsic lifespan. However, evolution of slow ageing under high-condition-dependent mortality is expected to result from reallocation of resources to different traits and such reallocation may be hampered by sex-specific trade-offs. Because same life-history trait values often have different fitness consequences in males and females, sexually antagonistic selection can preserve genetic variance for lifespan and ageing. We previously showed that increased condition-dependent mortality caused by heat shock leads to evolution of long-life, decelerated late-life mortality in both sexes and increased female fecundity in the nematode, Caenorhabditis remanei. Here, we used these cryopreserved lines to show that males evolving under heat shock suffered from reduced early-life and net reproduction, while mortality rate had no effect. Our results suggest that heat-shock resistance and associated long-life trade-off with male, but not female, reproduction and therefore sexually antagonistic selection contributes to maintenance of genetic variation for lifespan and fitness in this population.

Summary Life-history theory predicts a trade-off between early-life fitness and life span. While the focus traditionally has been on the fecundity-life span trade-off, there are strong reasons to expect trade-offs with growth rate and/or development time. We investigated the roles of growth rate and development time in the evolution of life span in two independent selection experiments in the outcrossing nematode Caenorhabditis remanei. First, we found that selection under heat-shock leads to the evolution of increased life span without fecundity costs, but at the cost of slower development. Thereafter, the putative evolutionary links between development time, growth rate, fecundity, heat-shock resistance and life span were independently assessed in the second experiment by directly selecting for fast or slow development. This experiment confirmed our initial findings, since selection for slow development resulted in the evolution of long life span and increased heat-shock resistance. Because there were no consistent trade-offs with growth rate or fecundity, our results highlight the key role of development rate – differentiation of the somatic cells per unit of time – in the evolution of life span. Since development time is under strong selection in nature, reduced somatic maintenance resulting in shorter life span may be a widespread cost of rapid development. A lay summary is available for this article.

SummaryMales and females age at different rates and have different life expectancies across the animal kingdom, but what causes the longevity “gender gaps” remains one of the most fiercely debated puzzles among biologists and demographers [1–7]. Classic theory predicts that the sex experiencing higher rate of extrinsic mortality evolves faster aging and reduced longevity [1]. However, condition dependence of mortality [8, 9] can counter this effect by selecting against senescence in whole-organism performance [5, 10]. Contrary to the prevailing view but in line with an emerging new theory [7–9, 11], we show that the evolution of sex difference in longevity depends on the factors that cause sex-specific mortality and cannot be predicted from the mortality rate alone. Experimental evolution in an obligately sexual roundworm, Caenorhabditis remanei, in which males live longer than females, reveals that sexual dimorphism in longevity erodes rapidly when the extrinsic mortality in males is increased at random. We thus experimentally demonstrate evolution of the sexual monomorphism in longevity in a sexually dimorphic organism. Strikingly, when extrinsic mortality is increased in a way that favors survival of fast-moving individuals, males evolve increased longevities, thereby widening the gender gap. Thus, sex-specific selection on whole-organism performance in males renders them less prone to the ravages of old age than females, despite higher rates of extrinsic mortality. Our results reconcile previous research with recent theoretical breakthroughs [8, 9] by showing that sexual dimorphism in longevity evolves rapidly and predictably as a result of the sex-specific interactions between environmental hazard and organism’s condition.

SummaryAging affects nearly all organisms, but how aging evolves is still unclear [1–5]. The central prediction of classic theory is that high extrinsic mortality leads to accelerated aging and shorter intrinsic life span [6, 7]. However, this prediction considers mortality as a random process, whereas mortality in nature is likely to be condition dependent. Therefore, the novel theory maintains that condition dependence may dramatically alter, and even reverse, the classic pattern [2–4]. We present experimental evidence for the evolution of longer life span under high condition-dependent mortality. We employed an experimental evolution design, using a nematode, Caenorhabditis remanei, that allowed us to disentangle the effects of mortality rate (high versus low) and mortality source (random versus condition dependent). We observed the evolution of shorter life span under high random mortality, confirming the classic prediction. In contrast, high condition-dependent mortality led to the evolution of longer life span, supporting a key role of condition dependence in the evolution of aging. This life-span extension was not the result of a trade-off with reproduction. By simultaneously corroborating the classic results [8–10] and providing the first experimental evidence for the novel theory [2–4], our study resolves apparent contradictions in the study of aging and challenges the traditional paradigm by demonstrating that condition-environment interactions dictate the evolutionary trajectory of aging.

While ageing is commonly associated with exponential increase in mortality with age, mortality rates paradoxically decelerate late in life resulting in distinct mortality plateaus. Late-life mortality plateaus have been discovered in a broad variety of taxa, including humans, but their origin is hotly debated. One hypothesis argues that deceleration occurs because the individual probability of death stops increasing at very old ages, predicting the evolution of earlier onset of mortality plateaus under increased rate of extrinsic mortality. By contrast, heterogeneity theory suggests that mortality deceleration arises from individual differences in intrinsic lifelong robustness and predicts that variation in robustness between populations will result in differences in mortality deceleration. We used experimental evolution to directly test these predictions by independently manipulating extrinsic mortality rate (high or low) and mortality source (random death or condition-dependent) to create replicate populations of nematodes, Caenorhabditis remanei that differ in the strength of selection in late-life and in the level of lifelong robustness. Late-life mortality deceleration evolved in response to differences in mortality source when mortality rate was held constant, while there was no consistent response to differences in mortality rate. These results provide direct experimental support for the heterogeneity theory of late-life mortality deceleration.

Organisms age because of the “selection shadow”—the decline of the force of natural selection with age. Seemingly straightforward corollary of this theory is the Medawar-Williams prediction, which maintains that increased extrinsic (non-aging) mortality will result in the evolution of accelerated aging and decreased longevity. Despite its centrality to modern thinking about the ultimate causes of aging, this prediction ignores the fact that mortality is often a non-random process depending on individual condition. Increased condition-dependent mortality inescapably results in increased selection for resistance against the agent of mortality. Provided that resistance to various stressors is commonly associated with increased longevity, the evolutionary outcome is no longer certain. We recently documented this experimentally by showing that populations of Caenorhabditis remanei evolved to live shorter under high extrinsic mortality, but only when mortality was applied haphazardly. On the contrary, when extrinsic mortality was caused by heat-shock, populations experiencing the same rate of increased mortality evolved greater longevities, notwithstanding increased “selection shadow.” Intriguingly, stress-resistant and long-lived worms were also more fecund. We discuss these results in the light of recent theoretical developments, such as condition-environment interactions and hyperfunction theory of aging.