Rates of mortality in populations of Caenorhabditis elegans

MORTALITY PLATEAUS AND THE EVOLUTION OF SENESCENCE: WHY ARE OLD-AGE MORTALITY RATES SO LOW?

Age-specific mortality rates level off far below 100% at advanced ages in experimental populations of Drosophila melanogaster and other organisms. This observation is inconsistent with the equilibrium predictions of both the antagonistic pleiotropy and mutation accumulation models of senescence, which, under a wide variety of assumptions, predict a "wall" of mortality rates near 100% at postreproductive ages. Previous models of age-specific mortality patterns are discussed in light of recent demographic data concerning late-age mortality deceleration and age-specific properties of new mutations. The most recent theory (Mueller and Rose 1996) argues that existing evolutionary models can easily and robustly explain the demographic data. Here we discuss the sensitivity of that analysis to different types of mutational effects, and demonstrate that its conclusion is very sensitive to assumptions about mutations. A legitimate resolution of evolutionary theory and demographic data will require experimental observations on the age-specificity of mutational effects for new mutations and the degree to which mortality rates in adjacent ages are constrained to be similar (positive pleiotropy), as well as consideration of redundancy and heterogeneity models from demographic theory.

Evolution of late-life mortality in Drosophila melanogaster

Aging appears to cease at late ages, when mortality rates roughly plateau in large-scale demographic studies. This anomalous plateau in late-life mortality has been explained theoretically in two ways: (1) as a strictly demographic result of heterogeneity in life-long robustness between individuals within cohorts, and (2) as an evolutionary result of the plateau in the force of natural selection after the end of reproduction. Here we test the latter theory using cohorts of Drosophila melanogaster cultured with different ages of reproduction for many generations. We show in two independent comparisons that populations that evolve with early truncation of reproduction exhibit earlier onset of mortality-rate plateaus, in conformity with evolutionary theory. In addition, we test two population genetic mechanisms that may be involved in the evolution of late-life mortality: mutation accumulation and antagonistic pleiotropy. We test mutation accumulation by crossing genetically divergent, yet demographically identical, populations, testing for hybrid vigor between the hybrid and nonhybrid parental populations. We found no difference between the hybrid and nonhybrid populations in late-life mortality rates, a result that does not support mutation accumulation as a genetic mechanism for late-life mortality, assuming mutations act recessively. Finally, we test antagonistic pleiotropy by returning replicate populations to a much earlier age of last reproduction for a short evolutionary time, testing for a rapid indirect response of late-life mortality rates. The positive results from this test support antagonistic pleiotropy as a genetic mechanism for the evolution of late-life mortality. Together these experiments comprise the first corroborations of the evolutionary theory of late-life mortality.

Age-specific demographic profiles of longevity mutants in Caenorhabditis elegans show segmental effects

Demographic profiles of several single-gene longevity mutants of the nematode Caenorhabditis elegans reveal segmental (age-specific) effects on mortality. The mortality profiles of wild-type worms were examined across multiple replicate cultures containing 100,000 or more nematodes and found to be quite replicable, although clear environmental effects are routinely found. The combined profile of wild type was compared with those of three long-lived mutants to determine how age-specific mortality is altered by mutations in age-1, clk-1, or spe-26. In all four genotypes, death rates fit a two-stage Gompertz model better than a one-stage Gompertz; that is, mortality levels off at later ages. The largest genetic effect on mortality was that of an age-1 mutation, which lowered mortality more than fivefold at most later ages. In contrast, a spe-26 mutant had a tenfold lower mortality until approximately 2 weeks of age but ultimately achieved a higher mortality, whereas clk-1 mutants show slightly higher mortality than wild type during the fertile period, early in life, but ultimately level off at lower mortality. Each mutant thus has a distinctive profile of age-specific mortalities that could suggest the time of action of each gene.

The fractionation experiment: reducing heterogeneity to investigate age-specific mortality in Drosophila

Age-specific mortality rates decelerate at older ages in both genetically homogenous and heterogeneous populations of Drosophila. One explanation proposed for deceleration is population heterogeneity. This hypothesis suggests that a population consists of sub-populations that differ in mortality characteristics and that the deceleration is the result of selective survival of stronger individuals. Here we describe an experiment that fractionates populations into several sub-populations without changing the physiological characteristics of the post-fractionated populations. Through a careful process of selection of Drosophila eggs, larvae, pupae and adults, we attempt to reduce as much as possible the degree of pre-adult, environmentally induced heterogeneity among individuals of a genetically identical cohort. We then ask whether such cohorts, when compared to non-fractionated populations, exhibit a lesser degree of mortality deceleration at advanced ages. From a total of 106 fractionated and control populations, consisting of 51331 individuals, 101 populations (93% of the fractionated populations and 100% of the control populations) exhibit a significant amount of mortality deceleration late in life. These observations suggest that environmental heterogeneity accrued during larval development is not a major factor contributing to mortality deceleration at older ages.

Deceleration of age-specific mortality rates in chromosomal homozygotes and heterozygotes of Drosophila melanogaster

Age-specific mortality trajectories were estimated in mixed-sex cohorts of D. melanogaster. We studied 22,000 flies that were either second-chromosome homozygotes or heterozygotes with a randomized genetic background. Broad-sense heritabilities for longevity were estimated to be 6% for males and 9% for females. Heterozygotes lived longer than homozygotes on average, but there were exceptions to the usual heterotic pattern; in several crosses parental homozygotes had average life spans as long as that of their F1 heterozygotes. Estimated age-specific mortality rates were found to decelerate at advanced ages in both homozygotes and heterozygotes. The mortality models that best fit the data are the logistic model and the two-stage Gompertz model, both of which produce mortality trajectories that level off at advanced ages. Old-age mortality deceleration is not peculiar to inbred Drosophila.