A demographic analysis of the fitness cost of extended longevity in Caenorhabditis elegans

Abstract

We monitored survival and reproduction of 1000 individuals of Caenorhabditis elegans wild type (N2) and 800 individuals of clk-1 and daf-2, and used biodemographic analysis to address fitness as the integrative consequence of the entire age-specific schedules of survival and reproduction. Relative to N2, the mutants clk-1 and daf-2 extended average life span by 27% and 111%, respectively, but reduced net reproductive rate by 44% and 18%. The net result of differences in survival and fertility was a significant differential in fitness, with both clk-1 (lambda = 2.74) and daf-2 (lambda = 3.78) at a disadvantage relative to N2 (lambda = 3.85). Demographic life table response experiment (LTRE) analysis revealed that the fitness differentials were due to negative effects in mutants on reproduction in the first 6-7 days of life. Fitness costs in clk-1 and daf-2 of C. elegans are consistent with the theory of antagonistic pleiotropy for the evolution of senescence.

Figure 1

Cohort survival (l_x) of Caenorhabditis elegans (strains N2, clk-1, and daf-2) maintained as individuals on nematode growth medium and OP-50 at 20°C (n=1000, 800, and 800 for N2, clk-1, and daf-2, respectively) with survival and reproduction monitored daily (bootstrap 95% confidence intervals shown).

Figure 2

Frequency of death (d_x) for cohorts of Caenorhabditis elegans (strains N2, clk-1, and daf-2) individuals maintained on nematode growth medium and OP-50 at 20°C (n=1000, 800, and 800 for N2, clk-1, and daf-2, respectively) with survival and reproduction monitored daily (bootstrap 95% confidence intervals shown).

Figure 3

Force of mortality (µ _x) for cohorts of Caenorhabditis elegans (strains N2, clk-1, and daf-2) individuals maintained on nematode growth medium and OP-50 at 20°C (n = 1000, 800, and 800 for N2, clk-1, and daf-2, respectively) with survival and reproduction monitored daily. The smoothed mortality rate curves were obtained using a locally weighted regression (LOESS) procedure.

Figure 4

Age-specific reproduction (m_x) from cohorts of Caenorhabditis elegans (strains N2, clk-1, and daf-2) maintained as individuals on nematode growth medium and OP-50 at 20°C (n = 1000, 800, and 800 for N2, clk-1, and daf-2, respectively) with survival and reproduction monitored daily (bootstrap 95% confidence intervals shown).

Figure 5

Event history diagrams showing daily survival versus age for Caenorhabditis elegans cohorts, with individual reproduction at each age indicated by color. Individuals of C. elegans (strains N2, clk-1, and daf-2) were maintained separately on nematode growth medium and OP-50 at 20°C (n = 1000, 800, and 800 for N2, clk-1, and daf-2, respectively) with survival and reproduction monitored daily.

Figure 6

Individual life span (in days) versus lifetime total reproduction for Caenorhabditis elegans (strains N2, clk-1, and daf-2) individuals maintained separately on nematode growth medium and OP-50 at 20°C (n=1000, 800, and 800 for N2, clk-1, and daf-2, respectively) and survival and reproduction monitored daily. Curves depicted include fit, 95% confidence intervals, and 95% prediction intervals (N2: y=312 − 3201/x², Fit Standard Error=45.4; clk-1: y=164 − 13045e^−x, Fit Standard Error = 37.1; daf-2: y = 245 − 2346/x², Fit Standard Error = 40.4).

Figure 7

Life table response experiment analysis of the survival differences and survival contributions to fitness in clk-1 and daf-2 relative to N2 (bootstrap 95% confidence intervals shown).

Figure 8

Life table response experiment analysis of the fertility differences and fertility contributions to fitness in clk-1 and daf-2 relative to N2 (bootstrap 95% confidence intervals shown).