Age specificity of inbreeding load in Drosophila melanogaster and implications for the evolution of late-life mortality plateaus

Abstract

Current evolutionary theories explain the origin of aging as a byproduct of the decline in the force of natural selection with age. These theories seem inconsistent with the well-documented occurrence of late-life mortality plateaus, since under traditional evolutionary models mortality rates should increase monotonically after sexual maturity. However, the equilibrium frequencies of deleterious alleles affecting late life are lower than predicted under traditional models, and thus evolutionary models can accommodate mortality plateaus if deleterious alleles are allowed to have effects spanning a range of neighboring age classes. Here we test the degree of age specificity of segregating alleles affecting fitness in Drosophila melanogaster. We assessed age specificity by measuring the homozygous fitness effects of segregating alleles across the adult life span and calculated genetic correlations of these effects across age classes. For both males and females, we found that allelic effects are age specific with effects extending over 1-2 weeks across all age classes, consistent with modified mutation-accumulation theory. These results indicate that a modified mutation-accumulation theory can both explain the origin of senescence and predict late-life mortality plateaus.

Figure 1.—

Change in mean SID with age. Males are represented by squares, and females are represented by circles. Error bars are standard deviation from the mean.

Figure 2.—

Change in mean ID with age. Dashed lines represent heterozygous reproductive success, dotted lines indicate homozygous reproductive success, and the solid line indicates ID. Error bars are standard deviation from the mean.

Figure 2.—

Change in mean ID with age. Dashed lines represent heterozygous reproductive success, dotted lines indicate homozygous reproductive success, and the solid line indicates ID. Error bars are standard deviation from the mean.

Figure 3.—

Correlations in inbreeding load among age classes. Each bar represents the correlation between two age classes; e.g., the first bar is the correlation in inbreeding load for age classes 1 week apart. Shaded bars represent correlations among age classes using SID values. Open bars represent correlations among age classes using ID values. Error bars are the standard deviation of the jackknifed data set.

Figure 3.—

Correlations in inbreeding load among age classes. Each bar represents the correlation between two age classes; e.g., the first bar is the correlation in inbreeding load for age classes 1 week apart. Shaded bars represent correlations among age classes using SID values. Open bars represent correlations among age classes using ID values. Error bars are the standard deviation of the jackknifed data set.