The genetic architecture of life span and mortality rates: gender and species differences in inbreeding load of two seed-feeding beetles

Abstract

We examine the inbreeding load for adult life span and mortality rates of two seed beetle species, Callosobruchus maculatus and Stator limbatus. Inbreeding load differs substantially between males and females in both study populations of C. maculatus--life span of inbred females was 9-13% shorter than the life span of outbred females, whereas the life span of inbred males did not differ from the life span of outbred males. The effect of inbreeding on female life span was largely due to an increase in the slope of the mortality curve. In contrast, inbreeding had only a small effect on the life span of S. limbatus--life spans of inbred beetles were approximately 5% shorter than those of outbred beetles, and there was no difference in inbreeding load between the sexes. The inbreeding load for mean life span was approximately 0.4-0.6 lethal equivalents per haploid gamete for female C. maculatus and approximately 0.2-0.3 for both males and females of S. limbatus, all within the range of estimates commonly obtained for Drosophila. However, contrary to the predictions of mutation-accumulation models, inbreeding load for loci affecting mortality rates did not increase with age in either species, despite an effect of inbreeding on the initial rate of increase in mortality. This was because mortality rates decelerated with age and converged to a mortality plateau for both outbred and inbred beetles.

Figure 1.—

The block design used to measure inbreeding depression. Each block is created by crossing beetles from two unrelated families, creating two outbred matings (reciprocal crosses between the two families) and two inbred matings (crosses between full-sibs within each family). Outbreds and inbreds within each block thus have, on average, the same set of alleles but differ in degree of homozygosity due to the mating treatment.

Figure 2.—

The effect of inbreeding on the adult life span of Callosobruchus maculatus (two replicates) and Stator limbatus. Means (± SEM) are calculated first by averaging across families in a block and then across blocks within a treatment. Each block is treated as an independent data point. Some error bars are smaller than the points.

Figure 3.—

Survivorship curves for inbred vs. outbred males and females of Callosobruchus maculatus (two replicates) and Stator limbatus.

Figure 4.—

Mortality curves, ln[u(t)], for inbred vs. outbred females of Callosobruchus maculatus (two replicates) and Stator limbatus. Parameter estimates for the logistic mortality curve are presented in Table 2.

Figure 5.—

Age-specific inbreeding load for the mortality rate of female Callosobruchus maculatus (two replicates) and both male and female Stator limbatus. The lines are smoothed by averaging 3-day intervals centered on the age on the x-axis.