The evolutionary costs of immunological maintenance and deployment

Abstract

Background: The evolution of disease resistance and immune function may be limited if increased immunocompetence comes at the expense of other fitness-determining traits. Both the maintenance of an immune system and the deployment of an immune response can be costly, and the observed costs may be evaluated as either physiological or evolutionary in origin. Evolutionary costs of immunological maintenance are revealed as negative genetic correlations between immunocompetence and fitness in the absence of infection. Costs of deployment are most often studied as physiological costs associated with immune system induction, however, evolutionary costs of deployment may also be present if genotypes vary in the extent of the physiological cost experienced.

Results: In this study we analyzed evolutionary and physiological costs of immunity in two environments representing food-limited and food-unlimited conditions. Patterns of genetic variation were estimated in females from 40 'hemiclone families' isolated from a population of D. melanogaster. Phenotypes evaluated included fecundity, weight measures at different time periods and resistance to Providencia rettgeri, a naturally occurring Gram-negative pathogen of D. melanogaster. In the food-limited environment we found a negative genetic correlation between fecundity in the absence of infection and resistance, indicative of an evolutionary cost of maintenance. No such correlation was observed in the food-unlimited environment, and the slopes of these correlations significantly differed, demonstrating a genotype-by-environment interaction for the cost of maintenance. Physiological costs of deployment were also observed, but costs were primarily due to wounding. Deployment costs were slightly exaggerated in the food-limited environment. Evolutionary costs of immunological deployment on fecundity were not observed, and there was only marginally significant genetic variation in the cost expressed by changes in dry weight.

Conclusion: Our results suggest that the costs of immunity may be an important factor limiting the evolution of resistance in food-limited environments. However, the significant genotype-by-environment interaction for maintenance costs, combined with the observation that deployment costs were partially mitigated in the food-unlimited environment, emphasizes the importance of considering environmental variation when estimating patterns of genetic variance and covariance, and the dubious nature of predicting evolutionary responses to selection from quantitative genetic estimates carried out in a single environment.

Figure 1

Genotype-by-environment interaction plot for the rank of hemiclone fecundity in yeast-limited and yeast-unlimited environments (ANOVA p < 0.0001; see Table 1 and text for details).

Figure 2

Genetic correlations between fecundity and resistance in yeast-limited and yeast-unlimited environments.

Figure 3

The effect of immune challenge on female fecundity. Stars indicate the significance of tests of the main effect of CHALLENGE from a model comparing pre-challenge fecundity (vial 3) to post-challenge fecundity (vials 4 – 7). See Tables 5 and 6 and the text for details on the statistical analysis. *** p < 0.001; * p < 0.05.

Figure 4

The effect of immune challenge on female dry weight.

Figure 5

Crossing scheme for the generation of paternal half sib sisters using 'clone generator' (CG) females. CG-females carry a compound × chromosome [C(1)DX, y, f], a Y chromosome, and are homozygous for a translocation of chromosomes 2 and 3 [T(2;3) rdgC st in ri p^P bw]. The translocated chromosomes are represented as a solid black bar because offspring are viable only if both chromosomes are inherited. The compound X ensures that during hemiclone capture and amplification, males receive their wild-type X chromosome from their father, and the absence of recombination in males ensures there is no mixing of wild-type and CG autosomes.

Figure 6

Outline of the experimental design. Eight standard (yeast-limited) vials and 5 yeast-supplemented vials were set up for each hemiclone family, consisting of 5 test females and 5 randomly sampled wild-type males. Females were collected at emergence (vial 1), transferred to a new vial 2 days later (vial 2) and then into vial 3 after 2 more days. Counts of emerging offspring were conducted for vials 3 – 7, with females transferred every 24 hours between these vials. Injections were carried out during the transfer of females from vial 3 to vial 4. Females receiving an injection of live Providencia rettgeri were homogenized 28 hours after injection and resistance quantified from bacterial counts. For each hemiclone, two vials were set up in each environment to evaluate resistance. Fecundity in vial 3 was used to evaluate the cost of immunological maintenance. Deployment costs were evaluated by comparing pre-challenge fecundity (vial 3) to post-challenge fecundity (vials 4–7). Dry mass was estimated for females collected at emergence and for females collected at the end of the experiment (exiting vial 7). Females from the three vials used in an analysis of constitutive gene expression were frozen at the time of injections. Data on gene expression is being published elsewhere. See text for details on the analyses.