Drosophila melanogaster Selection for Survival after Infection with Bacillus cereus Spores: Evolutionary Genetic and Phenotypic Investigations of Respiration and Movement
Laboratory populations of D. melanogaster have been subjected to selection for survival after live spores of B. cereus were introduced as a pathogenic agent. The present study was designed to investigate correlated traits: respiration as a metabolic trait and movement as a behavioral trait. An underlying hypothesis was that the evolution of increased survival after B. cereus infection exerts a metabolic cost associated with elevated immunity and this would be detected by increased respiration rates. There was support for this hypothesis in the male response to selection, but not for selected-line females. Two phenotypic effects were also observed in the study. Females especially showed a marked increase in respiration after mating compared to the other assay stages regardless of whether respiration was measured per fly or adjusted by lean mass or dry weight. Given that mating stimulates egg production, it is feasible that elevated metabolism was needed to provision oocytes with yolk. Females also moved less than males, perhaps due to behaviors related to oviposition whereas elevated male activity might be due to behaviors associated with seeking females and courtship. Relatively low movement of females indicated that their elevated respiration after mating was not due to a change in locomotion. 1. Introduction A tradeoff between immune function and reproduction has been observed in a range of studies. For example, an increase in reproductive effort is correlated with increased parasite incidence and disease [1, 2]. More generally, an increase in reproduction is associated with decreased immune system function [3, 4]. Two mechanisms have been proposed for this relationship. One is the Y model [5] in which there is an energetic competition between somatic function (immunity in the present study) and reproduction. The arms of the Y represent competition for energy between traits and the long axis represents resource input. Another potential mechanism for a tradeoff is a negative pleiotropic effect of hormones acting on two traits. An example is the effect of insulin signaling as a stimulator of reproduction and a suppressor of life span in D. melanogaster [6, 7]. In support of the role of hormones on insect life history tradeoffs, juvenile hormone (JH) has been shown to have negative pleiotropic effects on immunity and reproduction. In the flour beetle, Tenebrio molitor, mating causes a decrease in an immune system enzyme (phenoloxidase) by increasing the level of JH [8]. In the bee, Apis mellifera, a caste behavior was experimentally altered which
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