Evolution of metabolic rate in a parasitic wasp: the role of limitation in intrinsic resources.

Metabolic rate, a physiological trait closely related to fitness traits, is expected to evolve in response to two main environmental variables: (1) climate, low metabolic rates being found in dry and hot regions when comparing populations originating from different climates in a common garden experiment and (2) resource limitations, low metabolic rates being selected when resources are limited. The main goal of this study was to investigate if differences in intrinsic resource limitations may have disrupted the expected evolution of metabolic rate in response to climate in a parasitic wasp. We compared CO(2) production of females from 4 populations of a Drosophila parasitoid, Leptopilina boulardi, as an estimate of their metabolic rate. Two populations from a hot and dry area able to synthesise lipids de novo at adult stage were compared with two populations originating from a mild and humid climate where no lipid accumulation during adult life was observed. These last females are thus more limited in lipids than the first ones. We observed that a high metabolic rate has been selected in hot and dry environments, contrarily to the results of a great majority of studies. We suggest that lipogenesis occurring there may have allowed the selection of a higher metabolic rate, as females are less limited in energetic resources than females from the mild environment. A high metabolic rate may have been selected there as it partly compensates for the long distances that females have to cross to find laying opportunities in distant orchards. We suggest that intrinsic resources should be integrated when investigating geographical variations in metabolism as this factor may disrupt evolution in response to climate.