Genetic and environmental effects on the scaling of metabolic rate with body size

Metabolic rate (MR) often scales with body mass (BM) following a power function of the form MR = aBMb, where b is the allometric exponent (i.e. slope on a log-log scale). The variational properties of b have been debated, but very few studies have tested for genetic variance in b, and none have tested for a genotype-by-environment (GxE) interaction in b. Consequently, the short-term evolutionary potentials of both b and its phenotypic plasticity remain unknown. Using 10 clones of a population of Daphnia magna, we estimated the genetic variance in b and assessed whether a GxE interaction affected b. We measured metabolic rate on juveniles of different sizes reared and measured at three temperatures (17, 22 and 28&[deg]C). Overall, b decreased with increasing temperature. We found no evidence of genetic variance in b at any temperature, and thus no GxE interaction in b. However, we found a significant GxE interaction in size-specific metabolic rate. Using simulations, we show how this GxE interaction can generate genetic variation in the ontogenetic allometric slopes of animals experiencing directional changes in temperature during growth. This suggests that b can evolve despite having limited genetic variation at constant temperatures.