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

Abstract

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