Allometric scaling of flight energetics in orchid bees: evolution of flux capacities and flux rates.

The evolution of metabolic pathways involved in energy production was studied in the flight muscles of 28 species of orchid bees. Previous work revealed that wingbeat frequencies and mass-specific metabolic rates decline in parallel by threefold as body mass increases interspecifically over a 20-fold range. We investigated the correlated evolution of metabolic rates during hovering flight and the flux capacities, i.e. V(max) values, of flight muscle enzymes involved in substrate catabolism, the Krebs cycle and the electron transport chain. V(max) at the hexokinase (HK) step scales allometrically with an exponent almost identical to those obtained for wingbeat frequency and mass-specific metabolic rate. Analysis of this relationship using phylogenetically independent contrasts supports the hypothesis of correlated evolution between HK activity and mass-specific metabolic rate. Although other enzymes scale allometrically with respect to body mass, e.g. trehalase, glycogen phosphorylase and citrate synthase, no other enzyme activities were correlated with metabolic rate after controlling for phylogenetic relatedness. Pathway flux rates were used with enzyme V(max) values to estimate fractional velocities (fraction of V(max) at which enzymes operate) for various reactions to gain insights into enzyme function and how this varies with body mass. Fractional velocity is highly conserved across species at the HK step, but varied at all other steps examined. These results are discussed in the context of the regulation and evolution of pathways of energy metabolism.