Metabolic rate and genomic GC: what we can learn from teleost fish.

Teleosts are a highly diverse group of animals occupying all kind of aquatic environment. Data on routine mass specific metabolic rate were re-examined correcting them for the Boltzmann's factor. Teleostean fish were grouped in five broad groups, corresponding to major environmental classifications: polar, temperate, sub-tropical, tropical and deep-water. The specific routine metabolic rate, temperature-corrected using the Boltzmann's factor (MR), and the average base composition of genomes (GC%) were calculated in each group. Fish of the polar habitat showed the highest MR. Temperate fish displayed a significantly higher MR than tropical fish, which had the lowest average value. These results were apparently in agreement with the cold adaptation hypothesis. In contrast with this hypothesis, however, the MR of fish living in deep-water environment turned out to be not significantly different from that of fish living in tropical habitats. Most probably, the amount of oxygen dissolved in the water directly affects MR adaptation. Regarding the different habitats, the genomic GC levels showed a decreasing trend similar to that of MR. Indeed, both polar and temperate fish showed a GC level significantly higher than that of both sub-tropical and tropical fish. Plotting the genomic GC levels versus the MR a significant positive correlation was found, supporting the hypothesis that metabolic rate can explain not only the compositional transition mode (e.g. amphibian/mammals), but also the compositional shifting mode (e.g. fish/fish) of evolution observed for vertebrate genomes.