Blood capillary geometry helps to explain the link between genome size and metabolic rates

In the past decades, several authors have investigated the possibility that genome size is correlated with metabolic rates, obtaining conflicting results. The main biological explanation among the supporters of this correlation was related to the nucleotypic effect of the genome size, which, determining the cellular volume and hence the surface area-to-volume ratio, influences cellular metabolism. In the present study, I tested a different hypothesis: genome size, influencing red blood cell (RBC) volume, is correlated with capillary density and diameter. These, in turn, are (directly and inversely) correlated with mass-specific metabolic rates, and this can explain the link between genome size and metabolic rates. I have found that these correlations are significant in vertebrates with nucleated RBCs (non-mammals), but not in species with enucleated RBCs (i.e. mammals). Although further research is needed (in particular to understand how to test the correlation between genome size and metabolic rates in ectotherms), my results show that genome size and metabolic rates act on each other through a physical constraint. Through enucleation, mammals (but also some fish and salamanders) removed this constraint.