Meeting Abstract
Vertebrate red blood cells (RBCs) display a range of sizes among species and higher clades, with some groups differing by orders of magnitude in volume. Although explanations for this disparity range from metabolic costs to constraints imposed by genome size and nucleus volume, patterns of RBC size evolution are rarely considered in their historical context, underscoring a need to understand whether proposed constraints are reflected in the character’s phylogenetic history. Here, we present a meta-analysis of RBC diameters from over 290 species in all major extant tetrapod clades to explore the tempo of RBC size evolution. Data were supplemented with measurements from several anurans, non-avian reptiles, birds, and small mammals. Results suggest considerable variation both among and within lineages, even in mammals where RBCs are enucleated. For example, mammals originated with modest RBC sizes but were largest in early-diverging clades and became progressively smaller, especially in some laurasiatheres. PGLS regressions show weak support for correlated evolution with body size. Phylogenetic model-fitting suggests that multi-parameter models better fit observed patterns among clades than the null Brownian model, with saltatory changes in RBC size occurring early and becoming progressively more constrained around an optimum in daughter lineages. The existence of upper (diffusion capacity/metabolic costs) and lower (nucleus size) limits on RBC size may have both contributed to an overall pattern of constrained evolution. More sampling of laurasiatheres would help to test whether the loss of the nucleus relaxed constraints, permitting trends toward smaller RBCs in some mammals. Precise models of RBC evolution will better inform approaches to retrodicting ancestral RBC sizes in extinct lineages.
