MUELLER, R. L.; University of California, Berkeley: Genome size, cell size, and the evolution of enucleated red blood cells in the salamander genus Batrachoseps

Plethodontid salamanders of the genus Batrachoseps have very unusual blood; a majority of their red blood cells, like those of mammals, do not contain a nucleus. This unusual feature is seen in at least four other lineages within the Plethodontidae. These five lineages do not form a monophyletic group, meaning these cells have evolved at least five times within plethodontids. In each instance where enucleated RBCs have evolved, salamanders are characterized by large genomes and a miniaturized or attenuate morphology. This study examines the hypothesis that these cells have evolved multiple times in response to a common physical constraint against the circulation of large, fully nucleated RBCs resulting from a large genome in these miniaturized organisms. This constraint on cell morphology results from the interaction between evolutionary changes at the genome level (size increase) and evolutionary changes at the organismal level (size decrease). This study comprises two datasets, each analyzing an unusual aspect of the blood of Batrachoseps in light of the most recent phylogenetic hypothesis. The first quantifies high levels of inter- and intra-specific variation in levels of enucleated red blood cells. The second presents data on genome size, nuclear area, and cell area and shows the deviation in Batrachoseps from the well-established positive correlation among these variables. The results of both of these datasets are consistent with a hypothesis of enucleated cells evolving in response to a physical constraint, implying a mechanism for the homoplastic appearance of these cells multiple times in the Plethodontidae.