Dissertation Research: Molecular Systematics and the Evolution of a Novel Red Blood Cell Morphology in Plethodontid Salamanders

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

The proposed research will focus on a multidimensional analysis of evolution at different hierarchical levels, from the molecular to the organismal and phylogenetic. The study organisms, plethodontid salamanders of the genus Batrachoseps, are unique among vertebrates in displaying great variation in the proportion of red blood cells in the circulating blood - from nearly all nucleated to nearly all enucleated. The variation appears to be correlated with degree of organismal specialization, in particular with elongation, attenuation, and miniaturization, in a group of organisms that is specialized for semifossorial existence in seasonally dry environments. A complicating factor is the very high genome sizes in these organisms, which raises the possibility that enucleation is a phenomenon associated with a fundamental conflict between large cells, a necessary correlate of large genomes, and small blood vessels, a necessary correlate of miniaturization. The study will investigate the possibility that enucleation is a mechanical outcome of miniaturization, by conducting ecological and morphological studies. In order to proceed it will be necessary to develop a robust phylogenetic hypothesis for the genus, using molecular markers. Mitochondrial genes have already been studied, and there is good resolution of many of the tips of the phylogenetic tree; new work will focus on more conservative nuclear genes and deeper branches in the tree. A phylogenetic hypothesis will permit a comparative biological analysis of this problem area and a determination of whether increases in body size within a miniaturized lineage are accompanied by increases in nucleation levels for red blood cells. The significance of this research lies in the focused effort to study how different levels of organization -- molecular, cellular, organismal, ecological, and phylogenetic -- interact during development to give form and substance to lineages of organisms. A simultaneous investigation of different levels provides a realistic assessment of how evolution proceeds in lineages through time, and permits an assessment of the nature of general adaptation processes and their implications. The study also holds promise for opening new avenues of research on the puzzling phenomenon of increases in genome size, which are so great in these organisms as to strongly effect cellular and organismal levels (the classic C-value paradox).