Morphological, nucleotypic, and genomic drivers of physiological variation in lungless salamanders: Respiration and water loss in the family Plethodontidae

Abstract

Physiological performance depends on the complex integration of traits across multiple levels of biological organization. Characterizing the contribution of underlying traits to variation in adaptive performance phenotypes thus offers a simplified framework to identify patterns of selection and adaptation in nature. Here, I took a trait-based approach to characterize variation in physiological performance, using metabolic and water loss physiology in lungless salamanders as a model. In the lungless salamanders (Urodela: Plethodontidae), respiratory metabolism and evaporative water loss occur through the integument, leading to clear theoretical predictions about the influence of body size, skin structure, and cell size to performance. Using a combination of morphological measurements, physiological experiments, and genomic sequencing, I found that physiological variation is well explained not only by body mass – a commonly measured trait in physiological ecology – but more so by body surface area-to-volume ratios (SA:V), which place an upper limit on transport rates of oxygen into the body. The strength of natural selection on genes encoding for metabolism was inversely related to SA:V in a small sample of species, suggesting a relationship between relative degree of oxygen limitation and selection for metabolic efficiency. Further, I found that water loss is also well explained by genome size, likely due to a nucleotypic effect of the genome in determining cell size. Genome size influences water loss in the Plethodontidae at least in part through the effects of biological size – the interaction of both cell and body size. Though the mechanisms responsible for this effect are unknown, this work marks the first demonstration that biological size is a functional trait. Its influence over water balance physiology in the Plethodontidae suggests it is of particular ecological importance in this group, and raises the possibility that this understudied trait may be important to physiological performance in animals more broadly. Collectively, this dissertation expands upon a paradigm in physiological ecology that focuses extensively on body mass effects, revealing greater detail in the types of morphological traits that influence how physiological performance varies between individuals and evolves between species.