An investigation into the factors influencing venom potency in snakes and spiders

Abstract

A fundamental question in ecology and toxicology is why some venomous species have more potent venoms than others. While there is great interest in venomous species from a medical perspective due to envenomation cases globally, from an ecological perspective, venom is still relatively poorly understood despite its key role in predator-prey interactions. To address this fundamental question, I investigate the role of factors associated with diet and prey capture in the evolution of venom potency in snakes and spiders, two diverse venomous predator groups, using both data collated from the literature and data collected in the field and lab over the course of my PhD. Additionally, my research investigates different venom extraction methods as a factor influencing how venom potency is measured. I investigate the role these factors play in venom potency across four research chapters that represent stand-alone publications consisting of: “Diet Breadth Mediates the Prey Specificity of Venom Potency in Snakes.”; “Spider Venom Potency is Prey-specific but does not trade-off with Body Size or Silk Hunting Strategy.”; “The Relationship between Median Lethal dose (LD50) and Median Effective Dose (ED50) in Spider Venoms” and “Venom Extraction Method Influences Venom Composition and Potency in the Giant House Spider Eratigena atrica (C. L. Koch, 1843)”. In Chapter 2, I use phylogenetic comparative approaches to show that the evolution of preyspecific venom potencies in snakes is contingent on the breadth of a species’ diet, only observing preyspecific patterns in species with taxonomically narrow diets (specialists). In Chapter 3, I extend this phylogenetic comparative approach to spiders and show that they have evolved prey-specific venom potencies and that surprisingly the degree of silk use in prey capture does not trade-off with venom potency. In Chapter 4, I use spider venom bioassay experiments to determine the relationship between different venom potency measures LD50 and ED50, showing that they have a near isometric relationship. Finally in Chapter 5, I demonstrate through venom bioassays and Coomassie-stained SDSPAGE gel electrophoresis that using different venom extraction methods, electrical stimulation and venom gland removal, on the same spider species (Eratigena atrica) can result in venom samples with significantly different potencies and visibly different protein compositions. Throughout this thesis I demonstrate the importance of including factors pertaining to diet and prey capture in the analyses of predator venom evolution along with factors relating to measuring the toxicity and functionality of venom. By investigating and comparing the multitude of factors potentially influencing venom evolution in future research, especially those pertaining to the natural targets of species venoms, we will gain a deeper understanding of the processes surrounding toxin evolution and why some species venoms are deadlier than others.