Adaptive Changes in Lampyridae Illuminate Evolutionary Trends: A Study of Toxin Resistance in Sodium-Potassium Pumps

Abstract

Two Lampyridae species, Photinus and Photuris, have evolved the ability to safely produce and consume, respectively, lucibufagin, a cardiac glycoside toxin which targets the alpha subunit (ATPα) of sodium-potassium protein pumps. Photuris, the consumers of exogenous lucibufagin, have accumulated adaptive mutations in their ATPα to resist cardiac glycoside binding. The path of Photuris ATPα’s evolution has been shaped by negative pleiotropic constraints and gene duplication events. Previous studies have examined how these constraints and forces direct the speed and orientation of evolution, but debate continues over early models of gene duplicate maintenance and the significance of negative pleiotropy. I examined the amino acid sequences, cardiac glycoside binding affinities, and tissue-specific expression patterns of Photuris’ ATPα gene copies to better parse the constraints and forces guiding adaptive evolution. My findings show that Photuris have employed many of the same molecular strategies to evolve lucibufagin resistance as other insects which also safely consume cardiac glycosides (Zhen et al., 2012). This parallelism across ecological systems suggests that there are general rules governing adaptive evolution. Independent of ecological context, it seems that gene duplication eases constraints imposed by negative pleiotropy to allow for fixation of nonsynonymous mutations. In addition to providing insight into evolutionary dynamics, my findings have implications for treatment of human diseases involving the sodium-potassium pump, including Digitalis therapy for Congestive Heart Failure and several neural disorders.