title: Widespread convergence in toxin resistance by predictable molecular evolution.
abstract: The question about whether evolution is unpredictable and stochastic or intermittently constrained along predictable pathways is the subject of a fundamental debate in biology, in which understanding convergent evolution plays a central role. At the molecular level, documented examples of convergence are rare and limited to occurring within specific taxonomic groups. Here we provide evidence of constrained convergent molecular evolution across the metazoan tree of life. We show that resistance to toxic cardiac glycosides produced by plants and bufonid toads is mediated by similar molecular changes to the sodium-potassium-pump (Na(+)/K(+)-ATPase) in insects, amphibians, reptiles, and mammals. In toad-feeding reptiles, resistance is conferred by two point mutations that have evolved convergently on four occasions, whereas evidence of a molecular reversal back to the susceptible state in varanid lizards migrating to toad-free areas suggests that toxin resistance is maladaptive in the absence of selection. Importantly, resistance in all taxa is mediated by replacements of 2 of the 12 amino acids comprising the Na(+)/K(+)-ATPase H1-H2 extracellular domain that constitutes a core part of the cardiac glycoside binding site. We provide mechanistic insight into the basis of resistance by showing that these alterations perturb the interaction between the cardiac glycoside bufalin and the Na(+)/K(+)-ATPase. Thus, similar selection pressures have resulted in convergent evolution of the same molecular solution across the breadth of the animal kingdom, demonstrating how a scarcity of possible solutions to a selective challenge can lead to highly predictable evolutionary responses.
authors.affiliation: Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, Geelong, VIC 3216, Australia; Faculty of Veterinary Science, University of Sydney, Sydney, NSW 2006, Australia;
authors.forename: Beata
authors.initials: B
authors.lastname: Ujvari
authors.affiliation: Alistair Reid Venom Research Unit, Department of Parasitology, Liverpool School of Tropical Medicine, Liverpool L3 5QA, United Kingdom; nicholas.casewell@lstmed.ac.uk madsen@uow.edu.au.
authors.forename: Nicholas R
authors.initials: NR
authors.lastname: Casewell
authors.affiliation: Department of Ecology, Evolution and Behavior, The Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel;
authors.forename: Kartik
authors.initials: K
authors.lastname: Sunagar
authors.affiliation: Institute of Integrative Biology, University of Liverpool, Liverpool L69 7ZB, United Kingdom;
authors.forename: Kevin
authors.initials: K
authors.lastname: Arbuckle
authors.affiliation: Molecular Ecology and Fisheries Genetics Laboratory, School of Biological Sciences, Bangor University, Bangor LL57 2UW, United Kingdom;
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Wüster
authors.affiliation: School of Biological Sciences, University of Sydney, Sydney, NSW 2006, Australia;
authors.forename: Nathan
authors.initials: N
authors.lastname: Lo
authors.affiliation: Institute for Applied Ecology, University of Canberra, Bruce, ACT 2601, Australia;
authors.forename: Denis
authors.initials: D
authors.lastname: O'Meally
authors.affiliation: Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, Geelong, VIC 3216, Australia;
authors.forename: Christa
authors.initials: C
authors.lastname: Beckmann
authors.affiliation: Institute for Molecular Bioscience, The University of Queensland, St Lucia, QLD 4072, Australia;
authors.forename: Glenn F
authors.initials: GF
authors.lastname: King
authors.affiliation: Institute for Molecular Bioscience, The University of Queensland, St Lucia, QLD 4072, Australia;
authors.forename: Evelyne
authors.initials: E
authors.lastname: Deplazes
authors.affiliation: Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, Geelong, VIC 3216, Australia; School of Molecular Biosciences, University of Sydney, Sydney, NSW 2006, Australia; School of Biological Sciences, University of Wollongong, Wollongong, NSW 2522, Australia nicholas.casewell@lstmed.ac.uk madsen@uow.edu.au.
authors.forename: Thomas
authors.initials: T
authors.lastname: Madsen
mesh_terms: D002018:Bufanolides / Q000737:chemistry / Q000633:toxicity; D002301:Cardiac Glycosides / Q000633:toxicity*; D019143:Evolution, Molecular; D007526:Isoelectric Point; D008969:Molecular Sequence Data; D010802:Phylogeny; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D000254:Sodium-Potassium-Exchanging ATPase / Q000737:chemistry / Q000378:metabolism
chemical_list: D002018:Bufanolides; D002301:Cardiac Glycosides; D000254:Sodium-Potassium-Exchanging ATPase; C022777:bufalin
keywords: bufotoxin cardenolide; constraint; genotype phenotype; ion transporters; parallelism