Among extant reptiles only two lineages are known to have evolved venom delivery systems, the advanced snakes and helodermatid lizards (Gila Monster and Beaded Lizard)1. Evolution of the venom system is thought to underlie the impressive radiation of the advanced snakes (2,500 of 3,000 snake species)2,3,4,5. In contrast, the lizard venom system is thought to be restricted to just two species and to have evolved independently from the snake venom system1. Here we report the presence of venom toxins in two additional lizard lineages (Monitor Lizards and Iguania) and show that all lineages possessing toxin-secreting oral glands form a clade, demonstrating a single early origin of the venom system in lizards and snakes. Construction of gland complementary-DNA libraries and phylogenetic analysis of transcripts revealed that nine toxin types are shared between lizards and snakes. Toxinological analyses of venom components from the Lace Monitor Varanus varius showed potent effects on blood pressure and clotting ability, bioactivities associated with a rapid loss of consciousness and extensive bleeding in prey. The iguanian lizard Pogona barbata retains characteristics of the ancestral venom system, namely serial, lobular non-compound venom-secreting glands on both the upper and lower jaws, whereas the advanced snakes and anguimorph lizards (including Monitor Lizards, Gila Monster and Beaded Lizard) have more derived venom systems characterized by the loss of the mandibular (lower) or maxillary (upper) glands. Demonstration that the snakes, iguanians and anguimorphs form a single clade provides overwhelming support for a single, early origin of the venom system in lizards and snakes. These results provide new insights into the evolution of the venom system in squamate reptiles and open new avenues for biomedical research and drug design using hitherto unexplored venom proteins.
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We thank the following persons and institutions who helped us or contributed tissue samples used in this study: A. Fry, Alice Springs Reptile Centre, Australian Reptile Park, M. A. G. de Bakker, R. L. Bezy, B. Branch, J. Campbell, N. Clemann, C. Clemente, C. Cicero, K. Daoues, A. S. Delmas, B. Demeter, J. Haberfield, A. Hassanin, Healesville Sanctuary, M. Hird, Louisiana State University Museum of Zoology, P. Moler, T. Moncuit, P. Moret, National Museum of Natural History Naturalis Leiden (J. W. Arntzen), T. Pappenfus, J.-C. Rage, C. Skliris, J. Smith, S. Sweet, Ultimate Reptiles (South Australia), University of California Museum of Vertebrate Zoology (Berkeley), J. Walker, R. Waters, J. Weigel and B. Wilson. We also thank A. Webb and T. Purcell for providing HPLC access; N. Williamson for help with preliminary mass spectrometry characterization; E. V. Grishin for help in obtaining the references in Russian; S. Edwards for comments; and T. van Wagner and V. Wexler for artwork. This work was funded by the Service de Systématique moléculaire of the Muséum National d'Histoire Naturelle, Institut de Systématique (N.V.) and by grants from the Australian Academy of Science (B.G.F.), Australian Geographic Society (B.G.F.), Australia & Pacific Science Foundation (B.G.F.), Australian Research Council (B.G.F.), CASS Foundation (B.G.F.), Commonwealth of Australia Department of Health and Aging (B.G.F.), Ian Potter Foundation (B.G.F.), International Human Frontiers Science Program Organisation (B.G.F.), Leiden University (F.J.V., M.K.R.), NASA Astrobiology Institute (S.B.H.), National Science Foundation (S.B.H.) and University of Melbourne (B.G.F.). We thank the relevant wildlife departments for granting the scientific permits for field collection of required specimens.
This file contains detailed descriptions of materials and methods not already described in the main article.