Early evolution of the venom system in lizards and snakes.

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). Evolution of the venom system is thought to underlie the impressive radiation of the advanced snakes (2,500 of 3,000 snake species). In contrast, the lizard venom system is thought to be restricted to just two species and to have evolved independently from the snake venom system. 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.

Evolution of an arsenal: structural and functional diversification of the venom system in the advanced snakes (Caenophidia).

Venom is a key innovation underlying the evolution of advanced snakes (Caenophidia). Despite this, very little is known about venom system structural diversification, toxin recruitment event timings, or toxin molecular evolution. A multidisciplinary approach was used to examine the diversification of the venom system and associated toxins across the full range of the approximately 100 million-year-old advanced snake clade with a particular emphasis upon families that have not secondarily evolved a front-fanged venom system ( approximately 80% of the 2500 species). Analysis of cDNA libraries revealed complex venom transcriptomes containing multiple toxin types including three finger toxins, cobra venom factor, cysteine-rich secretory protein, hyaluronidase, kallikrein, kunitz, lectin, matrix metalloprotease, phospholipase A(2), snake venom metalloprotease/a disintegrin and metalloprotease, and waprin. High levels of sequence diversity were observed, including mutations in structural and functional residues, changes in cysteine spacing, and major deletions/truncations. Morphological analysis comprising gross dissection, histology, and magnetic resonance imaging also demonstrated extensive modification of the venom system architecture in non-front-fanged snakes in contrast to the conserved structure of the venom system within the independently evolved front-fanged elapid or viperid snakes. Further, a reduction in the size and complexity of the venom system was observed in species in which constriction has been secondarily evolved as the preferred method of prey capture or dietary preference has switched from live prey to eggs or to slugs/snails. Investigation of the timing of toxin recruitment events across the entire advanced snake radiation indicates that the evolution of advanced venom systems in three front-fanged lineages is associated with recruitment of new toxin types or explosive diversification of existing toxin types. These results support the role of venom as a key evolutionary innovation in the diversification of advanced snakes and identify a potential role for non-front-fanged venom toxins as a rich source for lead compounds for drug design and development.