Reptile Venom Evolution


The diversification of the reptile venom system has been an area of major research but of great controversy. Due to significant differences in anatomy of the venom delivery system and distant phylogenetic relatedness, it had been long assumed that the venom systems of snakes and lizards were independently evolved. However, my research has revealed that both lineages are members of a clade (Toxicofera) that also includes several lineages of other lizards also shown to be venomous. In contrast to the hypothesis of independent origins, this new perspective revealed that lizard and snake venom systems are homologous but highly differentiated descendants of an early-evolved venom system in squamates which possessed incipient glands on both the mandibular and maxillary regions; with snakes favouring the maxillary venom glands and secondarily losing the mandibular ones, while the anguimorph lizards (anguiids, helodermatids and varanids) did the reverse, resulting in the modern condition seen today. The lizard venom delivery system is less sophisticated than the high-pressure injection mechanism of the front-fanged advanced snakes, and the vast majority of the species pose trivial direct medical risks to human (with the exception of helodermatids and large varanids such as the Komodo Dragon [Varanus komodoensis]). However, the effects of envenomation from medically important species such as Heloderma (for example) may be clinically complex with symptoms including extreme pain, acute local swelling, nausea, fever, faintness, myocardial infarction, tachycardia, hypotension and inhibition of blood coagulation. The Iguania lizards retain the incipient arrangement but it appears to have little ecological relevance.

Mapping the timing of toxin recruitment events over the toxicoferan organismal evolutionary tree demonstrated that a core set of venom genes evolved in the common venomous ancestor shared between venomous lizards and all snakes and subsequent toxin recruitment events facilitated diversification into the more complex venoms seen today.

Fry BG, Wroe S, Teeuwisse W, van Osch MJP, Moreno K, Ingle J, McHenry C, Ferrara T, Clausen P, Scheib H, Winter KL, Greisman L, Roelants K, van der Weerd L, Clemente CJ, Giannakis E, Hodgson WC, Luz S, Martelli P, Krishnasamy K, Kochva E, Kwok HF, Scanlon D, Karas J, Citron DM, Goldstein EJC, Mcnaughtan JE, Norman JA (2009) A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) prisca. Proceedings of the National Academy of Science 106(22):8969-8974.

Fry BG, Vidal N, Van der Weerd L, Kochva E, Renjifo C. (2009) Evolution and diversification of the Toxicofera reptile venom system. Journal of Proteomics 72:127-136

Fry BG, Vidal N, Norman JA, Vonk FJ, Scheib H, Ramjan R, Kuruppu S, Fung K, Hedges SB, Richardson MK, Hodgson WC, Ignjatovic V, Summerhayes R and Kochva E (2006) Early evolution of the venom system in lizards and snakes. Nature 439(7076):509-632; Advance online publication November 17, 2005 doi:10.1038/nature04328.  Cited by Faculty of 1000.