Toxicofera

Phylogenetic of the venomous reptiles is a broadly debated. Many different classifications previously presented used morphological features to define clades. The Toxicofera theory was born after genetic testing of the Squamata order showed serious discrepancies between this morphological grouping and their genetic relationships (Vidal & Hedges. 2005). Further testing of this group gave greater support for the new classification of toxicofera based on the genetic similarities, primarily the presence of venom coding genes within three of the Squamata groupings (Fry et al. 2009a). 

The extant taxa forming this clade includes all Ophidia (snakes), Anguimorpha (lizards including monitors) and the Iguanians (lizards including dragons, chameleons and of course iguanas). All the known venomous reptiles belong to this group, however some families in the group are not venomous. It is thought that the non-venomous members have simply lost the venom production ability to some degree (Fry et al. 2009a).

New evidence is constantly being uncovered to support this clade. A study on anguimorphs showed a toxin homologous with those of snakes and functional venom glands have been discovered in the jaw of Komodo dragons Varanus komodoensis dispelling the previously held belief that it was only bacteria that forms the toxic bite by these gargantuous lizard (Fry et al. 2010; Fry et al. 2009b).

The common ancestor to all toxicoferan had a host of core venom genes. Along with other toxin recruitment, these are the original genes which have diversified into the venoms we see in many species today (Fry et al. 2009a).

References 

Fry, BG, Winter, K, Norman, JA, Roelants, K, Nabuurs, RJA, et al. 2010, ‘Functional and structural diversification of the Anguimorpha lizard venom system’, Molecular & Cellular Proteomics, Vol: 9, No: 11, pp. 2369-2390.

Fry, BG, Vidal, N, van der Weerd, L, Kochva, E & Renjifo, C. 2009a, ‘Evolution and diversification of the Toxicofera reptile venom system’, Journal of Proteomics, Vol: 72, No: 2, pp. 127-136.

Fry, BG, Wroe, S, Teeuwisse, W, van Osch, MJP, Moreno, et al. 2009b, ‘A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) priscus’, Proceedings of the National Academy of Sciences of the United States of America, Vol:106, No: 22, pp. 8969-8974.

Vidal, N & Hedges, SB. 2005, ‘The phylogeny of squamate reptiles (lizards, snakes, and amphsbaenians) inferred from nine nuclear protein-coding genes’, Comptes rendus – Biologies, Vol: 328, No: 10, pp. 1000-1008.

Image

http://www.bbc.com/earth/story/20160226-the-islands-where-dragons-are-real, accessed 5/4/2015