Isolation of a Neurotoxin (?-colubritoxin) from a Nonvenomous Colubrid: Evidence for Early Origin of Venom in Snakes

Abstract

The evolution of venom in advanced snakes has been a focus of long-standing interest. Here we provide the first complete amino acid sequence of a colubrid toxin, which we have called alpha-colubritoxin, isolated from the Asian ratsnake Coelognathus radiatus (formerly known as Elaphe radiata), an archetypal nonvenomous snake as sold in pet stores. This potent postsynaptic neurotoxin displays readily reversible, competitive antagonism at the nicotinic receptor. The toxin is homologous with, and phylogenetically rooted within, the three-finger toxins, previously thought unique to elapids, suggesting that this toxin family was recruited into the chemical arsenal of advanced snakes early in their evolutionary history. LC-MS analysis of venoms from most other advanced snake lineages revealed the widespread presence of components of the same molecular weight class, suggesting the ubiquity of three-finger toxins across advanced snakes, with the exclusion of Viperidae. These results support the role of venom as a key evolutionary innovation in the early diversification of advanced snakes and provide evidence that forces a fundamental rethink of the very concept of nonvenomous snake.

Experimental ophitoxemia produced by the opisthoglyphous lora snake (Philodryas olfersii) venom

“…Nevertheless, neurotoxic activity in venoms is most reliable in the role of killing or debilitating a victim, rather than for digestion or some other purpose 9 .…”

“…Many authors defined this venom as a Duvernoy´s gland secretion, but this term no longer applies to the colubrid families, as the glands were shown to be the same venom gland found in Atractaspididae, Elapidae and Viperidae 9 .…”

Dynamic evolution of venom proteins in squamate reptiles

“…15) 36 ). Each data set was run in duplicate using four chains simultaneously (three heated and one cold) for 5×10 6 generations, sampling every 500th cycle from the chain and using default settings in regards to priors. Tracer v1.4 (ref.…”

“…The homology of the venom apparatus across the advanced snakes (Caenophidia) is robustly supported by anatomical evidence [1][2][3][4] , as well as comparative embryology and developmental genetics 5 . A recent addition to the body of evidence supporting the single early evolution of venom in snakes has been the use of protein amino acid or DNA gene sequences from toxins, and their homologues among non-venom body proteins 6,7 . Venom toxins belong to multiple multi-locus gene families, evolving according to the birth and death model 8 , and first acquired their role within venom following recruitment from protein families fulfilling ordinary physiological functions 9 .…”

Evolution of an Arsenal

“…It has only become evident recently that venom in snakes is a basal characteristic and that the three front-fanged venom delivery system architectures are each independent secondary derivations (1)(2)(3)(4). In earlier schemes, the "colubrid," or "non-front-fanged," snakes were seen as a monophyletic transitional group to the presumably advanced front-fanged lineages Atractaspis, Elapidae, and Viperidae (e.g.…”