Background to the spitting cobra project

Venomous snakes are limbless animals and yet despite this apparent impediment are highly successful predators. To achieve this they have evolved extraordinary biological adaptations of dentition (hollow fangs evolved from teeth to efficiently deliver venom into prey), salivary glands (developed into venom glands). And non-toxic physiological proteins normally expressed in other tissues were (i) recruited for expression in the venom glands and (ii) modified to generate venom that causes rapid, catastrophic cardiovascular or neurological immobilisation of their prey. It is thought that predation was the evolutionary driving force of these substantial gene, protein and anatomical changes.

Throughout biological history the effectiveness of systems to defend against aggression/predation is also an important determinant of evolutionary success or failure. Our study seeks to understand whether defense has also played an important role in determining the evolution of snakes.

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 Distinct snake lineages have evolved different defensive strategies, including audible and visual ‘danger signals’, such as the rattle of a rattlesnake or the hood of a cobra. A few venomous snakes (‘spitting’ cobras), the subjects of our study, have evolved specially-adapted dentition and associated behavioral adaptations to spit venom into the eyes of potential predators/attackers as a novel predator-defence mechanism (Figure 1).

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Venom ‘spitting’ is restricted to some cobra (Naja) and closely related rinkhals (Hemachatus) species. Our preliminary data indicates that ‘venom spitting’ evolved independently on three occasions and necessitated a narrowing of the fang aperture. These convergent adaptations to dentition, enabling cobras/rinkhals to spit venom, are presumably a direct consequence of defensive selection pressures - and question the consensus view. Venom ‘spat’ into eyes of predators/attackers is an effective defense because it causes immediate, severe blindness or distraction without requiring physical contact with the adversary, thus permitting escape. Biological logic suggests that venoms causing increased inflammation/irritation to the eyes of predators offer greater defense from predation/aggression than those that do not. Given the specific defensive adaptations of fang morphology and behavior in ‘spitting’ cobras and rinkhals (Figure 1), we (i) hypothesise that defensive selection pressures also constitute an important additional driving force on the evolution of venom composition in these snakes, and (ii) that these will manifest as distinctions in gene sequence and venom protein-function between ‘spitting’ and non-’spitting’ cobras.
 
With Leverhulme Trust funding, we can now define the transcriptomic and proteomic composition of, and severity of ocular pathology induced by, venoms from multiple lineages of ‘spitting’ and non-’spitting’ cobras/rinkhals. The three independently replicated origins of ‘spitting’ provide an ideal model system to test the hypothesis that defensive selection pressures are an evolutionary driving force dictating the toxin composition of these snake venoms.

Dr Robert Harrison
Liverpool School of Tropical Medicine

Robert was awarded a Research Project Grant in June 2012.