Endless forms most beautiful: the dynamic diversification of snake venom
24th February 2017
Speaker: Byran Fry
Location: 10:30 am to 11:30 am in Building 6, Level C, Room 12
Snake venom is subjected to a myriad of evolutionary selection pressures. The classic interaction is of a predator-prey chemical arms race, with prey specificity being the driving force shaping the venom. We have shown for example that neonate Australian brown snakes (Pseudonaja species) specialise on lizards and have neurotoxic venoms which subjugate prey through the induction of flaccid paralysis brought about by blockage of post-synaptic nicotinic acetylcholine receptors. In contrast, adult brown snakes and taipans (Oxyuranus species) of any age specialise upon mammals and have procoagulant venoms that rapidly subjugate prey through the induction of stroke brought about by activation of thrombin. We have recently discovered that the long-glanded blue coral snake (Calliophis bivirgatus) has a novel form of neurotoxicity not seen before: activation of sodium channels to produce extremely fast developing muscle spasms. These snakes, which have venom glands extending over 20% the length of their body (the longest in the world), specialise upon other snakes, including venomous snakes such as kraits (Bungarus species) and young king cobras (Ophiophagus hannah). Prey species which in turn are specialists upon other snakes and thus capable of lethal retaliatory envenomations.
The use of venom for defensive purposes is less investigated but no less nuanced. Defensive spitting evolved twice within the cobras (Naja species), once within Africa and again within Asia. While it is conventional wisdom that spitting cobras have extremely cytotoxic venoms capable of profound local tissue destruction, the evolution of this unique defensive chemical cocktail has remained enigmatic. We have revealed that cytotoxicity did not emerge parallel to the spitting behaviour but preceded it. Defensive cytotoxins are instead linked to the evolution of hooding defensive displays. In African cobras, the relative degree of cytotoxicity indeed increased parallel to that subsequent evolution of spitting. However in the Asian cobras, there was an increase in cytotoxicity that preceeded spitting, instead increasing with the secondary evolution of ornate hood markings not found in African cobras. Other African/Asian elapids lack cytotoxic venoms except for the king cobra (Ophiophagus hannah) which in addition to evolving a hood independently of the Naja cobras, has also convergently evolved venom which contains defensive cytotoxins.