While previous studies suggest snake venom differences are largely driven by variation in diet, a new analysis in three lineages of "spitting cobra" points to a different mechanism - the need to defend - as the driver of these pain-inducing venoms' unique traits. "Spitting cobras highlight how similar selection pressures...can drive convergence at molecular, morphological, behavioral, and functional levels," say the authors. For the vast majority of snakes, venom is primarily used for predation - to disable or dispatch a would-be meal. The evolution of venom "spitting" in cobras, by contrast, plays no role in prey capture. Rather, it targets specific sensory tissues and is the only long-distance, injurious defensive adaptation among almost 4,000 species of snakes. Unexpectedly, three lineages of venomous snakes in the family Elapidae have independently evolved the ability to "spit" their venom over several meters and into the eyes of a perceived threat, causing pain as a means of defense. These few spitting snakes offer an ideal system for exploring the selective drivers underlying the repeated evolution of this similar, yet rare, defensive trait. Leveraging transcriptomic, proteomic and functional analysis approaches, Taline Kazandjian and colleagues evaluated the evolution of defensive venom spitting in the three spitting cobra groups. While each lineage exhibits its own distinct venom composition, Kazandjian et al. found that all spitting cobra venoms are more effective in causing pain than their non-spitting counterparts. The authors show that similar molecular adaptions occurred within these lineages that enhance the action of pre-existing venom cytotoxins to activate mammalian sensory neurons and cause immediate pain to deter an aggressor. This, say the authors, points to a role for defense as a driver for snake venom evolution in these lineages.
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Science