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From offense to defense
The venom in snakes is widely used to subdue and / or kill prey, and most venoms have clear actions that facilitate death or paralysis. In a group of snakes, however, the venom evolved from predation to protection. Specifically, in three different lineages of “spitting” snakes, the venom is used to deter predators. Kazanjian et al. show that similar adaptations have occurred within these lines which transform cytotoxic components into a mixture which acts on mammalian sensory neurons and causes pain. The authors argue that increased predation on these lineages led to similar changes in venom function.
Science, this issue p. 386
Abstract
Convergent Evolution provides insight into the selective factors underlying evolutionary change. Snake venoms, with a direct genetic basis and a clearly defined functional phenotype, provide a model system for exploring the repeated evolution of adaptations. While snakes use the venom primarily for predation, and the composition of the venom often reflects the specifics of the diet, three lines of cobras have independently developed the ability to spit venom at opponents. Using genetic, protein and functional analyzes, we show that the three spitting lineages possess venoms characterized by upregulation of phospholipase A2 (PLA2) toxins, which potentiate the action of pre-existing venom cytotoxins to activate mammalian sensory neurons and cause increased pain. These repeated independent changes provide a fascinating example of convergent evolution across multiple phenotypic levels driven by selection for defense.
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