Phylogenetic analyses of toxin gene families have revolutionised our understanding of the originand evolution of reptile venoms, leading to the current hypothesis that venom evolved once insquamate reptiles. However, because of a lack of homologous squamate non-toxin sequences,these conclusions rely on the implicit assumption that recruitments of protein families intovenom are both rare and irreversible. Here we use sequences of homologous non-toxin proteinsfrom two snake species to test these assumptions. Phylogenetic and ancestral-state analysesrevealed frequent nesting of ‘physiological’ proteins within venom toxin clades, suggestingearly ancestral recruitment into venom followed by reverse recruitment of toxins back tophysiological roles. These results provide evidence that protein recruitment into venoms fromphysiological functions is not a one-way process, but dynamic, with reversal of function and/orco-expression of toxins in different tissues. This requires a major reassessment of our previousunderstanding of how animal venoms evolve.
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