Restriction and recruitment - gene duplication and the origin and evolution of snake venom toxins

Abstract

Snake venom has been hypothesised to have originated and diversified via a process that involves duplication of genes encoding body proteins with subsequent recruitment of the copy to the venom gland, where natural selection acts to develop or increase toxicity. However, gene duplication is known to be a rare event in vertebrate genomes and the recruitment of duplicated genes to a novel expression domain (neofunctionalisation) is an even rarer process that requires the evolution of novel combinations of transcription factor binding sites in upstream regulatory regions. Therefore, whilst this hypothesis concerning the evolution of snake venom is therefore very unlikely and should be regarded with caution, it is nonetheless often assumed to be established fact, hindering research into the true origins of snake venom toxins. To critically evaluate this hypothesis we have generated transcriptomic data for body tissues and salivary and venom glands from five species of venomous and non-venomous reptiles. Our comparative transcriptomic analysis of these data reveals that snake venom does not evolve via the hypothesised process of duplication and recruitment of genes encoding body proteins. Indeed, our results show that many proposed venom toxins are in fact expressed in a wide variety of body tissues, including the salivary gland of non-venomous reptiles and that these genes have therefore been restricted to the venom gland following duplication, not recruited. Thus snake venom evolves via the duplication and subfunctionalisation of genes encoding existing salivary proteins. These results highlight the danger of the elegant and intuitive ?just-so story? in evolutionary biology.