Snakes represent a diverse reptile lineage, with the evolutionary innovation of venom allowing them to exploit and thrive in different ecological niches. Snake venom has frequently been proposed to be highly complex, having evolved a single time at the base of squamate reptiles with venom genes being "recruited" from multiple body tissues.
However, the genetic mechanisms behind these processes and those underpinning the regulation of venom gene expression are poorly understood. Therefore, 2nd generation sequencing was utilised in order to investigate and evaluate these processes. Several methods were first used in order to assess the optimal methodology for genome and transcriptome assembly. Comparative transcriptomic analyses revealed that venom is likely to be a simple mixture containing products from a few gene families. The Toxicofera hypothesis, proposing a single early origin of venom in reptiles, was found to be
unsupported in a number of regards. Evaluation of venom gene recruitment revealed that this hypothesis was never supported originally, and newly generated data suggests that venom genes are ancestrally expressed in multiple tissues, including the salivary gland of non-venomous reptiles. Venom genes likely arise through restriction of their expression to the venom gland following gene duplication, and venom can be considered to simply be
a modified version of saliva. The transcription factors (TFs) and signalling pathways in the venom and salivary glands are highly similar, with only one transcription factor found in the venom gland but not the salivary gland. Utilising a comparative genomics approach it was found that transcription factor binding sites are conserved between members of the same venom gene families, but not between families, suggesting multiple gene regulatory networks are behind snake venom production. Temporal variation in venom gene expression also appears to support this hypothesis. In short, snake venom has evolved via much simpler processes than previously thought.