Snakes: The Evolution of their Venomous Bite

A new study details the evolution of snakes and how they came to boast a venomous bite, challenging the conventional view of how the species got their toxic label.

Scientists believe that if oral glands exhibit some of the 20 gene families associated with "venom toxins, then that species can be classified as venomous. But a new model developed by a team at the University of Texas at Arlington suggests that the expression of genes related to venom toxins in oral glands alone isn't enough to definitively say that a snake or lizard is venomous.

That is based on an in-depth analysis comparing groups of related genes, or "gene families," in tissue from different parts of the Burmese python (Python molurus bivittatus). The results found similar levels of these so-called toxic gene families in python tissue from the python brain, liver, stomach and several other organs, in addition to their oral glands.

"Research on venom is widespread because of its obvious importance to treating and understanding snakebite, as well as the potential of venoms to be used as drugs, but, up until now, everything was focused in the venom gland, where venom is produced before it is injected," lead researcher Todd Castoe said in a statement. "There was no examination of what's happening in other parts of the snake's body. This is the first study to have used the genome to look at the rest of that picture."

Researchers hope that by better understanding the origin of venom could lead to improved anti-venom treatments as well as shed light on gene evolution in humans.

Castoe and his colleagues looked at the 24 gene families associated with venom that are shared by pythons, cobras, rattlesnakes and Gila monsters. Popular opinion among scientists is that a core venom system developed at one point in the evolution of snakes and lizards - referred to as the Toxicofera - and that the evolution of highly venomous snakes came afterward. However this fails to explain why just 24 genes out of a whopping 25,000 evolved to make highly toxic venom-encoding genes.

For example, in pythons venom gene families are "expressed at lower levels overall, expressed at moderate-high levels in fewer tissues and show among the highest variation in expression level across tissues," Castoe explained.

This means that over the course of evolution venom genes were expressed based on their intensity and where they first were turned on in the body. Such insight will help researchers gain a different perspective on these venomous creatures.

The findings are described in further detail in the journal Molecular Biology and Evolution.  

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