How The Scorpion Got His Sting: Divergent Evolution Explains How Non-Toxic Protein Became A Deadly Venom

By Ajit Jha on January 14, 2014 8:00 PM EST

Scorpion stinger
At some point, an ancestor of the scorpion realized that non-toxic proteins could become a deadly defense mechanism. (Photo: Shutterstock)

Scientists have been trying to unravel the evolutionary history of venoms in invertebrate species, like scorpions, centipedes, spiders, and more. All they know is that non-toxin proteins — called "defensins" in the literature — eventually developed into a toxin. These proteins, found in both plants and animals, help them defend against bacterial, viral and fungal pests. In course of time, these proteins evolved into venoms to attack their prey. 

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In a recent study, published in the advanced online edition of Molecular Biology and Evolution, researches have attempted to address how defensins developed into venoms. More specifically, they focused on venom-derived neurotoxins, known as αKTxs. The study has offered the first evidence of an evolutionary link between insect defensins and scorpion αKTxs. As it turns out, the deadly sting of scorpions comes from a small genetic mutation leading to a change in protein function.

The team analyzed the αKTxs in an attempt to locate neurotoxin structure and function, which they called "Scorpion Toxin Signature" (abbreviated as STS). They looked specifically for insect defensins with STS to unlock molecular mechanisms that went into the STS evolution. The researchers found that a single genetic deletion event or mutation is responsible for conversion of defensin into the scorpion αKTxlike neurotoxin.  

The most important findings of this study, according to lead author Dr. Shunyi Zhu are the "the predictability of scorpion toxicity evolution." This work, according to Zhu, is an excellent example of divergent evolution. The most famous case of this type of evolution is Darwin's finches — a group of about fifteen different species of birds, which had a common ancestor, but formed separate groups. Because each group had a different food source, the different groups evolved different beaks, eventually becoming so different that they could be considered different species.  

In this case, the researchers demonstrated that a change in structure led to change in function. While the structural change occurred in an ancestral scaffold, the consequent functional change occurred in proteins. The proteins earlier role was to fight against microbes, which eventually changed to attacking prey. 

Scorpion image via Shutterstock. 

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