Living amphibians include frogs and salamanders and the limbless worm-like caecilians (order Gymnophiona). Caecilians have cylindrical bodies with a compact, bullet-shaped skull that helps them burrow underground. Now exclusively home to South and Central America, Africa, and southern Asia, they spend their lives burrowing in leaf-litter or soil searching for prey such as worms and insects.
Caecilians are a group of limbless, vermiform or serpentine amphibians. Image credit: Marco Mancuso.
“Our research provides a textbook example of how a single predatory pressure can trigger an evolutionary cascade where the same way of fighting back arises independently multiple times in a species’ different lineages,” said University of Queensland’s Dr. Bryan Fry, senior author on the study.
“In this case, the key predatory pressure was the rise of the elapid snakes, such as cobras and coral snakes, characterized by the evolution of a new way of delivering venom via their hollow, fixed, syringe-like fangs.”
“Despite being quite slippery, caecilians are worm-like in their locomotion and speed and were incredibly easy prey to cobras and other snakes, which used their fangs to kill them and eat them later.”
“It would have been absolute carnage to the point where elapids were basically grazing on caecilians, contributing to the rapid spread of elapid snakes across Africa, Asia, and the Americas.”
“The caecilian’s ability to persevere and evolve despite these pressures is like a movie — like the survivors of Judgement Day fighting back by changing the chemical landscape.”
In the study, Dr. Fry and colleagues sequenced a part of the neuromuscular receptor in 37 caecilian species, representing all currently known families of caecilians from across the Americas, Africa, and Asia, including species endemic to the Seychelles islands never reached by elapid snakes.
They showed that resistance to elapid snake venom neurotoxins has evolved on at least 15 times.
“A particularly interesting validation of the theory was that the caecilians on the Seychelles islands were not resistant to snake venom, which is consistent with elapid snakes never reaching those islands,” said first author Marco Mancuso, a researcher at Vrije Universiteit Brussel.
“It’s an extraordinary signal for response to such severe selection pressure, where the survivors of the onslaught were those who were a bit less sensitive to the venom and some had mutations that made them completely immune.”
“These were the ones that repopulated the Earth after the elapid snake plague.”
“Caecilians were able to achieve this never-before-seen venom resistance by deploying three different kinds of biological methods,” Dr. Fry added.
“One kind is putting up a form of barricade that blocks the ability of the toxins to reach receptors that would normally elicit a deadly reaction.”
“A second form of resistance is changing the physical shape of the receptor.”
“As the toxins have evolved to be like keys and insert into the lock-like receptor, changing the shape means the toxin no longer fits.”
“Lastly, caecilians essentially deploy an electromagnetic ‘weapon’ which reverses the charge during this toxin-receptor interaction.”
“The positive-to-positive charge repulsion increases exponentially the closer the objects come together, like trying to force two magnets together.”
“This pocket of the receptor is normally negatively charged, so snake toxins have evolved with a positive charge to help guide the binding.”
“The mutation where the receptor is now positively charged like the toxins, electrostatically repels the toxins.”
“While the results won’t lead to any new direct human benefits, such as new antivenom, the results have the benefit of showcasing an important evolutionary interaction in an engaging way to the next generation of scientists.”
“Animals killing other animals, and the prey evolving to escape the predators, is something that I think is always fascinating to people, especially young people just getting into science.”
The results appear in the International Journal of Molecular Sciences.
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Marco Mancuso et al. 2023. Resistance Is Not Futile: Widespread Convergent Evolution of Resistance to Alpha-Neurotoxic Snake Venoms in Caecilians (Amphibia: Gymnophiona). Int. J. Mol. Sci 24 (14): 11353; doi: 10.3390/ijms241411353
