Worm-like amphibians known as caecilians may have lost their legs thanks to the same genetic tweaks that snakes have.
An analysis of the amphibians鈥� genomes suggests that caecilians and snakes 鈥� separated by hundreds of millions of years of evolution 鈥� converged on the same mutations when ditching their limbs. The findings provide insights into the genetic underpinnings of the evolution of limblessness.
The 200-some species of caecilian are totally limbless, and live secretive lives in tropical soils or waterways. They have small or nonexistent eyes and can sometimes resemble large, brightly coloured earthworms. At least one species may have a venomous bite, and some feed their own skin to their offspring.
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Being the earliest group to branch off the amphibian tree of life, caecilians are key to understanding vertebrate evolution, says Mary O鈥機onnell at the University of Nottingham in the UK. To learn more about these poorly known amphibians, O鈥機onnell and her colleagues sequenced and assembled the full genomes of two caecilian species 鈥� the Gaboon caecilian (Geotrypetes seraphini) and the tiny Cayenne caecilian (Microcaecilia unicolor) 鈥� adding to the single previously published caecilian genome.
When analysing all three caecilian genomes and comparing them with the genomes of 22 other vertebrates, the team found 1150 gene groups that were unique to caecilians. They seem to be related to chemical sensing and smell, traits that were probably important as caecilians first adapted to a dark, subterranean life.
The researchers also found that a key enhancer 鈥� a genetic sequence that can boost the activity of another gene 鈥� was absent in caecilians: the ZRS enhancer element. The ZRS is crucial for the growth of legs in vertebrates. For this reason, ZRS is ubiquitous and largely unaltered across vertebrate life. But snakes have a mutant form of ZRS, and when mice are engineered with a mutant ZRS, they develop with severely stunted limbs.
That the caecilian ZRS is either missing or so mutated that it is no longer recognisable suggests that the amphibians and snakes independently lost their legs through the same genetic route.
Independently evolved physical traits are pretty common, says O鈥機onnell. 鈥淭hink about the body plans for sharks and dolphins, for example,鈥� she says. 鈥淏ut having the same genomic element implicated is far less common.鈥�
鈥淲hat is surprising about this is that snakes and caecilians are only very distantly related, sharing a common ancestor almost 300 million years ago,鈥� says at Clark University in Massachusetts. 鈥淚t is often thought that parallelism 鈥� the evolution of convergent traits like loss of limbs via the same genetic mechanism 鈥� happens primarily in closely related [species]. This is a great counter example.鈥�
But, O鈥機onnell cautions, it is also possible that caecilians lost their legs first through a different genetic process, and their ZRS waned out of existence since there was no selective pressure to keep it functioning. Some legless lizards still have their , so there are alternative genetic paths to leaving legs behind.
Bergmann says it would be interesting to see if other ancient legless vertebrates 鈥� like worm lizards and blind skinks 鈥� have mutations in their ZRS enhancer.
bioRxiv
