Snakes on a plain: Chinese-led study shows when the serpents shed their legs

Comprehensive genome survey has determined that the reptiles descended from lizards when dinosaurs roamed the Earth
Researchers who identified factors in limb loss, hope discoveries boost knowledge of biological development, antivenom drugs

Science

Echo Xie

Published: 9:25pm, 22 Jun 2023

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New research has shown that snakes – those creepy coils of countless nightmares – used to look a lot different.

According to the largest genome survey of its kind, snakes used to have legs.

The new study, led by researchers at the Chinese Academy of Sciences, has found that snakes originated during the Early Cretaceous – some 118 million years ago – when dinosaurs dominated the Earth.

The ancestors of snakes are lizards, a group of reptiles that rapidly diversified after a mass extinction event about 65 million years ago – a catastrophe that claimed three-quarters of life on Earth, and which included the extinction of dinosaurs.

Snakes have undergone various evolutionary changes, including limb loss, lengthened bodies, and changes in their asymmetrical lungs and sensory systems, according to the new study, which was published Monday in the peer-reviewed journal Cell.

The authors identified genes, regulatory elements and structural variations that have contributed to evolutionary changes in snakes by analysing 14 genomes from 12 families of snakes, which account for about 84 per cent of all snake species.

Li Jiatang, the corresponding author of the study, and a researcher with the academy’s Chengdu Institute of Biology, said previous studies analysed a specific snake species or family, but his team wanted to capture a more comprehensive picture of snake genomes.

“Moreover, we measured genome-wide data and obtained more genetic information to support our hypothesis. Previous studies have only measured a few genetic segments of a snake species,” Li was quoted as saying in state-run China Science Daily on Tuesday.

Snakes can be found on all continents except for the polar regions. They live on land and sea, and in forests and deserts. There are more than 300 families of snakes consisting of 4,000 species.

“We want to explain how snakes adapt to these complex environments from a genetic point of view,” Li said.

The research project was officially launched in 2018. To obtain fresh samples, the team said it spent two to three months each year in the field to gather samples from deserts in China’s western region of Xinjiang or from highlands in northern China.

Along the way, the researchers identified some key genes that affected the evolutionary changes in snakes.

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One gene – PTCH1 – played an essential role in limb development. The team also investigated how the deletion of this gene affected limb development in mice.

Using CRISPR-Cas9 gene-editing technology, the authors edited a mouse to delete the PTCH1 gene. When it was one week old, the researchers observed that the mouse had significantly lower body mass and shorter bodies than normal mice. Additionally, the digital bones were significantly shorter in the adult PTCH1-deleted mice.

“These results suggest that the deletions of this gene inhibit skeletal growth, including the growth of the limb digits,” the authors wrote.

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The loss of limbs resulted in an elongated body blueprint for snakes, according to the study. The authors also identified the key genes that explain the evolution of those lengthened bodies.

The elongated body was accompanied by changes in internal organs, such as the lungs, the study said.

In modern-day snakes, or extent snakes, the left lung is either absent or much smaller than the right lung. The authors found that a gene that plays a role in left-right balance of development was absent in snakes. Another gene that facilitates lung development, was also lost in snakes.

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The authors said their study may improve understanding of the genetic basis underpinning the evolution of various snakes and “enhance our knowledge of reptile biology and developmental biology”.

Li said his team would carry out research on snake venom to provide scientific support for the development of antivenom drugs.