Skeleton turns snake evolution on its head

"Snakes have a lot more vertebrae compared to lizards and they have lost the shoulder girdle, but they are just as regionalized," says P. David Polly. (Credit: Daniel Oines/Flickr)

The evolution of the snake’s elongated, legless body appears to be the exact opposite of what scientists have long assumed, new research suggests.

Hox genes, which establish the boundaries of the neck, trunk, lumbar, sacral and tail regions in birds, lizards, crocodiles, and mammals, were previously thought to have been disrupted in snakes, resulting in a loss of regions in their seemingly simplified body form as they evolved from four-legged lizard ancestors.

snake skeleton
(Credit: John Bridges/Flickr)

For a study published in the journal Nature, researchers examined regional differences in the shapes of individual vertebral bones in snakes, lizards, alligators, and mice.

Snakes and lizards

Snakes are different from these other amniote groups in that they lack forelimbs, shoulder girdles, and sternal skeletons. Snake vertebrae were assumed to have become less regionalized when the limbs were lost.

“If the evolution of the snake body was driven by simplification or loss of Hox genes, we would expect to see fewer regional differences in the shapes of vertebrae,” says Jason Head of the University of Nebraska-Lincoln. “Instead, what we found was the exact opposite.

“Snakes have the same number of regions and in the same places in the vertebral column as limbed lizards.”

Not only did Head and coauthor P. David Polly of Indiana University find that snakes were as differentiated as lizards, but when they compared regions in snakes with Hox gene expression, they found the two matched.

“This suggests that Hox genes are functioning in the evolution and development of the vertebral column in snakes; but instead of patterning distinct, ribless regions like the neck and lumbar spine of mice, they control more subtle, graded changes in shape,” Head says.

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The findings combined with information from fossils indicate that the direction of snake evolution is the opposite of what has been concluded from developmental genetics alone.

“Our findings turn the sequence of evolutionary events on its head,” Polly says. “It isn’t that snakes have lost regions and Hox expression; it is that mammals and birds have independently gained distinct regions by augmenting the ordinary Hox expression shared by early amniotes.

“Snakes have a lot more vertebrae compared to lizards and they have lost the shoulder girdle, but they are just as regionalized.”

The researchers arrived at their conclusions using a method called geometric morphometrics and a regression-based analysis of the size and shape of vertebral structures. To determine where one segment ends and the next begins, they use a statistical method called maximum likelihood estimation.

“Analyses of gene functions are necessary, but not sufficient in studying evolutionary transitions,” Head says. “In order to fully understand the mechanisms by which new body forms evolve, it is crucial to study the anatomy of modern and fossil organisms.”

Source: Indiana University