|
Snake
skeletons are just as regionalized as lizards, despite loss of
limbs and increase in number of vertebrae. Photo Credit: Craig Chandler,
Angie Fox, Jason
Head, University of Nebraska-Lincoln
|
Sakes may not have shoulders, but their bodies
aren't as simple as commonly thought, according to a new study that could
change how scientists think snakes evolved.
Paleobiologists Jason Head of University of Nebraska-Lincoln and
P. David Polly of Indiana University Bloomington found distinctions among
snakes' vertebral bones that matched those found in the backbones of four-legged
lizards.
Rather than snakes evolving from a lizard ancestor to a more
simplified body form, the researchers say their findings suggest other animals
gained more complex vertebral columns as they evolved.
The study provides new perspective on Hox genes,
which govern the boundaries of the neck, trunk, lumbar, sacral and tail regions
of limbed animals. The functions of Hox genes previously were
thought to have been disrupted in snakes, resulting in seemingly simplified
body forms.
Snakes differ from mammals, birds and most other reptiles
because they lack forelimbs, shoulder girdles and breastbones. It was thought
that when they lost their limbs, they also lost the regional distinctions that
separated their backbones into neck, trunk, lumbar and other regions.
Yet when Head and Polly examined the shapes of individual
vertebral bones in snakes, lizards, alligators and mice, they found snakes had
regional differentiation like that of lizards.
"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," Head said.
"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 Polly 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, rib-less regions like the neck and lumbar
spine of mice, they control more subtle, graded changes in shape," Head
said.
When combined with information from fossils, these findings
indicate that the direction of snake evolution is the opposite of what had been
concluded from developmental genetics alone, Head and Polly say.
"Our findings turn the sequence of evolutionary events on
its head," Polly said. "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."
Amniotes are the group of vertebrates that lay shelled eggs.
They include reptiles, mammals and their predecessors.
"Snakes have a lot more vertebrae compared to lizards and
they have lost the shoulder girdle, but they are just as regionalized,"
Polly said.
Head and Polly reached 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.
"Analysis of gene functions are necessary, but not
sufficient in studying evolutionary transitions," Head concludes. "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."
Citation
Jason J. Head, P. David Polly. Evolution of the snake body form reveals
homoplasy in amniote Hox gene function. Nature, 2015; DOI:10.1038/nature14042