More complex than thought? From ScienceDaily:
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.”
“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.
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.
Here’s the abstract:
Hox genes regulate regionalization of the axial skeleton in vertebrates1, 2, 3, 4, 5, 6, 7, and changes in their expression have been proposed to be a fundamental mechanism driving the evolution of new body forms8, 9, 10, 11, 12, 13, 14. The origin of the snake-like body form, with its deregionalized pre-cloacal axial skeleton, has been explained as either homogenization of Hox gene expression domains9, or retention of standard vertebrate Hox domains with alteration of downstream expression that suppresses development of distinct regions10, 11, 12, 13. Both models assume a highly regionalized ancestor, but the extent of deregionalization of the primaxial domain (vertebrae, dorsal ribs) of the skeleton in snake-like body forms has never been analysed. Here we combine geometric morphometrics and maximum-likelihood analysis to show that the pre-cloacal primaxial domain of elongate, limb-reduced lizards and snakes is not deregionalized compared with limbed taxa, and that the phylogenetic structure of primaxial morphology in reptiles does not support a loss of regionalization in the evolution of snakes. We demonstrate that morphometric regional boundaries correspond to mapped gene expression domains in snakes, suggesting that their primaxial domain is patterned by a normally functional Hox code. Comparison of primaxial osteology in fossil and modern amniotes with Hox gene distributions within Amniota indicates that a functional, sequentially expressed Hox code patterned a subtle morphological gradient along the anterior–posterior axis in stem members of amniote clades and extant lizards, including snakes. The highly regionalized skeletons of extant archosaurs and mammals result from independent evolution in the Hox code and do not represent ancestral conditions for clades with snake-like body forms. The developmental origin of snakes is best explained by decoupling of the primaxial and abaxial domains and by increases in somite number15, not by changes in the function of primaxial Hox genes9, 10. (paywall)