A comprehensive analysis of genetic relationships among snakes, lizards and other reptiles has produced a new family tree of these animals, an international research group says.
The new findings are expected to rewrite the evolutionary history of the reptiles, and answer the question when and how some snakes become venomous.
Evolution of the venom system was thought to underlie the impressive radiation of the advanced snakes. In contrast, the lizard venom system is thought to be restricted to just two species and to have evolved independently from the snake.
But in this new study, scientists from Australia, Switzerland and the US said the common ancestor of lizards and snakes already owned a venom system, and when snakes first evolved 100 million years ago, their venom genes were already 100 million years old.
Their new research paper was published in the latest issue of the journal Nature.
The researchers compared nine genes in 19 species of lizards and snakes. Snakes, they found, were closely related to a group of lizards that include iguanas, Komodo dragons, bearded dragons and Gila monsters.
It's only among these lineages that the researchers have discovered venom. More distantly related lizards like geckos and skinks have no venom genes.
"We used to think that venom evolved relatively recently, but this study shows that it evolved very early in the history of these species, about 200 million years ago, when dinosaurs were just getting started," said Blair Hedges, a professor of the Pennsylvania State University and co-author of the paper.
The researchers collected and analysed the largest genetic data set ever assembled for the scaly reptiles known as squamates. The resulting family tree has revealed a number of surprising relationships.
For example, the molecular-genetic evidence shows that the primitive-looking iguanian lizards are close relatives of two of the most advanced lineages, the snakes on the one hand and the monitor lizards and their relatives on the other.
"The iguanian lizard Pogona barbata retains characteristics of the ancestral venom system, namely serial, lobular non-compound venom-secreting glands on both the upper and lower jaws, whereas the advanced snakes and anguimorph lizards have more derived venom systems characterised by the loss of the mandibular (lower) or maxillary (upper) glands," the paper said.
The researchers suggested iguanian lizards, snakes, and the monitor lizards should be classified as a separate group named 'Toxicofera'.
"And the iguanians, a huge group of 1,440 species of primitive-looking lizards, are near the top of our new tree in the Toxicofera group," indicated the researchers. Earlier, biology textbooks placed the iguanians at the base of the reptile family tree.
"These results provide new insights into the evolution of the venom system in squamate reptiles and open new avenues for biomedical research and drug design using hitherto unexplored venom proteins," it added.
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