Limb Blueprint Exists in Snake Genome; Surprising Finding Revealed by Enhancer Sequence Analysis in Boa Constrictor, Burmese Python, and King Cobra

When researchers at the University of Georgia (UGA) examined the genome of several different snake species, they found something surprising. Embedded in reptiles' genetic code was DNA that, in most animals, controls the development and growth of limbs--a strange feature for creatures that are famous for their long, legless bodies and distinctive slither. Now, they've found an explanation. In a paper published online on October 1, 2015 in the journal Developmental Cell, the scientists show that the same genetic tools responsible for limb development also control the formation of external genitalia, and that may help explain why snakes have held on to this limb circuitry through the ages. The article is titled “Shared Enhancer Activity in the Limbs and Phallus and Functional Divergence of a Limb-Genital cis-Regulatory Element in Snakes.” Snakes weren't always legless; they evolved the loss of limbs over 100 million years ago, said Douglas Menke, an Assistant Professor of Genetics in UGA's Franklin College of Arts and Sciences and senior author of the Developmental Cell paper. "There have been many millions of snake generations since they evolved a legless body, and we would generally expect the DNA associated with limb development to fade away or mutate to do another job, but that doesn't seem to have happened," he said. "Naturally, we wanted to know why snakes had retained DNA that they don't appear to need." In their experiments, Dr. Menke and postdoctoral researcher Dr. Carlos Infante examined specific regions of non-coding DNA known as enhancers--a kind of switch that controls the expression of genes, telling them when to turn on or off during embryonic development. The researchers followed patterns of enhancer activity in embryonic limbs and genitalia of mice and lizards.
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