Snakes are legless creatures, right? Well, embedded in the genetic code of several snake species was DNA that, for most animals, controls the development and growth of limbs, according to a recent University of Georgia (UGA) study.
So what does this mean for our slithering friends? Researchers found that the same genetic tools responsible for limb development also control the formation of external genitalia. This may explain why snakes have held on to this DNA after evolving without limbs over 100 million years ago.
"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," Douglas Menke, an assistant professor of genetics in UGA's Franklin College of Arts and Sciences and senior author of the paper, said in a news release. "Naturally, we wanted to know why snakes had retained DNA that they don't appear to need."
For their study, researchers examined specific regions of noncoding DNA known as enhancers, which control the expression of genes. Essentially, enhancers tell genes when to turn on or off during embryonic development. When following the patterns of enhancers during embryotic development of limbs and genitalia in mice and lizards, the researchers found that many of the same enhancers were activated in both species. However, this enhancer in a snake's genome only functions during development of genitalia.
"What this means is that much of the genetic circuitry that controls the development of limbs is also important for the formation of genitalia," Menke said in a statement. "And we think that's why snakes still have the genetic blueprints for limb development in their genome."
Evolutionary biologists generally believe that limbs evolved from fins. However, the phallus -- external genitalia that includes the penis and clitoris -- is thought to be a much more recent development, Menke added. Also, previous studies suggest that the genes initially used to grow limbs were later co-opted for the development of a phallus.
"We're only just beginning to understand the various roles of many of these enhancers," Menke said in the release. "But what we generally refer to as 'limb enhancers' should probably be more broadly categorized as 'appendage enhancers,' because they clearly perform more than one job."
Their findings were recently published in the journal Developmental Cell.
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