Scientists discovered the genomes of a number of snake species contained DNA that controls the development of limbs in most animals, even though the creatures are legless and slither on their stomachs.
Snakes are believed to have lost their legs about 100 million years ago. To explain why snakes still held onto the gene circuitry for limbs, a team of researchers demonstrated that the same genetic tool responsible for limb development in most animals also drives the development of genitalia, the University of Georgia reported.
"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," said Douglas Menke, an assistant professor of genetics in UGA's Franklin College of Arts and Sciences . "Naturally, we wanted to know why snakes had retained DNA that they don't appear to need."
To make their findings, the researchers looked at regions of noncoding DNA known as enhancers, which work to control expression in genes. They pinpointed patterns of enhancer activity in embryonic limbs and genitalia of mice and lizards. They noticed many of these same enhancers appeared to be activated during the formation of appendages in both species. The scientists engineered mice that did not possess one of these limb-genital enhancers, and noticed defects in both their limbs and 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. "And we think that's why snakes still have the genetic blueprints for limb development in their genome."
Scientists have generally believed limbs originally evolved from ancient fins, but external genitalia has been thought of as a more recent development. Research suggests genes originally used to grow limbs later co-opted to include the development of the phallus.
"We're only just beginning to understand the various roles of many of these enhancers," Menke said. "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."
The findings were reported in a recent edition of the journal Developmental Cell.
