Snake’s Genome Has Design for Limbs

Earlier this year, researchers David M. Martill, Helmut Tischlinger and Nicholas R. Longrich published a paperon a four-legged snake fossil dating from the early Cretaceous period.
Found in Brazil’s Crato Formation,Tetrapodophis’ skeleton displayed the sinuous body found in modern day snakes, but small hinged limbs jutted from its sides.

The find supported the evolutionary theory snakes evolved from burrowing lizards, eventually elongating their bodies and losing their limbs to aptly maneuver through the ground.

The switch to a limbless body is believed to have occurred over 100 million years ago.

“Some older lineages of living snake species, such as members of the Booidea (for example, boas and pythons), retain a highly reduced hindlimb, but most other modern snakes lack evidence of any forelimb or hindlimb development,” write Univ. of Georgia researchers in a paper appearing in Developmental Cell.

Investigating the genome of three different snake species, the researchers found snakes still possess the genetic blueprint for limb formation. However, the area of the genetic code is primarily responsible for the formation of external genitalia.

“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 senior author Douglas Menke, an assistant professor at Univ. of Georgia.

While the phallus differs from limbs in both form and function, their developments share certain features, the researchers write.

“Both limbs and external genitalia form through the establishment of tissue outgrowths from the main body axis during embryogenesis, and both possess regional signaling centers that direct growth and patterning,” they write.

Regions of noncoding DNA known as enhancers were observed in embryonic limbs and genitalia of mice and lizards.

According to the researchers, many of the same enhancers are activated during these different appendage formations.

Next, the team engineered mice lacking one of the enhancers, and found limb and genitalia development were stunted.

“Much of the genetic circuitry that control the development of limbs is also important for the formation of genitalia,” said Menke. “We think that’s why snakes still have the genetic blueprints for limb development in their genome.”

“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,” he added.

The researchers used the genomes of a boa constrictor, Burmese python and king cobra in their experiments.

Source : R & D


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