Sharks Have Genes for Fingers and Toes
for National Geographic News
|August 15, 2007|
The basic process for developing fingers and toes in land animals may have existed for more than 500 million years in shark genes, according to a new study.
Researchers identified genetic activity in spotted catsharks embryos that signal the creation of digits.
The discovery pushes back the date of the evolutionary "fin to limb" advance by some 135 million years.
When a gene—essentially a set of instructions—is translated into a trait, such as red hair or an arm, it is said to be expressed.
Scientists have long believed that the gene for digit development was first expressed some 365 million years ago in the earliest tetrapodsthe first vertebrates to walk on land. (Related: "Ancient Fish Fossil May Rewrite Story of Animal Evolution" [October 18, 2006].)
But the new study suggests the finger-and-toe gene was first expressed much earlier, in fish—though not to such an extent that it yielded actual digits.
"We've uncovered a surprising degree of genetic complexity in place at an early point in the evolution of appendages," study leader Martin Cohn of the University of Florida said in a statement.
The findings appear this week in the journal PLos ONE.
Limb development, which happens in the fetal stage of all limbed animals, is driven by the so-called Hox gene. (Get the basics on the human genome.)
The early stage of the Hox gene expression regulates the development of limbs down to the forearm and shin. The later phase is responsible for forming fingers and toes.
"It has long been thought that fish fins exhibit only the early wave of Hox expression..." Cohn told National Geographic News.
The discovery of the early and late waves of Hox gene activity in the fin buds of shark embryos suggests that both phases were present in the common ancestor of sharks and bony fishes, Cohn said.
"Thus, the late wave is not uniquely associated with the origin of tetrapod digits," he added.
The reason sharks carry the genes but don't actually grow digits, the team speculated, is because the gene expression occurs briefly and only on a narrow band of cells.
An earlier study had demonstrated that the patterns of Hox genes—once thought to be unique to the arms and legs of land animals—are in fact much more ancient.
"We demonstrated this pattern in a primitive living fish related to sturgeons, called the paddlefish," said Marcus C. Davis, a researcher on the earlier study, which was published in May in the journal Nature. Davis was not involved in the PloS One study. (Related: "China's Giant River Fish, the World's Largest, Feared Extinct" [July 26, 2007].)
"What [Cohn's] group has done is confirmed these results in an animal considered even further down the family tree of vertebrates."
These studies demonstrate that the genesis of hands, feet, fingers, and toes in animals did not require new genes or even new patterns of gene expression.
"Dramatically different ways of being—new forms, new functions—may evolve through relatively minor adjustments to existing genes and gene functions," Davis said.
"It only requires modifications—'tweaks'—if you will, to previously existing genetic systems," he said.
"A symphony can play dramatically different compositions by changing the role each musician plays, [but] only on occasion are instruments added or lost."
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