News: Sharks Have Genes for Fingers and Toes.
- From: Ye Old One <usenet@xxxxxxxxx>
- Date: Thu, 16 Aug 2007 20:19:02 GMT
Sharks Have Genes for Fingers and Toes
Sara Goudarzi
for National Geographic News
August 15, 2007
http://news.nationalgeographic.com/news/2007/08/070815-shark-gene.html
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].
http://news.nationalgeographic.com/news/2006/10/061018-fossil-fish.html)
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
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.
he 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.
Genetic Tweaks
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.
"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."
--
Bob.
.
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