Mar. 25, 2009 — The genetic toolkit that
animals use to build fins and limbs is the same genetic toolkit that
controls the development of part of the gill skeleton in sharks,
according to a new study.
The research is published in Proceedings of the National Academy of Sciences
on March 23, 2009, by Andrew Gillis and Neil Shubin of the University
of Chicago, and Randall Dahn of Mount Desert Island Biological
"In fact, the skeleton of any appendage off the body of an animal is
probably patterned by the developmental genetic program that we have
traced back to formation of gills in sharks," said Andrew Gillis, lead
author of the paper and a graduate student in the Department of
Organismal Biology and Anatomy at the University of Chicago. "We have
pushed back the evolutionary origin of the developmental genetic
program that patterns fins and limbs."
This new finding is consistent with an old theory, often discounted
in science textbooks, that fins and (later) limbs evolved from the
gills of an extinct vertebrate, Gillis added. "A dearth of fossils
prevents us from definitely concluding that fins evolved from gills.
Nevertheless, this research shows that the genetic architecture of
gills, fins and limbs is the same."
The research builds on the breakthrough discovery of the fossil
Tiktaalik, a "fish with legs," by Neil Shubin and his colleagues in
2006. "This is another example of how evolution uses common
developmental programs to pattern different anatomical structures,"
said Shubin, who is the senior author on the PNAS paper and Professor
and Associate Dean of Organismal and Evolutionary Biology at the
University of Chicago. "In this case, shared developmental mechanisms
pattern the skeletons of vertebrate gill arches and paired fins."
The research also showed for the first time that the gill arch
skeleton of embryonic skates (a living relative of sharks that has gill
rays) responds to treatment with the vitamin A derivative retinoic acid
in the same way a limb or fin skeleton does: by making a mirror image
duplicate of the structure as the embryo develops. According to the
researchers, the genetic circuitry that patterns paired appendages
(arms, legs and fins) has a deep evolutionary origin that actually
predates the origin of paired appendages themselves.
"These findings suggest that when paired appendages appeared, the
mechanism used to pattern the skeleton was co-opted from the gills,"
Gillis said. "Perhaps we should think of shark gills as another type of
vertebrate appendage—one that’s patterned in essentially the same way
as fins and limbs."
The deep structural, functional, and regulatory similarities between
paired appendages and developing gill rays, as well as the antiquity of
gills relative to paired appendages, suggest that the signaling network
that is induced by retinoic acid had a patterning function in gills
before the origin of vertebrate appendages, the research concludes. And
this function has been retained in the gill rays of living
Source : University of Chicago Medical Center