(Sujin Ko, sujinko93@gmail.com)
Tuesday, March 22, 2011
Fins to Hands
In chapter 2, Shubin describes the patch of tissue called the zone of polarizing activity (ZPA) that will produce a mirror image duplication of the limb if vitamin A is injected in the right stage and concentration, as well as the Sonic hedgehog gene that "made one end of the body segment look different from the other" (52). The ZPA and Sonic hedgehog gene together form the hand of humans (making every finger look different from one another) and the fin of a shark/skate. If the Sonic hedgehog gene has the same general function in two radically different species such as humans and sharks, why do they form hands in humans and fins in sharks, two completely different appendages? Where in evolutionary history does this change occur, where the Sonic hedgehog gene goes from forming a shark fin to a human hand with a full set of moving, functional fingers? How does this change occur?
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ReplyDeleteShubin says that the hedgehog gene controls growth in different parts of the body, much as the tissue (named the ZPA) controlled the growth of fingers in the chick. The main example used was the one of a fruit fly- The Hedgehog gene in a fly controls the growth of different sections of the embryo, and controls how those genes are different from one another. The hedgehog gene is so named because the researchers noticed that when there was a small mutation, it caused the fly to have small spikes all over it.
ReplyDeleteThe genes come from an ancient ancestor shared by sharks and humans. "It shows that this genetic program for building limbs has its origins in the median fin structures of very early vertebrates.” The study also found that the genes are vital for the formation of the ribbon-like fins on the back of lampreys, a primitive jawless fish that does not have paired side fins.
Sharks and lampreys belong to groups of fish that diverged many millions of years ago, so the new finding suggests genes important for the development of fins, and eventually limbs, were in place long before the different kinds of fishes evolved and went their separate ways.
Hox genes were active along in a fin on an axis that grew away from the fish along the back edge. Along this axis, cells proliferated rapidly, condensed into cartilage, and split into the branches of radial bones that fish use to paddle their fins. But then the Hox genes shut off. Stopping it from forming and developing into a “hand” of sorts but by stopping it their the fish was allowed to have an adapted fin that would be advantageous to them.
“This pattern of growth is reminiscent of the first stages of limb development in a land vertebrate. Experiments on lab mice have shown that at first the pattern of Hox genes marks a fishlike axis going out along the back edge of the bud. In a mouse’s front leg or your arm, this part of the axis became the humerus, radius, and ulna. But at the point where the Hox genes shut down in a fish, they keep going in a land vertebrate, and the skeletal bone of the limb keeps growing. The end of the limb hooks around to become wrist bones; rapidly dividing cells branching off this hook become fingers.”
(http://discovermagazine.com/1996/jan/fromfintohand653)
-Adnan Jahan
(adnanjahan@gmail.com)