Tuesday, 11 September 2007

Ancient gene kit came in handy for limbs

The master genes controlling development in the primitive animal known as the paddlefish turn out to be unexpectedly similar to those controlling the development of limbs in land animals. Rather than evolving a new set of control genes for their limbs, it seems that our amphibian ancestors adapted the genes their own ancestors used to develop fins.
The paddlefish Polyodon spathula is often referred to as a "living fossil", an organism that is similar to no known species apart from fossils. Most previous genetic work on fish has been done on the more highly evolved zebrafish. This appeared to show that the Hox family of control genes in land animals and fish were different, implying land animals had evolved new genes to control growth of hands and feet.
Hox genes control the alignment and polarisation of body structures in all animals, separating head from tail. In fish they arrange the structures of fins, and in land animals the structure of limbs. Zebrafish develop their fins in a single stage, in which Hox genes produce parallel stripes that underlie fin structures. Mice and chickens have a second phase of development in which Hox genes turn on only in the regions that become a hand or foot.
"The logical explanation was that since fish don't have hands, they don't have a second stage of Hox gene development, so [addition of] the second stage should correlate with evolution of the hand," says Marcus Davis of the University of Chicago. But zebrafish are highly evolved, so he wondered if they had lost the ancestral form of fin development.
Davis and colleagues looked at Hox genes in the paddlefish, a primitive relative of the sturgeon, because it is relatively unevolved. They found that the little arm fins of paddlefish develop in two phases, implying that the second phase of Hox gene expression had evolved long before arms and legs, but was lost in zebrafish (Nature, vol 447, p 473).
That ancient set of genes played a key role in helping vertebrates crawl onto land. Tiktaalik, the fish with feet discovered last year (New Scientist, 8 April 2006, p 14), "already had the toolkit needed to modify the part of the limbs furthest out", Davis says. As ancestral amphibians moved onto land, they used their existing genetic tools to adapt their limbs.
“As amphibians moved onto land they used their existing genetic tools to adapt their limbs to the new environment”
"Here's something we thought was invented from scratch, but it was there in a deep ancestor of tetrapods," says Sean Carroll, a developmental biologist at the University of Wisconsin, Madison.

From issue 2605 of New Scientist magazine, 23 May 2007, page 18

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