Long before animals with limbs (tetrapods) came onto the scene about 365 million years ago, fish already possessed the genes associated with helping to grow hands and feet (autopods) report University of Chicago researchers in the May 24, 2007, issue of Nature.
This finding overturns a long-held, but much-debated, theory that limb acquisition was a novel evolutionary event, requiring the descendents of lobed-fin fish to dramatically alter their genes to adapt their bodies to their new environments of streams and swamps.
The paper, "An autopodial-like pattern of Hox expression in the fins of a basal actinopterygian fish," shows that the genetic and developmental toolkit that builds limbs with fingers and toes was around long before the acquisition of limbs, according to the scientists, and that this toolkit exists in some primitive form in a living primitive bony fish, the paddlefish.
"We found that the genetic capability seen in tetrapods to build limbs is present in even more primitive fish," said lead author Marcus Davis, PhD, a postdoctoral fellow in Neil Shubins lab at the University of Chicago.
Instead of using zebrafishthe hallmark animal for laboratory development studiesthe scientists used paddlefish as a proxy for a more primitive ancestor. Unlike the simple fins of zebrafish, paddlefish have an elaborate fin skeletal pattern similar to that seen in more primitive vertebrates such as sharks and many fossil fish. This sturgeon-type fish is farm raised for caviar, which gives scientists relatively easy access to the animal for study.
The fin of paddlefish resembles that of zebrafish. The interior arrangement is the same, but the back part of the paddlefish fin has longer elements. Accepted theory among scientists has been that the pattern of Hox gene expression seen in zebrafish represents the primitive condition for the fin in any vertebrate, and the group leading to tetrapods elaborated on this Hox expression by ad
Contact: Catherine Gianaro
University of Chicago Medical Center