Public Release: 

New genetic data overturn long-held theory of limb development

Study reveals genes present long before fish needed hands, feet

University of Chicago Medical Center




"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 Shubin's lab at the University of Chicago.

Instead of using zebrafish--the hallmark animal for laboratory development studies--the 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 team looked at Hox genes--which play a vital role in limb development--in the pectoral fins of paddlefish. To track where the Hox genes are active in the fin, the team inserted molecular markers and showed the activity pattern has similarities to patterns of these same genes in tetrapod limbs.


This is the first molecular support for the theory that the genes to help make fingers and toes have been around for a long time--well before the 375-million-year-old Tiktaalik roseae, the newly found species discovered in 2004 by Shubin and colleagues. Tiktaalik provided a missing evolutionary link between fish and tetrapods and was among the first creatures that walked out of water onto land.

Prior to this find, scientists had little evidence of where the wrist came from. A popular theory, one Shubin himself subscribed to, was that it was a novel development--that genetic variance gave rise to an entirely new function.




"When you try to directly compare zebrafish fins and tetrapod limbs, it's like comparing apples to oranges because each retains portions of the skeleton lost by the other," he said. "Just as tetrapods went off and did something crazy with their fin by adding to it, zebrafish went off and did something crazy by losing part of their fin.


Although this pattern of genes clearly helps to make hands and feet today in tetrapods, that may not be their original intent, Shubin said. "Here's a fish that doesn't have an autopod but is still using those genes in a second phase to help pattern out a fin that doesn't have fingers, never did, and is very far removed from tetrapods."


According to Shubin, in the Late Devonian, animals like Tiktaalik and its descendants acquired limbs with fingers using this primitive design, largely because their ecosystem--the small streams that they lived in--was new.





Davis-Media01.jpg: Paddlefish fins exhibit a unique pattern of Hox expression previously thought present only in the developing hands and feet of land vertebrates (tetrapods). This result supports the notion that fossil fish already possessed the genetic toolkit needed to evolve hands, feet, fingers and toes.

Davis-Media02.jpg: Larval and juvenile specimens of the North American paddlefish, Polyodon spathula, stained for developing cartilage. Inset shows the skeleton of the pectoral fin, the equivalent of the arms of land vertebrates (tetrapods). Photo by M.C. Davis

Davis-Media03.jpg: Underside and side views of a juvenile specimen of the North American paddlefish, Polyodon spathula, stained for developing cartilage. Photo by M.C. Davis
