Evolution: Paddlefish clues to limb development
The limbs of tetrapods — land-living vertebrates — are usually thought to have been evolutionary innovations unique to that group. However, traces of limbs can be found in the development of primitive ray-finned fishes, according to a report.
The perceived uniqueness of the tetrapod limb may be a reflection of comparisons with fishes, notably the zebrafish (Danio rerio), which is often used as a model organism in embryological studies. However, the zebrafish is rather highly evolved, meaning that rather than lacking vestiges of limbs to begin with, it could have lost them during its ancestry. This latter possibility seems most likely following Neil Shubin and colleagues' study of Hox-gene expression in the development of the fins of a 'living fossil', the paddlefish Polyodon spathula.
Polyodon is one of the few relics of a type of bony fish common in the seas in the Palaeozoic era, more than 250 million years ago. Hox-gene expression in the developing fins of Polyodon shows patterns long considered to be tetrapod hallmarks. This finding demonstrates that some aspects of limb development are primitive and held in common by all bony fish — but have been lost in highly evolved fishes such as the zebrafish.
CONTACT
Neil Shubin (University of Chicago, IL, USA)
Tel: +1 773 834 7472; E-mail: nshubin@uchicago.edu