The researchers then interbred these finned offspring to produce fish with a range of hind fin lengths. Some fish lacked hind fins entirely while others had the fully formed fins of their saltwater grandparents. Still other fish had partially formed hind fins or structures that were slightly larger on one side.

Graduate student and co-first author Melissa Marks and former postdoctoral scholar Katie Peichel, PhD, found that the presence of hind fins seemed to correlate with changes at one particular location of the fish chromosomes. A few other chromosome regions had smaller effects on the length and shape of pelvic features, but most of the major evolutionary change could be attributed to a single region. Mike Shapiro, PhD, a postdoctoral scholar and co-first author, found that a gene located at that region is the stickleback version of a gene in mice called Pitx1 that, when mutated, causes mice to have greatly reduced hind limbs. These mice often have asymmetric limb and pelvic reductions, much like the sticklebacks.

These experiments helped explain how mutations at Pitx1 could nudge one population of fish toward losing hind limbs, but they don't hint at whether these mutations were a one-time solution or a widely used strategy.

The researchers addressed this larger evolutionary question using a second population of freshwater sticklebacks discovered by co-author Bjarni Jonsson from the Institute of Freshwater Fisheries in Iceland. The Icelandic fish also lack hind fins, but evolved thousand of miles away from the Vancouver fish. Marks bred the two freshwater sticklebacks and produced an aquarium full of fish lacking hind fins.

Breeding the Icelandic sticklebacks to a four-finned saltwater fish once again generated offspring with hind fins. Together, these experiments
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Contact: Amy Adams
amyadams@stanford.edu
650-723-3900
Stanford University Medical Center
14-Apr-2004