"People have been interested in limbs for a long time because they show such variability in different animals," said David Kingsley, PhD, professor of developmental biology, who led the work. "The debate has been how many genes account for these differences."

The study, published in the April 15 issue of Nature, took advantage of a unique species of fish called the threespine stickleback. Pockets of sticklebacks were isolated by geologic changes at the end of the Ice Age 10,000 years ago, with each newly separated population evolving in response to local ecological conditions. A handful of the thousands of populations around the world lost their hind fins and associated spines, probably to avoid local predators that grabbed the fish by those spines.

Kingsley, who is also an associate investigator with the Howard Hughes Medical Institute, said the debate over how limbs evolve has been stymied because most animals that evolved to have fewer or altered limbs also have a host of other genetic changes, making it hard for scientists to tease out the number and location of genetic changes most important for altering the limb. Sticklebacks, with their recent divergence into many distinct populations, present an opportunity to study recent limb evolution.

The group looked at two populations of freshwater threespine sticklebacks that had lost their hind fins. Working with senior co-author Dolph Schulter of the University of British Columbia, the group crossbred a population of Vancouver freshwater sticklebacks with their four-finned marine relatives. All the resulting offspring had hind fins. These four-finned offspring, which had o
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Contact: Amy Adams
amyadams@stanford.edu
650-723-3900
Stanford University Medical Center
14-Apr-2004