By Paul Rincon BBC News Online science staff

The study illustrates how rapid body plan changes can appear

Some sticklebacks have a spine in their pelvic fin to protect against predators but others lost it through evolution.

The team compared genetic maps of both types of fish to find places in their DNA code that controlled this feature.

The gene Pitx1 was found to map exactly to the part of the genome associated with most variation in the feature.

Scientists say the genetic mechanism could be a "smoking gun" for the way rapid changes in body plan can appear over time in the evolution of new animal species.

What is more, the mechanism appears to work without the harmful side effects sometimes seen with gene mutations.

Evolutionary importance

The work complements studies of Pitx1 in mice. But there is a crucial difference: in mice Pitx1 mutations are often lethal, causing abnormalities of the head, face and some glands.

The researchers think this is because Pitx1 mutations in mice involved molecular changes to the protein "encoded" by the gene. In sticklebacks, the protein is identical in fish with pelvic spines and those without.

However, the way Pitx1 was expressed in different tissues in the body differed markedly between the two populations of stickleback.

Gene expression patterns are responsible for the loss of pelvic spines

In larvae of fish with pelvic spines, Pitx1 was expressed strongly in a variety of tissues, including those in the pelvic area.

In fish without the spines, Pitx1 was not expressed in the area where the pelvic limb bud normally develops, but it was active in other tissues.

Thus, the mutation altered the gene's expression pattern in a specific part of the fish's body, changing the limb number without affecting other tissues.

Limb loss is implicated in a number of big steps in evolution.

For example, the loss of hindlimbs in whales is thought to have been crucial in their evolutionary transition from four-legged land-dwellers to streamlined marine mammals.

The genetic mechanism is one of several known to affect the body plan of animals.

In vertebrates, bunches of genes known as Hox clusters are known to play a crucial role in the development of the skeleton and soft tissue.