Stained nuclei in this brine shrimp highlight developing limbs.
Photograph courtesy of University of California at San Diego
Four hundred million years ago, multi-legged crustaceans gave rise to terrestrial insects possessing a drastically different six-legged body plan. Scientists have long considered this transformation one of the great mysteries of evolution, but William McGinnis, a biologist at the University of California at San Diego, has an explanation. The loss of just one peptide, or chemical unit, from a protein called Ubx can repress leg development over most of an organism's body. This discovery "is the first evidence that small mutational changes in a gene can contribute to very large jumps in evolution," he says.

McGinnis and graduate student Matthew Ronshaugen discovered the role of Ubx while studying Hox genes—those that carry the instructions determining which cells develop into which body part—in 22-legged brine shrimp and six-legged fruit flies. The researchers tested the protein's effect by mutating the Hox gene from the shrimp to create the variant of Ubx that they saw in the flies. They found that this variant, with the single peptide removed, performed its normal role in fruit fly development. "This describes a likely explanation for how a major change in animal morphology took place. It's kind of like an archaeological story, digging back through ancient DNA to infer what happened," Ronshaugen says.