Evolution Re-Sculpted Animal Limbs By Genetic Switches Once Thought Too Drastic For Survival

Homeotic, or Hox genes specify the identity of segments along the embryo's body axis and regulate the formation of major structures in every animal studied. But because laboratory mutations in these genes can cause monstrosities--such as a fly with legs where its antennae should be--many scientists doubted that natural variation of homeotic genes could underlie the incremental, survivable changes that accrued over eons as animals gradually evolved improved body parts. Today's finding marks the first time that changes in the control of homeotic genes have been shown to underlie an evolutionary trend leading to novel body structures.

Nipam Patel, assistant professor of organismal biology and anatomy in the University of Chicago's Howard Hughes Medical Institute, and Michalis Averof, currently at the European Molecular Biology Laboratory in Heidelberg, showed that changes in the pattern of activity of two Hox genes in crustaceans are linked to the relatively sudden evolutionary development of useful, distinctive feeding limbs called maxillipeds (literally jaw-feet) where swimming or walking legs once were.

The finding is a landmark in the new field of evolutionary developmental biology, or "evo-devo," as its proponents call it, in which scientists study the patterns of gene expression in embryos to peer backward in time. In the past few years, researchers have made remarkable progress in identifying genes that specify gross changes in body shape early in development. But others have argued that any naturally arising variants in such genes would quickly die. "The question has always been did evolution actually fiddle with this stuff to generate diversity of body plan?" says Patel.

Many scientists looked to insects for natural homeotic gene variation, Patel explained, because insects have such diverse body plans. "But it turns out that much of their diversity arises after this step in development," he said. "The diversity comes not so much from controlling where in the embryo the homeotic genes are initially expressed, but how other genes respond to them."

Instead, Patel and Averof turned to crustaceans. They collected a wide range of specimens--13 separate species from nine different orders--many of them at the Smithsonian Field Station in Belize.

"Crustaceans have far more variation than insects in their body plan," Patel said. "If that's surprising, it's because people only think of the two or three closely related kinds of crustaceans we eat."

The researchers used an antibody that labels the proteins made by two closely linked homeotic genes, Ubx and abdA, to show in which segments of the embryo the genes are turned on. The Ubx-abdA genes--like other homeotic genes, including the one that can cause a fruit fly to grow a leg out of its head--orient the embryonic tissues as to where they lie in the body.

In the case of that particular fly mutant, if the homeotic gene called Antennapedia is turned on in the head segment, "the cells that bud off from the fly's head think, 'oh, we're sitting in the thorax, we're supposed to grow into a leg,'" Patel explained.

In crustaceans, if Ubx-abdA is turned on in a segment, it tells the limb buds they lie in the part of the thorax that should grow locomotory legs. The researchers found variation in the pattern of segments in which Ubx-abdA is turned on, and that pattern corresponded with anatomic changes that traced ancestral relationships and evolution.

"In primitive kinds of crustaceans, we found Ubx-abdA is turned on in the first thoracic segment and is 'on' from there back to the tail," Patel said. "In these animals, there's a marked difference between the appendages of the head and the thorax--the head has tiny appendages used for pushing food into the mouth, and the thoracic appendages are long and feathery for swimming."

But in more advanced crustaceans, the first segment in which Ubx-abdA is turned on lies farther back along the body. In these animals, the first few thoracic segments have appendages that look like those of the head. These maxillipeds are not just misplaced jaw parts but have new capabilities because they are attached to the powerful muscles and the nervous system of the thorax. Crustaceans that have them include the decapods--shrimp and lobsters and their relatives.

"If you watch, animals with maxillipeds can feed in a different way," Patel said. "They can walk and hold and move their food at the same time. This has clear implications for the organism and can be an advantage, depending on environment."

Patel said the finding legitimizes the comparative genetics of early development as a tool for studying evolution.

"This variation is what everyone had hoped to find for this class of genes," he said. "They hoped you would find a change in morphology that mirrored what was happening with respect to expression of the gene. They weren't finding that in insects, but in crustaceans we show a very striking example of this correlation."

The research was funded by the Howard Hughes Medical Institute and the Carnegie Institute.