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.