Scientists have never known just what to make of flying insects. The fossil record shows that they arose on the scene some 500 million years ago, but just how insects managed to evolve wings has remained a mystery. In tomorrow's issue of Nature, however, a pair of researchers in Germany reports finding the first genetic evidence that insect wings evolved from gills, evidence that very well may settle the debate.
Some entomologists have considered wings to be an evolutionary novelty, first sprouting from tiny nubs of tissue that bud during development. Others have thought that wings were modified from existing structures, probably one of the many appendages that branch out from insect limbs. Gills are a likely candidate for such ancestral appendages because, like wings, they have joints and muscles, and in the larvae of some species, gills even resemble miniature wings.
Michalis Averof, a geneticist at the European Molecular Biology Laboratory in Heidelberg, and his colleague Stephen Cohen reasoned that if gills and wings are indeed related, the genes for the two structures would likely be similar. To test this notion, they compared genes in flies and crustaceans, which are insects' closest relatives with gills. Using a common technique for searching through DNA, known as the polymerase chain reaction, the researchers fished out equivalent genes from ground-up larvae of brine shrimp and crayfish. They discovered that two previously discovered insect wing genes, called pdm and apterous, were hidden in the DNA of the flightless crustaceans.
This isn't definitive proof, because animals carry many anachronistic genes--information perhaps once useful for some long-extinct ancestor, but expressed no more. So, to see what, if any, role the genes for fly wings played in shrimp and crayfish, the scientists developed a chemical marker that stains the proteins made by the pdm and apterous genes. When they injected the marker into the shrimp larvae, a black stain formed in the upper part of the developing limb: the area that would become the gill.
For many observers, the new evidence settles the matter. The origin of wings in insects "is an old chestnut in evolutionary biology," says Michael Akam, a developmental geneticist at Cambridge University. "It's nice to see this question come back and be resolved."