Birds: lightweights in the genetic sense.

Birds are the star athletes of the vertebrate world, pushing their bodies to metabolic extremes in order to defy gravity. Evolution has given them an edge by creating a lightweight skeleton, an aerodynamic coat of feathers, and a highly efficient respiratory system.

Their advantages even extend to the molecular realm. According to a new study, birds cast off excess genetic baggage long ago and in the process developed a much leaner genome.

Biologists have long known that bird cells contain less DNA than those of reptiles, mammals, and amphibians. But they did not know where the genetic differences lay. Austin L. Hughes and Marianne K. Hughes of Pennsylvania State University in University Park explored that question by comparing the sequences of 31 equivalent genes in humans and chickens--two animals for which this information exists.

They found that all of the chickens' sequences were shorter because they contain shorter introns, the regions of genes that contain so-called nonsense, or noncoding, DNA. Exons, the regions that contain the blueprints for protein formation, were roughly the same size in chickens and humans, the scientists report in the Oct. 5 Nature.

A single genetic change could not have shortened all chicken introns, the researchers argue. Instead, evolution gradually trimmed avian DNA, perhaps as an adaptation for flight.

Because the typical cell size of an animal tends to match the length of its genome, the development of shorter introns could account for the relatively small size of bird cells. Smaller cells should make birds more metabolically efficient by speeding up the diffusion of oxygen into the interior portion of cells.

To support their theory, the investigators note that bats tend to have less DNA than other mammals. Furthermore, eagles, pigeons, and other strong flyers have smaller genomes than weak flyers or flightless birds. "It would seem that reduced genome size is an adaptation for flight in vertebrates," conclude the Penn State scientists.

Intron researcher Stephen R. Palumbi of the University of Hawaii in Honolulu warns that the smaller intron size in birds might not have evolved in connection with flight. "There are lots of differences between chickens and humans [aside from flight ability] that may explain that pattern," says Palumbi. Nonetheless, he finds it compelling that bats also have smaller genomes, whereas flightless birds have larger ones.

Evolutionary geneticist Robert C. Fleischer of the National Zoological Park in Washington, D.C., adds that "birds apparently have very efficient enzymes for correcting mistakes in DNA." These enzymes prevent segments of noncoding DNA from multiplying and thereby enlarging the bird genome as they do in other animals. Fleischer had jokingly suggested that the correction system might reduce the weight of birds, although he did not pursue the idea. "I didn't think that anyone would take it seriously," he says. -