To stay high in the sky, birds filled their bones with air -- and cut the air out of their genes, according to Penn State biologists Austin and Marianne Hughes.

That birds have less DNA in their cells than mammals was known. The question was, why? Was it an accident, a chance mutation in a small group of ancestors millions of years ago? Or did it evolve, providing modern birds with an unknown benefit?

"If there is some adaptive reason why birds have a reduced genome size," Austin Hughes says, "it should be reflected in just about every gene. We would expect to find that each gene sequence is smaller -- not that a huge single chunk of DNA is missing."

Which is what the Hugheses found when they compared 31 chicken genes to similar human ones, zeroing in on segments of genes known as introns. Introns -- sometimes called "junk DNA" -- mark the boundaries between the active, protein-coding bits of a gene. "Introns basically just sit there," says Hughes, "so if you were looking for a way to make the genome smaller, that would be a good place to cut."

The researchers found that "small intron segments were missing in every gene, which indicates to me," says Hughes, "that there is some sort of overall pressure in the direction of reducing the genome size in birds." That the best flyers, out of 40 families of birds ranked, also had the smallest genomes suggested to the Hugheses that this adaptive pressure came from the rigors of flight.

How could less DNA help birds fly? Smaller genes can mean smaller cells, Hughes notes. "A good metabolism for flight requires each cell to exchange carbon dioxide for oxygen really fast, which is easier to do in a small cell because it has a relatively large surface area." Gene replication would also be faster.

To see if their hypothesis holds up, the Hugheses "next want to compare the introns of other birds -- like a penguin, which can't fly at all, and a hummingbird, which is a great flyer," says Austin Hughes. "It also would be great to do the same thing with bats, which have less DNA than the average mammal."

Austin L. Hughes, Ph.D, is assistant professor of biology and Marianne K. Hughes, Ph.D., is research associate in the Eberly College of Science, 208 Mueller, University Park, PA 16802; 814-865-5013. This project was supported by the National Institutes of Health and published in the October 5 issue of Nature. Reported by Barbara K. Kennedy.