Tiny Bird, Tiny Genome

By Rachel Ehrenberg, Science News

Flying with excess baggage is a drag, but hummingbirds have mastered efficient packing. The tiny hoverers have less DNA in their cells than any other previously studied birds, reptiles or mammals, researchers report online August 5 in Proceedings of the Royal Society B. Among hummingbird species, however, genome size doesn’t vary along with body size, suggesting that birds’ DNA was pared down before the diversification of today’s hummers.

Scientists have long noted the link between small genome size and high metabolic rates — a notion first put forth in 1970 by Polish scientist Henryk Szarski. Bats and birds have the smallest genomes of backboned creatures, and flightless birds tend to have bigger genomes than fliers. The genome, or full book of genetic instructions, is typically present in every cell. So lugging around a smaller genome means you can have smaller cells, the thinking goes. Smaller cells mean a larger surface-to-volume ratio and more efficient gas exchange, all the better to fly with. (The metabolic rate of hummingbirds is thought to approach the theoretical maximum.)

Ryan Gregory of the University of Guelph in Canada and colleagues estimated genome size in 37 species of hummingbirds by staining DNA in the nucleus and then analyzing DNA density. Bird weight, body size, red blood cell concentration and other factors associated with metabolism were also examined in the birds. Hummingbird genomes averaged a mere 1.03 picograms per cell, or 1.03 trillionths of a gram, the researchers report. (A picogram in this case is made up of nearly 1 billion base pairs, the chemical units that make up DNA.) The average for previously examined birds is 1.42 picograms, for reptiles it is 2.24 picograms and for humans it is 3.5 picograms. Some salamanders have more than 100 picograms.

The findings are consistent with Szarski’s hypothesis and “make perfect sense,” says molecular evolutionist Austin L. Hughes of the University of South Carolina in Columbia. An analysis of dinosaur bones published in Nature in 2007 suggests that the trimming down of the avian genome may have begun in ancestors as far back as Tyrannosaurus rex (SN: 3/31/2007), though hummingbirds have taken the trimming to an extreme. Genome size and other avian traits — such as lighter bones, feathers and small body size — appear to have accumulated piecemeal, Hughes says. “That’s the way evolution happens.”