Each of the model organisms brings advantages the others don't have. A simpler organism like yeast, for instance, is easier to understand, but it is harder to extrapolate that knowledge to the development of humans and human diseases.

For some types of research, it is essential to use multicellular creatures, such as the roundworm or the fly. Yet the roundworm is a pretty simple system, Rubin points out, compared with the fruit fly, which exhibits certain behaviors that can be found in humans and other mammals.

"Things like biological clocks and circadian rhythms, like learning and memory, have all been studied in Drosophila," Rubin says. "But you can't study them in a worm because worms are not complex enough. So there are processes that you can study in a fly that you can't study in a worm, and there are things in a worm that you can't study in yeast, and there are things in a mouse you can't study in a fly."

The fly in question, Drosophila melanogaster, is about as common a fruit fly as you'll ever come across. While there are some 900 species of Drosophila worldwide, melanogaster is the one you've probably seen yourself—the tiny flies, maybe an eighth of an inch long, that you'll find hovering around your overripe bananas.

These flies first auditioned as lab animals at the turn of the century, but they were not an immediate success. Biologists and geneticists tended to work on them as a "last resort," after their other breeding experiments had failed, writes University of Pennsylvania historian of science Robert Kohler in Lords of the Fly. They wrote to their research colleagues complaining that their laboratories were covered with "bugs, bugs, bugs" or the "pesky little beasts." But these early Drosophila pioneers persevered, if for no other reason than that fruit flies are ideal for student projects—the colonies are inexpensive to keep and easy to replace if lost to carelessness, an invasion of fly mites, or simply summer vacation.

The fruit fly also has a small genome—about 13,600 genes, compared with the estimated 40,000 genes of humans. It takes only 24 hours for a fruit fly to develop from a single fertilized cell to a larva with a complex musculature, nervous and digestive systems, eyes, and mobility. It takes only 11 days for an egg to develop into an adult female fly that will lay hundreds of eggs. "So it's very easy to generate a lot of offspring in a very short period of time," says Steller. "And you can see the embryos develop, because flies just lay eggs—which is not the case with human and mouse embryos, where the early stages develop inside the mother, in utero, and it's very hard to observe what's going on."

— Gary A. Taubes

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This fruit fly grew an extra pair of wings through mutations in bithorax, a gene that controls development in the rear half of the fly's body.

Photo: E.B. Lewis

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