Discovery Overturns Long-Held Genetic Belief

Researchers from the University of Chicago report in the January 4, 2002, issue of Science that, contrary to expectations, the tiny fourth chromosome of the fruit fly, believed to be identical in every member of the species, actually has several regions that vary.

"This classic conviction of genetics and evolution, this rock-bottom-solid conclusion, which has become a textbook example of natural selection's propensity to eliminate variation from closely linked genes, just doesn't hold up," said Manyuan Long, Ph.D. assistant professor of ecology and evolution at the University of Chicago and director of the study.

Since 1906, Drosophila melanogaster, the common fruit fly, has been at the center of genetic research. The proof that genes exist on chromosomes like beads on a string, that they are linked together, that they can be mapped, even the method of naming genes, comes from early research on fruit flies.

The synthesis of Darwin's theories about evolution and Mendel's discoveries about genetics was made using fruit flies. And the discovery of recombination, in which paired chromosomes can exchange genes as they form egg or sperm cells and thus increase genetic diversity, derived from early work on the four chromosomes of the fruit fly.

Despite the fly's century in the laboratory limelight, it's tiny fourth chromosome -- which contains only one percent of the insect's coding DNA -- has been comparatively neglected. Based on limited evidence available at the time, the pioneers, including Calvin Bridges and Nobel Prize winner Hermann Muller, determined that there were no genetic crossovers -- meaning no exchange of hereditary information -- on this tiny chromosome.

This belief helped to inspire elaborate theories about the consequences of a nonrecombinant chromosome, introducing the concepts of "selective sweeps" and "genetic hitchhiking." If a mutated gene on Chromosome 4 conferred a sufficient survival advantage, the experts posited, the entire advantageous chromosome, rather than the single gene, would sweep through a population, carrying along the founder's version of every gene on that chromosome, thus eliminating all variation.

In 1991, as a graduate student in Martin Kreitman's laboratory at the University of Chicago, one of the authors of the current Science paper tested the theory. Andrew Berry (now a researcher at Harvard's Museum of Comparative Zoology) sequenced the cubitus interruptus gene, on Chromosome 4, from 10 natural lines of Drosophila melanogaster. All ten were identical.