Absent protein causes chromosomal breakup.

Perhaps even more than fans of Godzilla films, fruit fly biologists love mutants. For decades, these investigators have studied strange and unusual examples of the fast-breeding insect, both natural mutants and those induced by chemicals or X rays.

In 1968, researchers captured an intriguing mutant near a winery outside Rome. As they studied this mutant strain, scientists found that the insects had problems making the cells that would become sperm or eggs. Sometimes too much genetic material would end up in sperm cells, for example; other times sperm were a bit light on DNA.

Now, a group led by Terry L. Orr-Weaver of the Whitehead Institute for Biomedical Research in Cambridge, Mass., has finally discovered the broken gene responsible for these reproductive troubles. It appears to describe a protein that briefly glues chromosomes together while sperm and egg cells are made.

Investigators hope the advance will help them understand why human sperm and egg cells often have inappropriate numbers of chromosomes. Conception with these sperm or eggs may result in early miscarriages or children with diseases such as Down's syndrome.

"This is really important work. [Orr-Weaver] is tying up this incredibly long, beautiful story," says R. Scott Hawley of the University of California, Davis, who coauthored a commentary that appears in the Oct. 20 Cell with the report of the Whitehead investigators.

In genetics shorthand, the mutant insects Orr-Weaver studies are known as mei-S332 flies because their difficulties arise during meiosis, the two-stage process in which a precursor cell divides twice to form four sperm or eggs.

Mei-S332 flies appear to have no problems with the first division. To begin with, the cell contains 4 pairs of chromosomes, one of each pair derived from the insect's father and the other from its mother.

Each chromosome then makes a copy of itself. Now called sister chromatids, the two identical chromosomes remain linked at their midpoint, an area called the centromere. Next, maternal sister chromatids pair up with their paternal counterparts, usually exchanging stretches of DNA before the first cell division in meiosis separates the maternal and paternal chromosomes into two cells.

In the second stage of meiosis, the sister chromatids, still tightly linked at their centromeres, gather at the centers of the two cells. As cell division begins again, an apparatus called the spindle forms within each cell and gently pulls the sister chromatids apart, just as the maternal and paternal pairs are separated for the first division. Once the sister chromatids part, the two cells divide, forming four cells, each with half the original number of chromosomes.

But in mei-S332 flies, "you can see the sisters prematurely disjoining," says Orr-Weaver. As a result, she explains, "some cells get too many chromosomes, others get too few."

When Orr-Weaver and her Whitehead colleagues Anne W. Kerrebrock, Daniel P. Moore, and Jim S. Wu found the mei-S332 gene, they discovered it contained instructions for a kind of protein never seen before. To explore the protein's function, they fused it to a fluorescent marker by joining the marker's gene to a working mei-S332 gene. They inserted the genetic blend into mei-S332 flies.

Adding these joined genes not only corrected the mutation in the insects, it enabled the Whitehead team to watch the gene's protein as they examined the formation of fruit fly sperm. During the first stage of meiosis, the protein shows up initially where the sister chromatids join and then disappears from the centromeres during the second stage, right before the sisters break apart.

The mei-S332 protein is the first protein shown to play a direct role in holding sister chromatids together, says Hawley. Orr-Weaver and her colleagues plan to look for similar proteins in mammals and examine what happens to the gluelike protein right before the sisters free themselves. It may simply disperse off the DNA, or another protein may degrade it, Orr-Weaver notes.