The Y copies another chromosome's gene.

The human Y chromosome is both a degenerate and a copycat.

The first accusation is an old one. Scientists believe this male chromosome once had thousands of genes in common with the X chromosome, the other mammalian sex chromosome. Early in mammalian evolution, however, the Y chromosome apparently began to degenerate rapidly. Today, it's a shadow of its former self, containing at most several dozen functional genes.

The copycat allegation is new. Researchers studying a recently discovered gene on the Y chromosome have concluded that it originated from a similar gene on chromosome 3. The new finding may support an old theory that the Y offers a home to genes beneficial to male fitness or fertility.

The gene in question is known as DAZ. A group headed by David C. Page of the Howard Hughes Medical Institute at the Whitehead Institute for Biomedical Research in Cambridge, Mass., has found that the region of the Y chromosome containing DAZ is sometimes missing in men with very low sperm counts and in men unable to make any sperm, a condition known as azoospermia (SN: 5/18/96, p. 310). The group contends that DAZ, which stands for deleted in azoospermia, may play a crucial role in the formation of sperm.

Other researchers have shown recently that the fruit fly and mouse versions of DAZ reside not on the sex chromosomes but on the animal's other chromosomes, known as autosomes.

Intrigued by this finding, Page's group looked through the human genome for copies of DAZ. They found an almost identical gene, which they call DAZH, on chromosome 3 and discovered that the Y chromosome actually holds several copies of the DAZ gene.

After analyzing the DNA sequences of DAZ and DAZH, the team found evidence that DAZH was the original gene. Sometime during primate evolution, a copy of DAZH apparently jumped to the Y chromosome, where the gene was subsequently duplicated several times to form the cluster of DAZ genes.

Page and his colleagues suggest that this finding is at odds with theories that the Y chromosome, once it started evolving separately from the X chromosome, only degenerated. The transfer of autosomal genes to the Y chromosome may have countered this deterioration somewhat, they say.

"The case of human DAZ challenges the prevailing view that most, if not all, Y chromosomal genes were once shared with the X chromosome.... We speculate that the direct acquisition of autosomal genes that enhance male fertility is an important component of Y chromosome evolution," the investigators conclude in the November NATURE GENETICS.

"The Y chromosome should be a hot spot for male-benefit genes," agrees William R. Rice of the University of California, Santa Cruz.

Rice notes that a theory arguing that the Y chromosome will accumulate genes that are beneficial to males but potentially detrimental to females dates back more than 60 years. In a recent test of this idea, Rice added synthetic Y-like chromosomes to fruit flies and bred the insects for many generations. The synthetic sex chromosome did indeed pick up genes that improved male fitness and would be harmful to female fitness, says Rice.