Y Chromosome Exposed
For eons the human Y chromosome has been shedding so many genes that some biologists think it could eventually vanish. Quashing that theory is the discovery that the Y has devised a way to survive, says David Page of MIT, who led the international team that in June announced the complete sequencing of the Y chromosome.
The story begins about 300 million years ago, when the Y chromosome was comparable in size to the X chromosome. At that time, it was not yet a male maker. That function emerged when male-making genes clustered on one chromosome. The congregation proved so effective that nearly all the genes stopped mingling with the X chromosome. Yet what the Y gained in male-making ability it lost in quality control. Without a partner chromosome with which to compare and correct the spelling of genes, the Y chromosome acquired lots of mistakes, and whole chunks of it were eventually lost. The once sizable Y is today but a sixth the size of its former partner, the X.
So when Page’s team started sequencing the chromosome, they expected to find a gene-free runt or a genetic jungle whose sequence would consist mostly of repetitive, difficult-to-read gobbledygook. And short, meaningless repeats did make up half the chromosome. The remainder was breathtakingly orderly, however: nine different genes, each in good working order and each present in multiple copies. Each copy of a gene was an almost perfect mirror replica of another copy. Eight enormous sequences, each containing genes, were repeated almost perfectly, forward and backward in palindrome fashion. The longest is 2.9 million letters long, more than half as long as the complete works of Shakespeare.
That area appeared to be a sanctuary of maleness: The diligently copied genetic material contained genes that are active only in the testes and are presumably devoted to sperm production. A quick look at the chimpanzee Y chromosome by Page’s MIT colleague, Steve Rozen, revealed similar coding, although spelled slightly differently. Page and Rozen concluded that some mechanism must correct the sequence of each arm of the palindrome so that it remains virtually identical to the other arm. The system, however, is clearly not at all perfect; genetic deletion on the Y chromosome is one of the best-known causes of faulty sperm production.
