The Y chromosome has long had an image problem. A male grasshopper lacks a Y, and a male bee stems from an egg that the queen deemed unfertilizable. Turtle eggs laid in the sun become sisters, their shaded brethren, brothers. And although most mammalian males do indeed have Ys, the two species that don't--mole voles--are apparently fine. They even copulate.
But the Y's lowly status has changed; now, it seems, the Y has been evolving, not dying, thanks to work by David Page and others at the Massachusetts Institute of Technology.1 How? Think sex.
Yes, the Y seems to have a mind of its own, and I feel shortchanged; we XXs are boring in comparison.
The Y's taint dates to 1965, when British geneticist Patricia Jacobs discovered that seven of 197 inmates at a high-security prison in Scotland had an extra Y. Similar investigations ensued into unsavory places where additional Ys might lurk. Newsweek ran a cover story on "congenital criminals." Newborn screens for the dreaded violent Y ensued in Denmark, Canada, England, and Boston, with psychologists dispatched to the homes of the marked infants to offer "anticipatory guidance." Fortunately, by 1974, the geneticists said: Wait--all that these males share is great height, acne, and learning disabilities. What might have happened had Jacobs studied basketball teams? Or worse, football players?
Meanwhile, geneticists searched for Y-linked traits, perhaps to address jealousy over the 1400 or so X loci. (Female mammals must shut off an X in each cell, so over-endowed are we.) For years the only such attribute was hairy ears, until it was discovered that girls born with the trait were simply hidden.
Fortunately, Jane Gitschier, University of California, San Francisco, took a behavioral approach, mapping such Y-linked traits as "spitting (P2E)" and "air guitar (RIF)," which mutates to "air violin" by middle age. (Inability to ask for directions is also a well-known human male trait, according to strong empirical evidence. Ask any woman.)
Circa 1990, the SRY gene, the hallmark of Y's maleness, was found. A gene called SOX3 begat SRY, and is expressed in the brain. (Did Woody Allen know this when he mumbled in his 1973 movie, Sleeper, that his brain is his second-favorite organ?) As we XXs near the end of embryohood, our lack of SRY catapults us, inevitably and inadequately, towards femaleness.
Besides SRY, a human Y did not seem like much, its puny gene contingent concerned mainly with male sexual development. "The Y is a pathetic little chromosome with lots of junk. It is gene-poor, prone to deletion, and useless. You can lack a Y and not be dead, just female," says Jennifer A. Marshall Graves, Australian National University in Canberra.
In her work, Graves found, in several species, a gradual exodus of genes from the Y to other chromosomes (about five genes per million years2), causing her to predict the eventual demise of the little chromosome that could. David Page, Dr. Y himself, and others noted the dire forecast. "It truly frightened the people in my lab." His team focused on the 23 megabases of the male- specific region.
Page recounted the saga at several recent conferences. "Back 300 million years ago, when we were reptiles, we had no sex chromosomes, only ordinary autosomes. Shortly after our ancestors parted company with the ancestors of birds, a mutation arose on one autosome to give rise to SRY. Shutting down XY crossing over began, in the vicinity of SRY, and then in an expanding region." And that spawned problems. "Y genes are not protected because they have lots of areas of no crossing over. Genes decayed, except for SRY and the tips."
The Y put up a stiff fight during sequencing; it's astounding symmetry, including eight massive palindromes, complicated the alignment phase. By chopping the Y into smaller bits, Page and colleagues discovered that many had a mate elsewhere on the Y, and that corresponding segments from one could be copied onto the other. The Y, in essence, fights shrinkage by having sex with itself. Graves dubs the newly recognized phenomenon "a desperate race to stave off disappearing altogether."
The recently unveiled Y sequence elevates the chromosome from genomic junkyard to evolutionary revelation. For in its mirror-like palindromes lie glimpses of its past and its future. The Y's stunning new view has altered its doomed future: It is a survivor. The males of our species need not fear the fate of the mole voles after all, but take pride in their mirror-like, self-recombining little Ys. Rather than being merely a default option, I wish I had a chromosome that cool.
Ricki Lewis (email@example.com) is a freelance writer in Scotia, NY.
1. H. Skaletsky et al., "The male-specific region of the human Y chromosome is a mosaic of discrete sequence classes," Nature, 423:825-37, June 19, 2003.
2. R.J. Aitken, J.A.M. Graves, "Human spermatozoa: The future of sex," Nature, 415:963, 2002.