Jennifer Hughes says the human Y has more or less stopped shrinking. Hughes, who works at the Whitehead Institute in Cambridge, Mass., drew this conclusion from a new comparison between human Y chromosomes and those from rhesus macaques. She and her colleagues found 19 common genes.
Since we and the macaque parted ways on the evolutionary tree 25 million years ago, both species must have kept these genes that long for a reason. Hughes believes this finding, published last week in Nature, shows the Y is down to its bare essentials and will shrink little more.
"Hopefully with this new piece of evidence, it's going to be hard to argue that the Y is doomed," she said.
Geneticist Scott Hawley, of the Stowers Institute for Medical Research in Kansas City, agrees. "Those genes have remained unchanged for 25 million years," he said. "While it is theoretically possible that the Y might disappear in another 50 million years, it is extremely unlikely," he said. "Quite frankly, it's hard for me to worry about that."
Jennifer Graves has a less optimistic view of the Y, though she isn't worried either. A biologist at the Australian National University, she has confidence that male humans are not going extinct, at least not before the female humans do. Even if the X goes away, she's excited about the possibility that we'd get a new sex-determining mechanism. With very few exceptions, she said, animals reproduce using two sexes. But there are many ways other than a Y chromosome to sort the boys from the girls.
In birds, it's the females who have a shrunken, withering chromosome. In turtles, it's the temperature of the eggs that determines sex. Some fish can switch sexes following an environmental trigger.
Hughes said the human Y chromosomes probably originated 200 million to 300 million years ago, when the lineage that was to become mammals was branching off from the one that became birds and reptiles.
The shrinkage that subsequently afflicted our Y can happen any time a male- or female-determining gene lands on a chromosome. In the case of our Y, any mutation that benefited males would spread, even if it came along with damage to other parts of the Y.
If it's any consolation to males, the Whitehead Institute's David Page, another author of the new paper, has hypothesized that the smaller the Y, the bigger the testicles. Chimp testicles are about two or three times the size of human ones, and male chimps have much higher sperm counts than men.
The male chimps benefit because female chimps tend to mate with multiple males during their fertile phases. That means it's not always the best-looking or fiercest or biggest male who passes on the most genes but the one who can release the most sperm.
Australia's Graves agrees that the human Y isn't going to shrink as fast now that it has lost most of its unessential genes. But it may yet be doomed, she said. For one thing, our Y is full of swaths of repetitive DNA that's copied forward and backward, forming palindromes of genetic code. "These monster things have taken over the Y in chimps and humans," she said. She sees them as a "desperate" attempt to keep the chromosomes viable by making extra copies of otherwise weakened genes.
The macaques don't have these genetic palindromes, she said, and perhaps that means their Y is more stable than ours.
She's fascinated by several of our fellow mammals whose males have lost the Y. Among three species of mole voles, two have no Y chromosome, and the other has one that has lost its ability to determine sex.
The Y also disappeared in two species of Japanese rats. The creatures survived because the sperm-making genes got stuck on other chromosomes, and some new male-determining gene cropped up.
She's also learning from the platypus sex chromosomes - females have 10 Xs and males five Xs and five Ys. And their Xs and Ys are not related to ours.
One thing that makes the Y so exciting to study, said Graves, is that it evolves much faster than the X and the 22 other non-sex chromosomes, known as autosomes. It's possible that losing the Y triggered the branching of those voles and rats into new species. And then there's the Japanese wrinkled frog, in which members of the same species use different sex chromosomes. In some, the female has a small chromosome, and in some, the males do.
Those populations may eventually separate themselves out and become different species, in which case the frogs give us a living example of a transitional animal - evolution in action.
Contact staff writer Faye Flam at 215-854-4977, firstname.lastname@example.org, on her blog at www.philly.com/evolution, or @fayeflam on Twitter.