Instead of doubling up to protect its genetic cargo like other chromosomes, the lone Y safeguards its genes by having sex with itself, an international consortium has found. Proving perhaps that nature has a sense of humor, scientists have discovered that the Y chromosome - the one that makes a man a man - has a remarkable ability to make do-it-yourself repairs.
June 20, 2003 — The human male sex chromosome does have the ability to repair itself and may not be headed for extinction as had previously been thought, according to a surprising new study.
A 40-strong team of researchers led by David Page of the Whitehead Institute at the Massachusetts Institute of Technology, report their findings in this week's issue of the journal Nature.
As well as having a previously unknown and elaborate back-up system for self-repair, the Y chromosome also carries 78 genes.
"The Y chromosome is a hall of mirrors," said Page, whose team has for the first time identified the full genetic sequence of a Y chromosome, from an anonymous donor.
Both the male Y and female X chromosomes are thought to have originally been the same size, but after the Y took on the sex-determining role for maleness it apparently began to lose genes. At this time it also lost the ability to pair up exactly with its partner and to swap faulty genes for good ones, as the other 22 pairs of non-sex chromosomes do.
Earlier studies had suggested that the Y chromosome carried only a few dozen genes, compared with more than 1,000 known on the X chromosome.
A team of Australian researchers led by Jenny Graves, of the Research School of Biological Sciences at the Australian National University in Canberra, previously found that the Y chromosome had been losing five genes per million years. Graves had thus predicted that the chromosome might be heading for extinction within five to 10 million years.
But Page said that the Y's full genome sequence has revealed that scientists generally had underestimated both its number of genes and its powers of self-preservation.
The team believes the Y has developed an apparently unique way of pairing up with itself. They found that many of its 50 million DNA "letters" occur in sequences known as palindromes. Like their grammatical counterparts, these sequences of letters read the same forward as backward but are arranged in opposite directions — like a mirror image — on both strands of the DNA double helix. This means that a back-up copy of each of the genes they contain occurs at each end of the sequence.
When the DNA divides during reproduction, the team believes, it opens an opportunity for genes to be shuffled or swapped and faulty copies to be deleted.
The death of the Y doesn't mean the death of men. Men need only look to the mole vole for comfort. Burrowing through the soil of western Asia are two species of these rodents (Ellobius tancrei and E. lutescens) that have lost all the genes from their Y chromosome--in fact, they no longer have a Y chromosome at all. In one of these species, both males and females have been left with just the unpaired X; in the other, both sexes have two X's. No one knows how mole voles ended up being the first mammals to cross over into the Y-free future. But along the way, they must have evolved new genes--on other chromosomes--that are responsible for making males. One of those genes took over the job of SRY, and the chromosome on which it resides is probably on its way to becoming the new Y.
If our species manages to survive for another 10 million years, our descendants will go on making men even after their Y chromosome vanishes. But the change may not be smooth. Graves speculates that several new systems for determining sex could emerge within the global human population. People conceived under one system might be genetically incompatible with those conceived under others. As a result, the human species could fragment into separate populations and, ultimately, separate species. Which of them will prefer football and which the ant nest, we'll have to wait and see.
Chromosome which is found in males. XY = male. XX = female. Sperm can be either an X or Y chromosome. During fertilization, if the Y chromosome sperm reaches the egg, then the baby is a boy. If the X chromosome sperm reaches the egg, then the baby is a girl.