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Male Y chromosome here to stay

x and y chromosomes

The Y chromsome (right): no longer longer puny despite being so much smaller than the X chromosome (left) (Pic: University of Illinois Laboratory High School)

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 Dr 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, almost double the previously known tally, the reseachers report.

"The Y chromosome is a hall of mirrors," says 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 Dr 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. Dr Graves had thus predicted that the chromosome might be heading for extinction within five to 10 million years.

But Page says 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.

But this deletion process may not have had entirely positive consequences because a proportion of cases of male infertility have been found to be the result of deletions of key genes in the Y chromosome. The team found, for example, that the 27 genes the Y still shares with the X often do not function on the Y, which means that it is still degrading.

Page predicts, however, that the new understanding of the Y's structure and vulnerability to decay will help to improve infertility testing for men.

"We have a greater knowledge of where the Y tends to break," says Page. "Testing needs to be updated to reflect our better understanding from the finished sequence."

Tags: health