Size matters in chromosomes

When it comes to the chromosomes that determine sex, it seems that often size is important and it is the male who winds up with far less to boast about than the female. At least in the cases of humans and fruit flies. Scientists are beginning to understand just why the "male" Y-chromosome is so much smaller than its female counterpart.

A study by Soojin Yi, at the University of Chicago, and Brian Charlesworth, at the University of Edinburgh, has shown this may all be down to male genes lacking the ability to weed out mutants.

As a result, growing numbers of Y's genes get damaged, large tracts eventually becoming useless, and, ultimately, degenerate chunks may fall away over millions of years of evolution.

The work, published in the latest issue of the journal Molecular Biology and Evolution, finds evidence for this gradual degeneration.

"Our study has confirmed that the variation in genes on the Y-chromosome was very small, showing the natural selection necessary to keep the chromosome active had become highly inefficient," said Dr Yi.

The scientists were able to watch Y degenerate through the generations by studying Drosophila miranda, a species of fruit fly in which this process has started relatively recently in evolutionary history.

The problem appears to come down to the inability of Y to swap genetic information as it is never paired with its like even in male organisms.

The X-chromosome, on the other hand, is paired with another X in female organisms and hence recombination, as the process is called, can occur.

For the handful of genes directly involved in sex determination, damaging mutations simply prevent reproduction and hence natural selection steps in right at the start. But this does not happen for the rest of the genes on Y, which pick up mutations they cannot shake off, become irreparably damaged and simply fall out of use.

Dr Yi's research suggests those genes therefore wind up with very little variation from one fruit fly to another. Natural selection has stopped working efficiently.

In humans and our ancestors this process has been ongoing for more than 100 million years. The result is painfully obvious - while X looks like a normal chromosome, little more than a stump of Y remains.