Sex Chromosomes Won’t Become Extinct After All, Say Researchers


(CORDIS) — Scientists have refuted recent claims that sex-linked chromosomes such as the male Y chromosome could become extinct. The new claims have been made in a genetic study into the sex chromosomes of chickens, published in the journal Proceedings of the National Academy of Sciences (PNAS).

The team, who hail from Sweden and the United Kingdom, looked at how genes on sex-linked chromosomes are passed down through generations and linked to fertility, using the specific example of the W chromosome in female chickens.

W chromosomes in female chickens are akin to Y chromosomes in men in that they are sex-limited and do not re-combine when males and females reproduce, as the other regions of the genome do. This means that any studies on chickens are relevant for humans too.

The recombination process allows chromosomes to break up linked genes, which makes selection more effective and helps get rid of faulty mutations. Some scientists believe that Y and W chromosomes are set to perish because of this lack of recombination.

But this new study shows that although these chromosomes have shrunk over millions of years, and have lost many of their original genes, those that remain are extremely important in predicting fertility and are, therefore, unlikely to become extinct. Lead study author Professor Judith Mank, from University College London, says: ‘Y chromosomes are here to stay, and are not the genetic wasteland that they were once thought to be.’

The study, which received a funding boost from the European Research Council (ERC), compared DNA regions on the W chromosome in different breeds of chickens, whose fertility rates are very easy to measure simply by counting eggs.

Genetic information from two breeds, the Minorca and Leghorn, which lay more than 250 eggs per year, were compared with two breeds selected for male traits (fighting and plumage) called Yokohama and Old English Game. The researchers also looked at Red Jungle Fowl, a tropical member of the Pheasant family that is ancestral to the domestic chicken.

The scientists measured gene expression levels from the W-linked genes in all the breeds, and showed that selection for laying lots of eggs has led to elevated gene expression for almost all the W-linked genes in the layer breeds. At the same time, relaxed female selection in the fighting and plumage breeds has led to a loss of W gene expression.

This means that female-specific selection related to fertility acts to shape the W chromosome, and also that the chromosome is able to respond to that selection despite all the problems surrounding the lack of recombination.

Professor Mank comments: ‘We have shown that Y and W chromosomes are very important in fertility – the Y in males and the W in females. It is the ability of the W-linked genes to evolve that is the key to their survival, and which suggests that both the Y and the W chromosomes are with us for the long haul.’

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