13 January 2014

Matthew Thomas By

Appeared in BioNews 737

The human Y chromosome may have some use, after all. Experts previously thought that the chromosome containing 'male' genes was shrinking to the point of extinction.

A study published in PLOS Genetics suggests that natural selection may be maintaining genes on the Y chromosome. This chromosome 'tells' an embryo to develop biologically male characteristics. Many of the tiny Y's 27 genes are linked to male fertility.

Most chromosomes come in pairs and swap DNA when cells divide into sex cells – a process known as recombination. The Y chromosome, however, has no genetic partner. Researchers think this can lead to damage from harmful mutations building up over time. This may cause DNA to be discarded and the chromosome to dwindle.

'The Y chromosome has lost 90 percent of the genes it once shared with the female X chromosome, and some scientists have speculated that the Y chromosome will disappear in less than five million years', said Dr Melissa Wilson Sayres, an evolutionary biologist from the University of California, Berkeley and lead author of the study.

'Our study demonstrates that the genes that have been maintained, and those that migrated from the X to the Y, are important, and the human Y is going to stick around for a long while'.

The researchers analysed the complete genomes of 16 men – eight from Africa, eight from Europe. They found that Y chromosome variation between the men was very small, suggesting a 'purifying' form of natural selection has whittled it down to the bare necessities.

The continued existence of the 27 Y chromosomes genes may mean they play a vital role. Seventeen of these genes have remained intact after 200 million years of evolution. The other ten, called ampliconic genes, were more recently acquired.

According to Dr Wilson Sayres: 'These ampliconic regions that we haven’t really understood until now are evidently very important and probably should be investigated and studied for fertility'.

A common alternative explanation for the lack of Y chromosome variation in humans is that relatively few men fathered a disproportionate number of children. This is popularly known as the 'Genghis Khan effect', after the Mongol leader whose Y chromosome can still be found in 0.5 percent of males worldwide.

The study demonstrated that if this explanation were true, fewer than one in four males could have passed on their Y chromosomes to future generations.