Is the male Y chromosome at risk of being lost? Recent work by Dr Wilson Sayres and colleagues at UC Berkeley, published in PLOS Genetics, demonstrates that the genes on the Y chromosome are important: they have probably been maintained by selection. This implies that despite its dwindling size, the Y chromosome will be sticking around.
The human Y chromosome contains 27 unique genes, compared to thousands on other chromosomes. Some mammals have already lost their Y chromosome (despite still having males, females and normal reproduction); this has led some researchers to speculate that the Y chromosome is superfluous.
As the X and Y chromosomes evolved, male-specific genes became fixed on the Y chromosome. Some of these genes were detrimental to females, so the X and Y chromosomes stopped swapping genes. This meant the Y chromosome was no longer able to correct mistakes efficiently and has thus degraded over time.
There is low genetic diversity in the human Y chromosome, and Dr Wilson Sayres and colleagues were able to precisely measure this by comparing variation on a person's Y chromosome with variation on that person's other 22 chromosomes, the X chromosome and the mitochondrial DNA. The researchers then showed that this low genetic diversity cannot be explained solely by a reduction in the number of males passing on their Y chromosome (successfully fathering male offspring). Instead, the low diversity must also result from natural selection, in this case purifying selection (the selective removal of deleterious alleles).
The movements of human populations around the world are tracked by variations in the Y chromosome. The increased understanding provided by this research will improve estimates of humans' evolutionary history.