In many animal species, the chromosomes differ between the sexes - the male has a Y chromosome. This contains genes which result in the development of male characters and reproductive organs. If there is no Y chromosome, the organism will be a female.

But in birds and some other animals, it is the other way round and females have their own sex chromosome, the W chromosome. 

In a new study, Linnea Smeds, Hans Ellegren and their colleagues show that, surprisingly enough, a bird's W chromosome does not contain genes that lead to the development of a female. The W chromosome seems instead to function as some kind of buffer for females since it contains genes similar to those in the X chromosome. In order for certain genes to work, it is critical that an individual has two copies of that gene. In this way, the W chromosome can serve as a complement for females who only have one copy of the X chromosome.

"Sex determination in birds and other animals with a W chromosome seems instead to depend upon the number of their equivalent to the X chromosome. Two copies of it produces a male, one copy (plus a W chromosome) produces a female," says Ellegren.

Researchers have discovered that the W chromosome changes at a slower rate than any other part of the genetic material.

"This is because it is only inherited on the maternal side and fewer mutations arise in females than in males," says Ellegren.

Most mutations occur during the formation of germ cells. Males produce a vastly greater number of germ cells than females and so the probability that a sperm contains a new mutation is much greater than for an egg cell.

The W chromosome is the only chromosome from the cell nucleus which is inherited on the maternal side. It shares this property with the small amount of DNA found in cell mitochondria.

Citation: Ellegren et al (2015) Evolutionary analysis of the female-specific avian W chromosome, Nature Communications 6, Article number:7330,doi:10.1038/ncomms8330