In many animal species, the chromosomes differ between the sexes. The male has a Y chromosome. In some animals, however, for example birds, it is the other way round. In birds, the females have their own sex chromosome, the W chromosome. For the first, researchers in Uppsala have mapped the genetic structure and evolution of the W chromosome.
Every individual of a species has the same sorts of chromosomes, with one exception. In many species, the way the sexes differ is that males have their own sex chromosome, the 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.
In birds, however, the situation is different. It is the females which have a unique sex chromosome – the W chromosome.
In a study published today in Nature Communications, 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.
Researchers have discovered that the W chromosome changes at a slower rate than any other part of the genetic material.
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.