London, June 5 (IANS): In a first, Swedish researchers have mapped the genetic structure and evolution of the female sex chromosome in birds.

In many animal species, the chromosomes differ between the sexes. The male has a Y chromosome. In birds, however, the situation is different. It is the females which have a unique sex chromosome - the W chromosome.

However, the researchers found that a bird's W chromosome does not contain genes that lead to the development of a female.

"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 produce a male, one copy (plus a W chromosome) produces a female," said one of the researchers Hans Ellegren from Uppsala University.

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.

The 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," Ellegren said.

The study was published in the journal Nature Communications.