Fungi may help unravel the origin of sex chromosomes

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Washington, March 18 : Swedish researchers have found that there are striking similarities between the parts of DNA that determine mating types in certain fungi and the parts of DNA that determine the sex of plants and animals.

Hanna Johannesson, an Assistant Professor in the Department of Evolutionary Biology at Uppsala University, says that her findings suggest that fungi may be used as new model organisms in studies of the evolutionary development of sex chromosomes.

In the plant and animal kingdoms there are individuals of different sexes -- bearers of many tiny sex cells are called males, while bearers of a few large ones are called females.

Whereas, in fungi kingdom, there are no sexes but rather a simpler and more primitive system of different so-called mating types, which are distinguished by different variants of a few specific genes.

In humans sex chromosomes help determine sex. It is thought that this sex difference arose in the plant and animal kingdom from the simpler system of mating types and that this happened several times independently of each other throughout evolution.

The change is believed to have happened with the inhibition of a step in the copying process in DNA, which led to two separate chromosomes, which then developed further over a long period of time.

"In humans, sex chromosomes are believed to have developed over the last 300 million years from a common 'proto-sex chromosome,'" says Hanna Johannesson, who directed the study.

Hanna claims that her study is the first to show that, even though fungi do not have sexes, there are many similarities between the parts of the genome that determine sex in plants and animals and the parts of the genome that control mating types in certain fungi.

Her research group specifically studied a spore sac fungus (Neurospora tetrasperma), and found that the similarities were great, regarding both present-day structure and the way in which it arose.

"It's hard to study the evolution of sex chromosomes, partly because so many different and important sex-specific characters are tied to them. But much of this can be avoided if we use simpler systems, like fungi, as models," Hanna says.

ANI

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