Males and females developed from hermaphroditic ancestors.

Washington, Nov 21 : In a new study on plants, scientists at the University of Pittsburgh have provided evidence of the first stages of the evolution of separate sexes - a theory which holds that males and females developed from hermaphroditic ancestors.

The results of the study have also explained how plants can serve as models for understanding early evolution.

These early stages are not completely understood because the majority of animal species developed into what has been considered to be less titillating separate-sex state too long ago for scientists to observe the transition.

But, led by Tia-Lynn Ashman, a plant evolutionary ecologist in the Department of Biological Sciences in Pitt's School of Arts and Sciences, researchers have explained early separate-sex evolution in a wild strawberry species still transitioning from hermaphroditism.

The findings also apply to animals (via the unified theory) and provide the first evidence in support of the theory that the establishment of separate sexes stemmed from a genetic mutation in hermaphroditic genes that led to male and female sex chromosomes.

The separate sexes flourished with the ability to breed but spared the inbred defects of hermaphrodites.

"This is an important test of the theory of the early stages of sex chromosome evolution and part of the process of understanding the way we are today," said Ashman.

She further said that the study also shows that plants can lend insight into animal and human evolution.

"We have the opportunity to observe the evolution of sex chromosomes in plants because that development is more recent. We wouldn't see this in animals because the sex chromosomes developed so long ago. Instead, we can study a species that is in that early stage now and apply it to animals based on the unified theory that animal and plant biology often overlaps," she said.

In the current study, the researchers worked with a wild strawberry species in which the evolution of separate sexes is not complete, so hermaphrodites exist among male and female plants.

Sex chromosomes in these plants have two loci-or positions of genes on a chromosome-one that controls sterility and fertility in males and the other in females.

Offspring that inherit both fertility versions are hemaphrodites capable of self-breeding. Plants that possess one fertility and one sterility version become either male or female. Those with both sterility versions are completely sterile, cannot reproduce, and, thus, die out.

The single-sex plants breed not only with one another but also with hermaphroditic plants and pass on the mutation, which can result in single-sex offspring.

Ashman said that if they take into account inbreeding depression in hermaphrodites, a gradual decline in the number of hermaphroditic plants is to be expected.

Consequently, fewer chromosomes with both fertility versions of the loci will be passed on and the frequency of single-sex individuals will increase.

The study is published in the latest edition of "Heredity."

ANI