What makes a persimmon a boy or a girl?

Only about 5 percent of plant species have separate sexes, a condition called dioecy. "Think of it as nature's best trick to ensure that reproduction involves two individuals, thus maximizing the mixing of genes," says Luca Comai. (Credit: Koshy Koshy/Flickr)

Persimmons belong to a small group of plants that are either male or female. Researchers say knowing how sex is determined could open up new possibilities for breeding plants.

Most plants have both male and female sex organs in the same individual. Some, like tomato, rice, beans, and other cultivated species, cast pollen from male to female organs in the same flower. Others employ ingenious schemes to ensure that one individual pollinates the flower of another.

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Only about 5 percent of plant species have separate sexes, a condition called dioecy, or “two houses.”

“Think of it as nature’s best trick to ensure that reproduction involves two individuals, thus maximizing the mixing of genes, says Luca Comai, professor of plant biology at University of California, Davis.

“Persimmon, pistachio, wild grapevine, kiwi, hops, spinach, and even marijuana are dioecious.”

In mammals, sex is determined by X and Y chromosomes: males have an X and a Y; females have two Xs. A single gene on the Y is responsible for triggering the development of male traits.

Molecular scissors

Most dioecious plants resemble the human system, with XY males and XX females. What gene may be responsible for determining plant sex has been a long-standing mystery.

For a new study, published in the journal Science, researchers worked on a family of persimmon trees (Diospyros lotus) established at Kyoto University. They combed through the genomes of some of these trees looking for genes that were exclusive to males and found an unusual gene they called OGI (Japanese for male tree).

Unlike most genes, OGI does not encode a protein—instead, it codes for a very small piece of RNA that acts as “molecular scissors,” cutting down expression of another gene, called MeGI (Japanese for female tree).

In females, MeGI builds to high level and acts like a neutering agent, repressing pollen formation. In males, OGI prevents accumulation of MeGI.

Plant hybrids

Regulation by RNA scissors can be fickle, and this may help explain why plants that are genetically one sex but functionally another can arise in dioecious species.

Discovery of the OGI-MeGI system in persimmon provides a comparison for parallel studies in other dioecious plant species, Comai says.

“Because separate sexes evolved independently many times in plants, we can effectively replay the evolutionary game and ask whether plants invent different solutions to the same problem or whether the same regulatory system is recruited over and over,” he says.

The findings may also have practical applications.

“Separate sexes are the most effective way to produce plant hybrids, and hybrids are key to agricultural productivity,” says Isabelle Henry.

“In the future, we may be able to breed dioecy into new species and facilitate hybrid production through exploitation of a natural system.”

The work was supported by the Japan Society for the Promotion of Science, the US Department of Energy, and the UC Davis Genome Center.

Source: UC Davis