Taboo might be enough to control intermarriage among people, but flowering plants equipped with both male and female reproductive organs have been forced to evolve some ingenious methods to avoid inbreeding.

Penn State University researchers have found a genetic missing link for one such method, called self-incompatibility, that is used by plants as diverse as apple trees, potatoes and snapdragons.

In these plants, the female organ, called the pistil, identifies and rejects its own pollen, accepting only pollen that is produced by other plants of the same species and transferred via wind or insects.

It's possible, said Teh-hui Kao, the molecular biologist who led the research team, that this mechanism could eventually be transferred to crops such as corn and soybeans that are prone to inbreeding.

Though detasseling of corn to prevent self-pollination has been the first summer job for many a teen in farm communities, Kao said he doesn't expect to put anyone out of a job anytime soon. "There's still a lot of work that needs to be done," he explained, before self-incompatibility can be transferred to today's hybrid crops.

Working with wild petunias, Kao's team identified the gene that accounts for the male half of this self-incompatibility system. The findings, reported last week in the journal Nature, complete the genetic puzzle Kao began 10 years ago, when his research team identified the gene that controls self-incompatibility in the pistil, the female organ.

"Given the new findings, research can now shift towards decoding the biochemical details of the exchanges between pollen and pistil," said Bruce McClure, a biochemist at the University of Missouri-Columbia, who wrote a commentary that accompanied Kao's paper.

Normally, when pollen lands on a petunia's stigma, it germinates to produce tubes that grow through the pistil to reach the plant ovaries, where fertilization occurs. But pollen tube growth ends abruptly if the pollen is incompatible.

Kao had earlier reported that self-incompatibility in the pistil was controlled by the S-RNase gene, which produces an enzyme that degrades RNA. Each species has 50 or 60 different versions of this particular gene and each plant will have up to two of them, one variant gene from each parent. Petunia pollen carries just one copy of this gene and the plant's pistil is primed to reject the pollen if that variant matches either of the variant genes in that plant.

But scientists knew this could take place only by an interaction between the proteins produced by the S-RNase gene and those of a genetic counterpart in the pollen. "It takes two to tango," Kao said. Because it is so crucial that these genes coincide, the two genes are all but certain to be located near each other on a chromosome.

Many researchers looked for the gene, but they found a number of candidates and it has proven difficult to determine which one was the one.

Kao was able to show that a gene called PiSLF, which produces a so-called "F-box" protein involved in protein degradation, was the long-sought gene. His experiment involved transferring the PiSLF gene from one plant to another and evaluating its effects on pollen compatibility. Its details, he admitted, "are complicated even for scientists to understand," but the transferred gene behaved exactly as predicted.

McClure agreed that the experiment proved conclusively that the PiSLF gene was the long-sought male counterpart to the S-RNase gene.

This mechanism is used by a wide variety of plants, including tomatoes, tobacco and cherry trees, but other plants, such as cabbage, have evolved different self-incompatibility methods. "Plants are very clever," Kao said. "They have come up with a number of ways to do this."

British researchers at the University of Birmingham, for instance, reported in the same Nature issue on a different mechanism used by the common poppy. It triggers programmed cell death in self-incompatible pollen.

Though inbreeding makes a species less diverse over time and more vulnerable to environmental changes, self-incompatibility was bred out of many crops as growers sought to breed for specific, desirable characteristics, Kao said. At the same time, hybrid crops have become a mainstay of modern agriculture because hybrids have proven more productive and more uniform in quality than plants that are self-pollinated or pollinated by plants of the same species.

If self-incompatibility can be added back into hybrid seed, the need for labor-intensive detasseling would be eliminated and hybrid seed production could be more efficient.