130-year-old pollination mystery solved.

Botanists have struggled to piece together how some plants avoid self-fertilization -- and the loss of genetic diversity that goes hand-in-hand with it -- ever since Charles Darwin noticed that some plants can fertilize themselves while others cannot.

Now, biologists at Pennsylvania State University in University Park have found the first direct evidence of a self-incompatibility gene, or S gene. Biochemist Teh-hui Kao and his colleagues confirm in the Feb. 10 NATURE a theory of genetic self-incompatibility underlying years of research in plant genetics.

According to this theory, a plant that cannot fertilize itself has an S gene that is turned on, causing it to make an S protein in the pistil that recognizes and rejects its own pollen. Most plants have at least two versions, or alleles, of this gene. Plants with an S gene that is turned off do not produce this S protein, so they can fertilize themselves.

Working with S2 and S3 alleles in petunias, Kao first triggered a loss of self-recognition by using bacteria to insert antisense DNA, which mirrors normal DNA, into the flower's genome. Just how antisense technology prevents translation of the normal DNA into protein remains a mystery, Kao says.

The genetically modified petunias no longer produced the S3 allele's protein and, because of the similarity between the two alleles, sometimes did not produce the S2 protein. When fertilized with genetically similar S3 pollen, Kao found, "they produced the same large number of seeds as you would get from compatible pollination."

In a second experiment, Kao's team slipped an extra allele, S3, into petunias containing S1 and S2 alleles. If these plants produced normal amounts of S3 protein, they formed no seeds at all when pollinated with S1, S2, or S3 pollen. Petunias with low S3 expression produced fewer seeds than usual. This, says Kao, shows that S proteins alone control a plant's ability to reject its own pollen.

More than half of all flowering plants depend on the S gene to prevent self-pollination. Others separate male and female parts so they develop on different plants or at different times. And a little bit of inbreeding isn't always a bad thing: If the plant population is changing rapidly, inbreeding can prevent the parent species from being swamped by its variable offspring.

Kao is still in search of his "holy grail" -- the precise biochemical mechanism behind self-incompatibility. He wants to understand exactly how the S proteins inhibit self-pollination. What he's already found, however, may give a big boost to commercial growers, whose major crops grow from hybrid seeds.

"The potential could be quite high to create hybrid seeds at a reasonable cost," says Robert Griesbach, a research geneticist at the U.S. Department of Agriculture in Beltsville, Md. "This basic work may allow us to produce hybrids in many crops where this technique has previously been inefficient or impossible."