green pistil surrounded closely by white anthers

A research team led by Teh-hui Kao, professor of biochemistry and molecular biology at Penn State University, in collaboration with a team lead by Professor Seiji Takayama at the Nara Institute of Science and Technology in Japan, has discovered a large suite of genes in the petunia plant that acts to prevent it from breeding with itself or with its close relatives, and to promote breeding with unrelated individuals.

In much the same way that human inbreeding sometimes results in genetic disease and inferior health, some inbred plants also experience decreased fitness, and therefore, have developed mechanisms to ensure that their offspring benefit from hybrid vigor — the mix that results when genetically distinct members of the same species breed. The team’s discovery of the multiple inbreeding-prevention genes will be published on 5 November 2010 in the journal Science. The identification of these genes comes on the heels of Kao’s earlier identification of two additional inbreeding-prevention genes in the same plant.

“Humans have mechanisms to prevent inbreeding that are in part cultural,” Kao explained. “But a plant can’t just get up and move to the next town to find a suitable, unrelated mate. Some other system must be at work.”

Kao began to unravel the mystery of what he calls a “non-self recognition system” in the mid 1980s by studying the genetic sequence of petunias. Petunias and many common garden plants are hermaphroditic, possessing both male and female reproductive organs, and these reproductive organs are located in close proximity in the same flower. This floral anatomy makes it easy for a plant’s pollen to land on itself, resulting in self-fertilization and genetically inferior, inbred offspring. To prevent self-fertilization, many flowering plants, including the petunia, have evolved a strategy called self-incompatibility, or the ability to recognize self and non-self components within both the male and female reproductive organs.

Prior Research

Because of the petunia’s hermaphroditic nature, Kao and his colleagues assumed that there had to be both male and female genetic strategies to prevent a plant from breeding with itself or with close relatives. In 1994, Kao’s team discovered the first piece of the self-incompatibility puzzle. In a paper published in Nature, he and his colleagues announced that they had identified a gene called S-RNase (S for self-incompatibility) in Petunia inflata, a wild relative of the garden petunia. The S-RNase gene controls self-incompatibility in the pistil — the plant’s female reproductive organ. Thanks to this gene, the pistil is able to distinguish between self and non-self pollen, which is analogous to sperm cells, and specifically kills self-pollen to prevent inbreeding.

Later, in another paper published in Nature in 2004, Kao’s team announced the discovery of the male counterpart of S-RNase — a gene called Type-1 SLF — that controls self-incompatibility in pollen by distinguishing between self and non-self pistil S-RNase proteins, and specifically detoxifying non-self S-RNase proteins, thereby allowing outcrossing.

That is, the team found that the S-RNase and the Type-1 SLF genes worked in concert to control the way in which the plant accepted or disallowed the introduction of particular pollen into its own reproductive system. In summary, they found that, thanks to the genetic interaction between the male-component and female-component genes, a plant pollinated by its own pollen or by pollen of a similar genotype failed to produce seeds. However, a plant pollinated by pollen of a sufficiently distinct genotype produced seeds and reproduced successfully.

More recently, Kao and his colleagues set out to fill in some important missing pieces in the self-incompatibility puzzle. “During previous research studies, other researchers who had studied the evolutionary histories of Type-1 SLF and S-RNase found no evidence of co-evolution, which was surprising as the male and female genes directly involved in controlling self/non-self recognition during sexual reproduction are expected to have co-evolved.” Kao said. “In fact, Type-1 SLF has a much shorter evolutionary history than S-RNase.”