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 evid
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