Humans aren't the only organisms that possess taboos about inbreeding, although for plants it's not cultural, it's genetic. in order to keep plants from producing weaker offspring, they have evolved many genes to ensure they don't mate with themselves.
Although the dangers of inbreeding are somewhat overstated, it is true that the practice can increase the risk of genetic diseases or general health problems. Partially in response to these concerns, many human societies have stigmatized inbreeding. Now it turns out that plants go to even more extreme lengths to bring new contributors into the gene pool, according to research by Penn State's Teh-hui Kao and the Nara Institute's Seiji Takayama. Kao explains:
"Humans have mechanisms to prevent inbreeding that are in part cultural. 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."
The pair examined the petunia plant, which can also experience health defects due to inbreeding. They discovered the plant has an entire suite of genes that act to prevent it from being fertilized by close relatives - including itself. Like many other plants, the petunia is hermaphroditic and so possesses both sets of reproductive organs. That means it can easily pollinate itself if shifting wind currents deposit its spores back onto its flower.
To get around this, the plant has evolved what is known as self-incompatibility, in which it is able to distinguish related and unrelated genetic material. Back in 1994, Kao's team discovered the petunia possesses a gene called S-RNase, which controls self-incompatibility in the female sexual organ, the pistil. S-RNase allows the pistil to recognize which pollen is its own and specifically kill the self-pollen before it has a chance to fertilize the plant.
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Ten years later, Kao discovered the male organ equivalent with the discovery of another gene, Type-1 SLF, which further ensures pollen only reaches pistils that aren't its own by working with S-RNase proteins. These genes are targeted to prevent self-fertilization, but the effect is slightly broader. Any closely related plants will have trouble fertilizing each other under these conditions, which means plants become much more likely to pollinate unrelated flowers.
Now, Kao and Takayama have identified many additional genes that are involved in the self-incompatibility process. There are five more types of SNF genes, all of which help to ensure the male organ doesn't accidentally fertilize itself. This helps explain how the SLF genes, which helps explain how they are able to work with so many different sets of non-self S-RNase proteins.
The idea of self-incompatibility may seem very far outside our experience, but Kao says humans and all other vertebrates have something very similar in our bodies - the immune system:
"The plant needs to distinguish between non-self and self to know which plants it should breed with and which it should reject as too similar. In the same way, our bodies distinguish between non-self and self to know what to attack and what to leave alone. When this system goes awry, our bodies misidentify self as non-self and attack it. These attacks on our own tissues are known as auto-immune disorders; arthritis and Lupus are just a couple of examples."
In much the same way auto-immune disorders can weaken our bodies, failure to prevent self-fertilization can result in plants that are far less fit than their hybrid counterparts. And so plants require this large collection of genes to work together to make self-incompatibility work and to create the strongest possible offspring.