How Plants Avoid Incest: Self-Incompatibility Explained

My brain is ded (junioritis?!?!?) so I’m just doing something easy for today. It’s still cool tho, and there was a head-scratcher about this topic on ARBC online quals individual round, so it’s still important and stuff. Anyways, enjoy :) And good luck if you’re at camp anywhere!!

Plants, like pretty much all organisms, are better off in populations with higher genetic diversity, as this allows resilience in adverse conditions. However, plants do not have many controllable behaviors that they can use to prevent self-fertilization, as they cannot move or think. Because plant reproduction consists of pollen meeting ovary, plants can avoid self-fertilization by interrupting this union. For example, dioecious species have the male and female reproductive organs on separate flowers, preventing self-fertilization (at least, within the single flower). Each flower, then, is imperfect, as it lacks one sex’s reproductive organs. Another mechanism by which plants “avoid incest” is by having male and female parts mature at different times, in a phenomenon called dichogamy.

Self-incompatibility is a cooler way for plants to reject their own genetic material, and doesn’t require any anatomical/“behavioral” adaptations. Essentially, a plant is able to reject its own pollen. There are two mechanisms for this: gametophytic and sporophytic, both of which rely on S alleles.

The gametophytic mechanism prevents sperm from reaching the egg if the pollen’s S allele matches any of the sporophyte’s. For example, imagine that a pollen comes from an S1S2 plant. Let’s say meiosis has decided the pollen’s fate to carry the S1 allele. This pollen floats over to a S2S3 flower. S1 doesn’t match any of the alleles on the S2S3 stigma, so fertilization is successful. Imagine, however, that meiosis made S2 the allele of the pollen grain. S2 matches an allele on the S2S3 flower; thus, this pollen would be rejected. The mechanism of rejection for gametophytic incompatibility is that the pollen’s RNA is broken down by a ribonuclease (S-RNase) while the tube attempts to grow into the style.

The sporophytic mechanism prevents pollen from traveling past the stigma if either of the pollen source plant’s S alleles match any of the stigma’s. This is determined before meiosis; any matching S alleles in the diploid sporophyte genotype that match between organisms will prevent germination, regardless of the pollen’s specific genotype. For example, an S1 grain from an S1S2 flower will not germinate on an S2S4 flower because the diploid genotypes are incompatible (common S2).

Here’s an image that summarizes this nicely:

So yeah, that’s pretty much it. As always, please lmk article ideas. Thanks :)

Also, here’s a helpful article (way more detailed than mine, great read): https://www.biology-pages.info/S/SelfIncompatibilty.html