Self-Incompatibility: Avoiding Inbreeding

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Plants, being anchored in position, have a special problem in this regard. Many employ the services of animals (e.g., insects, birds, bats) to transfer pollen from plant to plant. But if the flowers have both sex organs: what is to prevent the pollen from fertilizing its own eggs?

A variety of solutions have been tried in the plant kingdom.

These include: But the vast majority of angiosperms have perfect flowers; that is containing both male and female sex organs. So how do they avoid self-fertilization?

Sporophytic Self-Incompatibility (SSI)

This form of self-incompatibility has been studied intensively in members of the mustard family (Brassica), including turnips, rape, cabbage, broccoli, and cauliflower.

In this system, The rules:
The explanation:

Gametophytic Self-Incompatibility (GSI)

This form of self-incompatibility is more common than SSI but not so well understood. It occurs in nearly one-half of all the families of angiosperms, including The rules: This appears to be the mechanism in the petunia:

An entirely-different mechanism of gametophytic self-incompatibility is found in poppies (Papaver rhoeas).

Switching from Cross-Pollination to Self-Pollination

A substantial minority of angiosperms have abandoned cross-pollination for self-pollination. For example, while its wild relatives continue to be cross-pollinated, the domestic tomato is not.

Two steps are needed for this change:

Unlike its wild relatives, the stigma of the domestic tomato does not protrude beyond the anthers. Of the several genes involved in this change, the most important one is Style2.1. The mutation in Style2.1 responsible for the change in phenotype in our cultivated tomatoes is found in the promoter region — the protein-encoding portion of the gene is exactly the same as in wild tomatoes.

Here, again [Link], is evidence that much of the diversity of life arises not from mutations in the protein-coding portion of the genes that we share but mutations in their regulatory regions (promoters and enhancers).

Self-Incompatibility in Animals

Some animals are both hermaphroditic (have both male and female sex organs) and sessile (anchored in one place). So, like the plants discussed above, they also face the problem of avoiding self-fertilization. The sea squirt, Ciona intestinalis, uses a SI system that functions much like SI in plants although the recognition molecules are entirely different.

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4 February 2019