Self-incompatibility is one strategy that evolved with the flowering plants to maintain genetic diversity in a species population and avoid inbreeding. It is not universal in the flowering plants, where a number of other morphological and/or physiological mechanisms are employed that achieve the same purpose. Indeed, a great many species are self-compatible.
Flowering plants that have hermaphroditic or perfect flowers (that is, have both male (anther) and female (pistil) reproductive organs) may be self-incompatabile to prevent self-fertilization. This self-incompatibility limits inbreeding and promotes out-breeding. The stigma of the pistil can recognise and reject self-pollen or pollen from very closely related individuals (sharing as little as one allele of one gene - not clones) so that only unrelated pollen grains achieve fertilization.
Some plants, such as Primula, have morphological differences between plants to prevent self-pollination. Primula flowers are heterostylous and the flowers are either pin (with the capita of the style excerted) or thrum (with the stamens excerted). Insect pollinators visiting Primula flowers are likely to encounter either stamens or the style but not both.
There are two types of self-incompatibility: gametophytic and sporophytic
- Gametophytic self-incompatibility is the most common type found in flowering plants.
- Sporophytic self-incompatibility involves the phenotype of the pollen being determined by the diploid genome of the parent plant.