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Self-incompatibility is a commonly employed mechanism in flowering plants to avoid inbreeding. Although other mechanisms have evolved (dichogamy, herkogamy, dioecy), they tend to either fail to prevent self-fertilization among flowers of the same plant, or dramatically reduce the reproductive efficiency. Self-incompatibility is not universal in flowering plants. Indeed, a great many species are self-compatible. Pollinator decline, variability in pollinator service, among other factors, may favor the loss of self-incompatibility. As a result, mutants that confer the breakdown of self-incompatibility (result in self-compatibility) may become common or entirely dominate in natural populations. Similarly, human-mediated artificial selection through selective breeding may be responsible for the commonly observed self-compatibility in cultivated plants. Self-compatibility enables far more efficient breeding techniques to be employed for crop improvement.
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. The stigma or style of the pistil can recognise and reject a plant's own 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 exserted style) or thrum (with exserted stamens exserted). 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.
These are two broad groups, classified based on the phenomenological occurences (i.e. whether recognition is based on the recognition of a haploid gametophyte or diploid sporophyte). Within each of these two groups, different genetic machinery may operate to cause the self-incompatibility response.
Gametophytic self-incompatibility in plants is a widespread mechanism preventing self-fertilization and the ensuing inbreeding depression. The evolution of inbreeding depression allows for a partial purging of nearly recessive lethal mutations by selfing, and accounting for pollen limitation and sheltered load linked to the S-locus. Individual variation in selfing rates may lead to individual variation in genetic load, such that lineages that practice more self-fertilization may have lower genetic load then those that primarily outcross from the exposure to deleterious recessives to selection. If there are genetic loci that modify the selfing rate (for example, loci that influence the separation or timing of female and male reproductive fertility), then loci determining rates of self-fertilization may become associated with loci influencing genetic load.
See also
Recent Journal Articles (As of Jan 23, 2006)
- Plant self-incompatibility systems: a molecular evolutionary perspective
- Progress in study on signal transduction of gametophytic self-incompatibility
- Gametophyte interaction and sexual reproduction: how plants make a zygote
External links
"Self-incompatibility in plants"