Self-incompatibility (SI) is a general name for several genetic mechanisms that prevent self-fertilization in sexually reproducing organisms, and thus encourage outcrossing and allogamy.
SI is best-studied and particularly common in flowering plants,[1] although it is present in other groups, including sea squirts and fungi.
SI is one of the most important means of preventing inbreeding and promoting the generation of new genotypes in plants and it is considered one of the causes of the spread and success of angiosperms on Earth.
The S-locus contains two basic protein coding regions – one expressed in the pistil, and the other in the anther and/or pollen (referred to as the female and male determinants, respectively).
This is a simplistic description of the general mechanism of SI, which is more complicated, and in some species the S-haplotype contains more than two protein coding regions.
[5] Within a decade of the initial confirmation their role in GSI, proteins belonging to the same RNase gene family were also found to cause pollen rejection in species of Rosaceae and Plantaginaceae.
Despite initial uncertainty about the common ancestry of RNase-based SI in these distantly related plant families, phylogenetic studies[7] and the finding of shared male determinants (F-box proteins)[8][9][10] strongly supported homology across eudicots.
[7][11] In the past decade, the predictions about the wide distribution of this mechanism of SI have been confirmed, placing additional support of its single ancient origin.
[5] The interaction between male and female determinants transmits a cellular signal into the pollen tube, resulting in strong influx of calcium cations; this interferes with the intracellular concentration gradient of calcium ions which exists inside the pollen tube, essential for its elongation.
At this stage, pollen inhibition is still reversible, and elongation can be resumed by applying certain manipulations, resulting in ovule fertilization.
[5] Subsequently, the cytosolic protein p26, a pyrophosphatase, is inhibited by phosphorylation,[19] possibly resulting in arrest of synthesis of molecular building blocks, required for tube elongation.
This form of SI was identified in the families: Brassicaceae, Asteraceae, Convolvulaceae, Betulaceae, Caryophyllaceae, Sterculiaceae and Polemoniaceae.
In Brassica, the pollen coat, derived from the anther's tapetum tissue, carries the translation products of the two S alleles.
Another protein essential for the SI response is MLPK, a serine-threonine kinase, which is anchored to the plasma membrane from its intracellular side.
The grass subfamily Pooideae, and perhaps all of the family Poaceae, have a gametophytic self-incompatibility system that involves two unlinked loci referred to as S and Z.
[45] However, as opposed to 'complete' or 'absolute' SI, in CSI, self-pollination without the presence of competing cross pollen, results in successive fertilization and seed set;[45] in this way, reproduction is assured, even in the absence of cross-pollination.
Criticizers claim, that absence of fruit set is due to genetic defects (homozygosity for lethal recessive alleles), which are the direct result of self-fertilization (inbreeding depression).
[54][55][56] Supporters, on the other hand, argue for the existence of several basic criteria, which differentiate certain cases of LSI from the inbreeding depression phenomenon.
[47][52] Self-compatibility (SC) is the absence of genetic mechanisms which prevent self-fertilization resulting in plants that can reproduce successfully via both self-pollen and pollen from other individuals.
However, when genetically similar SI cultivars are bred, inbreeding depression can cause a cross-incompatible form of SC to arise, such as in apricots and almonds.