- These mechanisms are less abundant and have received only limited attention in scientific research. Therefore, they are still poorly understood.
Heteromorphic self-incompatibility
- A distinct SI mechanism exists in heterostylous flowers, termed heteromorphic self-incompatibility. This mechanism is probably not evolutionarily related to the more familiar mechanisms, which are differentially defined as homomorphic
- self-incompatibility.
- Almost all heterostylous taxa feature SI to some extent. The loci responsible for SI in heterostylous flowers are strongly linked to the loci responsible for flower polymorphism, and these traits are inherited together. Distyly is determined by a single locus, which has two alleles; tristyly is determined by two loci, each with two alleles. Heteromorphic SI is sporophytic, i.e. both alleles in the male plant, determine the SI response in the pollen.
- SI loci always contain only two alleles in the population, one of which is dominant over the other, in both pollen and pistil. Variance in SI alleles parallels the variance in flower morphs, thus pollen from one morph can fertilize only pistils from the other morph. In tristylous flowers, each flower contains two types of stamens; each stamen produces pollen capable of fertilizing only one flower morph, out of the three existing morphs.
- A population of a distylous plant contains only two SI genotypes: ss and Ss. Fertilization is possible only between genotypes; each genotype cannot fertilize itself. This restriction maintains a 1:1 ratio between the two genotypes in the population; genotypes are usually randomly scattered in space. Tristylous plants contain, in addition to the S locus, the M locus, also with two alleles. The number of possible genotypes is greater here, but a 1:1 ratio exists between individuals of each SI type.
Cryptic self-incompatibility (CSI)
- Cryptic self-incompatibility (CSI) exists in a limited number of taxa (for example, there is evidence for CSI in Silene vulgaris, (Caryophyllaceae). In this mechanism, the simultaneous presence of cross and self pollen on the same stigma, results in higher seed set from cross pollen, relative to self pollen. 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; in this way, reproduction is assured, even in the absence of cross-pollination.
- CSI acts, at least in some species, at the stage of pollen tube elongation, and leads to faster elongation of cross pollen tubes, relative to self pollen tubes. The cellular and molecular mechanisms of CSI have not been described. The strength of a CSI response can be defined, as the ratio of crossed to selfed ovules, formed when equal amounts of cross and self pollen, are placed upon the stigma; in the taxa described up to this day, this ratio ranges between 3.2 and 11.5.
Late-acting Self-Incompatibility (LSI)
- Late-acting Self-Incompatibility (LSI) is also termed Ovarian Self-Incompatibility (OSI). In this mechanism, self pollen germinates and reaches the ovules, but no fruit is set. LSI can be pre-zygotic (e.g. deterioration of the embryo sac prior to pollen tube entry, as in Narcissus triandrus) or post-zygotic (malformation of the zygote or embryo, as in certain species of Asclepias and in Spathodea campanulata.
- The existence of the LSI mechanism among different taxa and in general is subject for scientific debate. 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). Supporters, on the other hand, argue for the existence of several basic criteria, which differentiate certain cases of LSI from the inbreeding depression phenomenon
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