Editing Self-incompatibility (section)

===Heteromorphic self-incompatibility===
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A distinct SI mechanism exists in [[heterostyly|heterostylous]] flowers, termed '''heteromorphic self-incompatibility'''. This mechanism is probably not [[evolution]]arily related to the more familiar mechanisms, which are differentially defined as '''homomorphic self-incompatibility'''.<ref name="ganders1979">{{cite journal | vauthors = Ganders FR |title=The biology of heterostyly |journal=New Zealand Journal of Botany |volume=17 |issue=4 |pages=607–635 |year=1979 |doi=10.1080/0028825x.1979.10432574|doi-access=free |bibcode=1979NZJB...17..607G }}</ref>

Many heterostylous [[taxon|taxa]] feature SI to some extent.{{cn|date=April 2025}} The loci responsible for SI in heterostylous flowers, are strongly linked to the loci responsible for flower [[Polymorphism (biology)|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 [[stamen]]s; each stamen produces pollen capable of fertilizing only one flower morph, out of the three existing morphs.<ref name="ganders1979"/>

A population of a distylous plant contains only two SI genotypes: ss and Ss.<ref name="ganders1979"/> Fertilization is possible only between genotypes; each genotype cannot fertilize itself.<ref name="ganders1979"/> This restriction maintains a 1:1 ratio between the two genotypes in the population; genotypes are usually randomly scattered in space.<ref>{{cite journal | vauthors = Ornduff R, Weller SG | title = Pattern diversity of incompatibility groups in ''Jepsonia heterandra'' (Saxifragaceae) | journal = Evolution | volume = 29 | issue = 2 | pages = 373–375 | date = June 1975 | pmid = 28555865 | doi = 10.2307/2407228 | jstor = 2407228 }}</ref><ref>{{cite journal |doi=10.1139/b76-271 | vauthors = Ganders FR |title=Pollen flow in distylous populations of Amsinckia (Boraginaceae) |journal=Canadian Journal of Botany |volume=54 |pages=2530–5 |year=1976 |issue=22 | bibcode = 1976CaJB...54.2530G }}
</ref> Tristylous plants generally contain, in addition to the S locus, the M locus, also with two alleles.<ref name="ganders1979"/> The number of possible genotypes is greater here, but a 1:1 ratio exists between individuals of each SI type.<ref>{{cite journal | vauthors = Spieth PT | title = A necessary condition for equilibrium in systems exhibiting self-incompatible mating | journal = Theoretical Population Biology | volume = 2 | issue = 4 | pages = 404–418 | date = December 1971 | pmid = 5170719 | doi = 10.1016/0040-5809(71)90029-3 | bibcode = 1971TPBio...2..404S }}</ref>