Editing Self-incompatibility (section)

==Other mechanisms of self-incompatibility==
These mechanisms have received only limited attention in scientific research. Therefore, they are still poorly understood.

===2-locus gametophytic self-incompatibility===
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''.<ref name=Baumann>{{cite journal| vauthors = Baumann U, Juttner J, Bian X, Langridge P |year=2000|title=Self-incompatibility in the Grasses|journal=Annals of Botany|volume=85|issue=Supplement A|pages=203–209|doi=10.1006/anbo.1999.1056|doi-access=free|bibcode=2000AnBot..85..203B }}</ref> If the alleles expressed at these two loci in the pollen grain both match the corresponding alleles in the pistil, the pollen grain will be recognized as incompatible.<ref name=Baumann/> At both loci, ''S'' and ''Z'', two male and one female determinant can be found. All four male determinants encode proteins belonging to the same family (DUF247) and are predicted to be membrane-bound. The two female determinants are predicted to be secreted proteins with no protein family membership.<ref>{{cite journal | vauthors = Rohner M, Manzanares C, Yates S, Thorogood D, Copetti D, Lübberstedt T, Asp T, Studer B | display-authors = 6 | title = Fine-Mapping and Comparative Genomic Analysis Reveal the Gene Composition at the S and Z Self-incompatibility Loci in Grasses | journal = Molecular Biology and Evolution | volume = 40 | issue = 1 | date = January 2023 | pmid = 36477354 | pmc = 9825253 | doi = 10.1093/molbev/msac259 }}</ref><ref>{{cite journal | vauthors = Lian X, Zhang S, Huang G, Huang L, Zhang J, Hu F | title = Confirmation of a Gametophytic Self-Incompatibility in ''Oryza longistaminata'' | journal = Frontiers in Plant Science | volume = 12 | pages = 576340 | date = 2021 | pmid = 33868321 | pmc = 8044821 | doi = 10.3389/fpls.2021.576340 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Shinozuka H, Cogan NO, Smith KF, Spangenberg GC, Forster JW | title = Fine-scale comparative genetic and physical mapping supports map-based cloning strategies for the self-incompatibility loci of perennial ryegrass (Lolium perenne L.) | journal = Plant Molecular Biology | volume = 72 | issue = 3 | pages = 343–355 | date = February 2010 | pmid = 19943086 | doi = 10.1007/s11103-009-9574-y | s2cid = 25404140 }}</ref>

===Heteromorphic self-incompatibility===
<!--[[Heteromorphic self-incompatibility]] redirects here -->
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>

===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]]<ref>Glaettli, M. (2004). Mechanisms involved in the maintenance of inbreeding depression in gynodioecious Silene vulgaris (Caryophyllaceae): an experimental investigation. PhD dissertation, University of Lausanne.</ref>). 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.<ref name="bateman">{{cite journal |doi=10.1038/hdy.1956.22 | vauthors = Bateman AJ |title=Cryptic self-incompatibility in the wallflower: ''Cheiranthus cheiri'' L |journal=Heredity |volume=10 |pages=257–261 |year=1956 |issue=2|doi-access=free }}</ref> 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;<ref name="bateman"/> 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. {{cn|date=July 2024}}

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.<ref>{{cite journal | vauthors = Travers SE, Mazer SJ | title = The absence of cryptic self-incompatibility in Clarkia unguiculata (Onagraceae) | journal = American Journal of Botany | volume = 87 | issue = 2 | pages = 191–196 | date = February 2000 | pmid = 10675305 | doi = 10.2307/2656905 | name-list-style = amp | jstor = 2656905 }}</ref>

===Late-acting self-incompatibility (LSI)===
'''Late-acting self-incompatibility (LSI)''' is also termed '''[[ovary|ovarian]] self-incompatibility (OSI)'''. In this mechanism, self pollen germinates and reaches the ovules, but no [[fruit]] is set.<ref name="seavey">{{cite journal |doi=10.1007/BF02861001 | vauthors = Seavey SF, Bawa KS | title=Late-acting self-incompatibility in angiosperms |journal=Botanical Review |volume=52 |pages=195–218 |year=1986 |issue=2 | bibcode = 1986BotRv..52..195S |s2cid=34443387 }}</ref><ref>{{cite book | vauthors = Sage TL, Bertin RI, Williams EG | chapter = Ovarian and other late-acting self-incompatibility systems | veditors = Williams EG, Knox RB, Clarke AE | title = Genetic control of self-incompatibility and reproductive development in flowering plants | series = Advances in Cellular and Molecular Biology of Plants | date = 1994 | volume = 2 | pages = 116–140 | doi = 10.1007/978-94-017-1669-7_7 | publisher = Kluwer Academic | location = Amsterdam | isbn = 978-90-481-4340-5 }}</ref> LSI can be pre-[[zygote|zygotic]] (e.g. deterioration of the [[embryo sac]] prior to pollen tube entry, as in ''[[Narcissus triandrus]]''<ref>{{cite journal | vauthors = Sage TL, Strumas F, Cole WW, Barrett SC | title = Differential ovule development following self- and cross-pollination: the basis of self-sterility in Narcissus triandrus (Amaryllidaceae) | journal = American Journal of Botany | volume = 86 | issue = 6 | pages = 855–870 | date = June 1999 | pmid = 10371727 | doi = 10.2307/2656706 | s2cid = 25585101 | doi-access = free | jstor = 2656706 }}</ref>) or post-zygotic (malformation of the [[zygote]] or [[embryo]], as in certain species of ''[[Asclepias]]'' and in ''[[Spathodea campanulata]]''<ref>{{cite journal | vauthors = Sage TL, Williams EG |title=Self-incompatibility in Asclepias |journal=Plant Cell Incomp. Newsl. |volume=23 |pages=55–57 |year=1991 }}</ref><ref>{{cite journal | vauthors = Sparrow FK, Pearson NL |title=Pollen compatibility in ''Asclepias syriaca'' |journal=J. Agric. Res. |volume=77 |pages=187–199 |year=1948 }}</ref><ref name="lipow">{{cite journal | vauthors = Lipow SR, Wyatt R | title = Single gene control of postzygotic self-incompatibility in poke milkweed, Asclepias exaltata L | journal = Genetics | volume = 154 | issue = 2 | pages = 893–907 | date = February 2000 | pmid = 10655239 | pmc = 1460952 | doi = 10.1093/genetics/154.2.893 | name-list-style = amp }}</ref><ref>{{cite journal | vauthors = Bittencourt NS, Gibbs PE, Semir J | title = Histological study of post-pollination events in Spathodea campanulata beauv. (Bignoniaceae), a species with late-acting self-incompatibility | journal = Annals of Botany | volume = 91 | issue = 7 | pages = 827–834 | date = June 2003 | pmid = 12730069 | pmc = 4242391 | doi = 10.1093/aob/mcg088 }}</ref>).

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]]).<ref>{{cite book | vauthors = Klekowski EJ | date = 1988 | title = Mutation, Developmental Selection, and Plant Evolution. | publisher = Columbia University Press | location = New York }}</ref><ref>{{cite journal |doi=10.2307/2444892 | vauthors = Waser NM, Price MV |title=Reproductive costs of self-pollination in ''Ipomopsis aggregata'' (Polemoniaceae): are ovules usurped? |jstor=2444892 |journal=American Journal of Botany |volume=78 |issue=8 |pages=1036–43 |year=1991 }}</ref><ref>{{cite book | vauthors = Lughadha N | veditors = Owen SJ, Rudall PJ | chapter = Preferential outcrossing in Gomidesia (Myrtaceae) is maintained by a post-zygotic mechanism. | title = Reproductive biology in systematics, conservation and economic botany | location = London | publisher = Royal Botanic Gardens, Kew | date = 1998 | pages = 363–379 | doi = 10.13140/RG.2.1.2787.0247 }}</ref> Supporters, on the other hand, argue for the existence of several basic criteria, which differentiate certain cases of LSI from the inbreeding depression phenomenon.<ref name="seavey"/><ref name="lipow"/>