self-incompatibility

The best studied mechanisms of SI act by inhibiting the germination of pollen on stigmas, or the elongation of the pollen tube in the styles. These mechanisms are based on protein-protein interactions, and the best-understood mechanisms are controlled by a single locus termed S, which has many different alleles in the species population. Despite their similar morphological and genetic manifestations, these mechanisms have evolved independently, and are based on different cellular components;{{cite journal | vauthors = Charlesworth D, Vekemans X, Castric V, Glémin S | title = Plant self-incompatibility systems: a molecular evolutionary perspective | journal = The New Phytologist | volume = 168 | issue = 1 | pages = 61–69 | date = October 2005 | pmid = 16159321 | doi = 10.1111/j.1469-8137.2005.01443.x |doi-access=free | bibcode = 2005NewPh.168...61C }} therefore, each mechanism has its own, unique S-genes.

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). Due to their physical proximity, these are genetically linked, and are inherited as a unit. The units are called S-haplotypes. The translation products of the two regions of the S-locus are two proteins which, by interacting with one another, lead to the arrest of pollen germination and/or pollen tube elongation, and thereby generate an SI response, preventing fertilization. However, when a female determinant interacts with a male determinant of a different haplotype, no SI is created, and fertilization ensues. 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. {{cn|date=July 2024}}

Following is a detailed description of the different known mechanisms of SI in plants. {{cn|date=July 2024}}

In gametophytic self-incompatibility (GSI), the SI phenotype of the pollen is determined by its own gametophytic haploid genotype. This is the most common type of SI.{{cite book |doi=10.1016/S0074-7696(08)62485-7 | vauthors = Franklin FC, Lawrence MJ, Franklin-Tong VE |author3-link= Vernonica Franklin-Tong|title=Cell and Molecular Biology of Self-Incompatibility in Flowering Plants |journal=Int. Rev. Cytol. |volume=158 |pages=1–64 |year=1995 |series=International Review of Cytology |isbn=978-0-12-364561-6 }} Two different mechanisms of GSI have been described in detail at the molecular level, and their description follows. {{cn|date=July 2024}}

In this mechanism, pollen tube elongation is halted when it has proceeded approximately one third of the way through the style.{{cite journal | vauthors = Franklin-Tong VE, Franklin FC | title = The different mechanisms of gametophytic self-incompatibility | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 358 | issue = 1434 | pages = 1025–1032 | date = June 2003 | pmid = 12831468 | pmc = 1693207 | doi = 10.1098/rstb.2003.1287 | name-list-style = amp }} The female component ribonuclease protein, termed S-RNase{{cite journal | vauthors = McClure BA, Haring V, Ebert PR, Anderson MA, Simpson RJ, Sakiyama F, Clarke AE | title = Style self-incompatibility gene products of Nicotiana alata are ribonucleases | journal = Nature | volume = 342 | issue = 6252 | pages = 955–957 | year = 1989 | pmid = 2594090 | doi = 10.1038/342955a0 | s2cid = 4321558 | doi-access = | bibcode = 1989Natur.342..955M }} probably causes degradation of the ribosomal RNA (rRNA) inside the pollen tube, in the case of identical male and female S alleles, and consequently pollen tube elongation is arrested, and the pollen grain dies.

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{{cite journal | vauthors = Igic B, Kohn JR | title = Evolutionary relationships among self-incompatibility RNases | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 98 | issue = 23 | pages = 13167–13171 | date = November 2001 | pmid = 11698683 | pmc = 60842 | doi = 10.1073/pnas.231386798 | name-list-style = amp | doi-access = free }} and the finding of shared male determinants (F-box proteins){{cite journal | vauthors = Qiao H, Wang F, Zhao L, Zhou J, Lai Z, Zhang Y, Robbins TP, Xue Y | display-authors = 6 | title = The F-box protein AhSLF-S2 controls the pollen function of S-RNase-based self-incompatibility | journal = The Plant Cell | volume = 16 | issue = 9 | pages = 2307–2322 | date = September 2004 | pmid = 15308757 | pmc = 520935 | doi = 10.1105/tpc.104.024919 | bibcode = 2004PlanC..16.2307Q }}{{cite journal | vauthors = Ushijima K, Yamane H, Watari A, Kakehi E, Ikeda K, Hauck NR, Iezzoni AF, Tao R | display-authors = 6 | title = The S haplotype-specific F-box protein gene, SFB, is defective in self-compatible haplotypes of Prunus avium and P. mume | journal = The Plant Journal | volume = 39 | issue = 4 | pages = 573–586 | date = August 2004 | pmid = 15272875 | doi = 10.1111/j.1365-313X.2004.02154.x | doi-access = free }}{{cite journal | vauthors = Sijacic P, Wang X, Skirpan AL, Wang Y, Dowd PE, McCubbin AG, Huang S, Kao TH | display-authors = 6 | title = Identification of the pollen determinant of S-RNase-mediated self-incompatibility | journal = Nature | volume = 429 | issue = 6989 | pages = 302–305 | date = May 2004 | pmid = 15152253 | doi = 10.1038/nature02523 | s2cid = 4427123 | bibcode = 2004Natur.429..302S }} strongly supported homology across eudicots. Therefore, this mechanism likely arose approximately 90 million years ago, and is the inferred ancestral state for approximately 50% of all plant species.{{cite journal | vauthors = Steinbachs JE, Holsinger KE | title = S-RNase-mediated gametophytic self-incompatibility is ancestral in eudicots | journal = Molecular Biology and Evolution | volume = 19 | issue = 6 | pages = 825–829 | date = June 2002 | pmid = 12032238 | doi = 10.1093/oxfordjournals.molbev.a004139 | name-list-style = amp | doi-access = free }}

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. Specifically, a style-expressed T2/S-RNase gene and pollen-expressed F-box genes are now implicated in causing SI among the members of Rubiaceae,{{cite journal | vauthors =Asquini E, Gerdol M, Gasperini D, Igic B, Graziosi G, Pallavicini A |title=S-RNase-like Sequences in Styles of Coffea (Rubiaceae). Evidence for S-RNase Based Gametophytic Self-Incompatibility?|journal=Tropical Plant Biology |volume=4|pages=237–249|year=2011|issue=3–4|doi=10.1007/s12042-011-9085-2|s2cid=11092131}} Rutaceae,{{cite journal | vauthors = Liang M, Cao Z, Zhu A, Liu Y, Tao M, Yang H, Xu Q, Wang S, Liu J, Li Y, Chen C, Xie Z, Deng C, Ye J, Guo W, Xu Q, Xia R, Larkin RM, Deng X, Bosch M, Franklin-Tong VE, Chai L | display-authors = 6 | title = Evolution of self-compatibility by a mutant S_m-RNase in citrus | journal = Nature Plants | volume = 6 | issue = 2 | pages = 131–142 | date = February 2020 | pmid = 32055045 | pmc = 7030955 | doi = 10.1038/s41477-020-0597-3 | bibcode = 2020NatPl...6..131L }} and Cactaceae.{{cite journal | vauthors = Ramanauskas K, Igić B | title = RNase-based self-incompatibility in cacti | journal = The New Phytologist | volume = 231 | issue = 5 | pages = 2039–2049 | date = September 2021 | pmid = 34101188 | doi = 10.1111/nph.17541 | name-list-style = amp | s2cid = 235370441 | doi-access = free }} Therefore, other mechanisms of SI are thought to be recently derived in eudicots plants, in some cases relatively recently. One particularly interesting case is the SI expressed in Prunus species, which functions through self-recognition{{Cite journal |vauthors=Matsumoto D, Tao R |date=2016 |title=Distinct Self-recognition in the Prunus S-RNase-based Gametophytic Self-incompatibility System |url=https://www.jstage.jst.go.jp/article/hortj/85/4/85_MI-IR06/_article |journal=The Horticulture Journal |language=en |volume=85 |issue=4 |pages=289–305 |doi=10.2503/hortj.MI-IR06 |issn=2189-0102 |doi-access=free |access-date=2022-09-28 |archive-date=2022-09-28 |archive-url=https://web.archive.org/web/20220928084909/https://www.jstage.jst.go.jp/article/hortj/85/4/85_MI-IR06/_article |url-status=live }} (the cytotoxic activity of the S-RNases is inhibited by default and selectively activated by the pollen partner S-haplotype-specific F-box protein (SFB) upon self-pollination), while SI in the other species with S-RNase functions through non-self recognition (the S-RNases are selectively detoxified upon cross-pollination).

In this mechanism, pollen growth is inhibited within minutes of its placement on the stigma. The mechanism is described in detail for Papaver rhoeas and so far appears restricted to the plant family Papaveraceae. {{cn|date=July 2024}}

The female determinant is a small, extracellular molecule, expressed in the stigma; the identity of the male determinant remains elusive, but it is probably some cell membrane receptor. 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.{{cite journal |doi=10.1046/j.1365-313X.1993.04010163.x | vauthors = Franklin-Tong VE, Ride JP, Read ND, Trewavas AJ, Franklin FC |title=The self-incompatibility response in Papaver rhoeas is mediated by cytosolic free calcium |journal=Plant J. |volume=4 |pages=163–177 |year=1993 |doi-access=free }}{{cite journal |doi=10.1046/j.1365-313x.1997.12061375.x | vauthors= Franklin-Tong VE, Hackett G, Hepler PK |title=Ratioimaging of Ca21 in the self-incompatibility response in pollen tubes of Papaver rhoeas |journal=Plant J. |volume=12 |pages=1375–86 |year=1997 |issue=6 |doi-access=free }}{{cite journal | vauthors = Franklin-Tong VE, Holdaway-Clarke TL, Straatman KR, Kunkel JG, Hepler PK | title = Involvement of extracellular calcium influx in the self-incompatibility response of Papaver rhoeas | journal = The Plant Journal | volume = 29 | issue = 3 | pages = 333–345 | date = February 2002 | pmid = 11844110 | doi = 10.1046/j.1365-313X.2002.01219.x | s2cid = 954229 | doi-access = free }} The influx of calcium ions arrests tube elongation within 1–2 minutes. At this stage, pollen inhibition is still reversible, and elongation can be resumed by applying certain manipulations, resulting in ovule fertilization.

Subsequently, the cytosolic protein p26, a pyrophosphatase, is inhibited by phosphorylation,{{cite journal | vauthors = Rudd JJ, Franklin F, Lord JM, Franklin-Tong VE | title = Increased Phosphorylation of a 26-kD Pollen Protein Is Induced by the Self-Incompatibility Response in Papaver rhoeas | journal = The Plant Cell | volume = 8 | issue = 4 | pages = 713–724 | date = April 1996 | pmid = 12239397 | pmc = 161131 | doi = 10.1105/tpc.8.4.713 }} possibly resulting in arrest of synthesis of molecular building blocks, required for tube elongation. There is depolymerization and reorganization of actin filaments, within the pollen cytoskeleton.{{cite journal | vauthors = Geitmann A, Snowman BN, Emons AM, Franklin-Tong VE | title = Alterations in the actin cytoskeleton of pollen tubes are induced by the self-incompatibility reaction in Papaver rhoeas | journal = The Plant Cell | volume = 12 | issue = 7 | pages = 1239–1251 | date = July 2000 | pmid = 10899987 | pmc = 149062 | doi = 10.1105/tpc.12.7.1239 | bibcode = 2000PlanC..12.1239G }}{{cite journal | vauthors = Snowman BN, Kovar DR, Shevchenko G, Franklin-Tong VE, Staiger CJ | title = Signal-mediated depolymerization of actin in pollen during the self-incompatibility response | journal = The Plant Cell | volume = 14 | issue = 10 | pages = 2613–2626 | date = October 2002 | pmid = 12368508 | pmc = 151239 | doi = 10.1105/tpc.002998 | bibcode = 2002PlanC..14.2613S }} Within 10 minutes from the placement on the stigma, the pollen is committed to a process which ends in its death. At 3–4 hours past pollination, fragmentation of pollen DNA begins,{{cite journal | vauthors = Jordan ND, Franklin FC, Franklin-Tong VE | title = Evidence for DNA fragmentation triggered in the self-incompatibility response in pollen of Papaver rhoeas | journal = The Plant Journal | volume = 23 | issue = 4 | pages = 471–479 | date = August 2000 | pmid = 10972873 | doi = 10.1046/j.1365-313x.2000.00811.x | doi-access = free }} and finally (at 10–14 hours), the cell dies apoptotically.{{cite journal | vauthors = Thomas SG, Franklin-Tong VE | title = Self-incompatibility triggers programmed cell death in Papaver pollen | journal = Nature | volume = 429 | issue = 6989 | pages = 305–309 | date = May 2004 | pmid = 15152254 | doi = 10.1038/nature02540 | name-list-style = amp | s2cid = 4376774 | bibcode = 2004Natur.429..305T }}

In sporophytic self-incompatibility (SSI), the SI phenotype of the pollen is determined by the diploid genotype of the anther (the sporophyte) in which it was created. This form of SI was identified in the families: Brassicaceae, Asteraceae, Convolvulaceae, Betulaceae, Caryophyllaceae, Sterculiaceae and Polemoniaceae.{{cite journal |doi=10.1038/hdy.1997.177 | vauthors = Goodwillie C |title=The genetic control of self-incompatibility in Linanthus parviflorus (Polemoniaceae) |journal=Heredity |volume=79 |pages=424–432 |year=1997 |issue=4|doi-access=free }} Up to this day, only one mechanism of SSI has been described in detail at the molecular level, in Brassica (Brassicaceae). {{cn|date=July 2024}}

Since SSI is determined by a diploid genotype, the pollen and pistil each express the translation products of two different alleles, i.e. two male and two female determinants. Dominance relationships often exist between pairs of alleles, resulting in complicated patterns of compatibility/self-incompatibility. These dominance relationships also allow the generation of individuals homozygous for a recessive S allele.{{cite journal | vauthors = Hiscock SJ, Tabah DA | title = The different mechanisms of sporophytic self-incompatibility | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 358 | issue = 1434 | pages = 1037–1045 | date = June 2003 | pmid = 12831470 | pmc = 1693206 | doi = 10.1098/rstb.2003.1297 | name-list-style = amp }}

Compared to a population in which all S alleles are co-dominant, the presence of dominance relationships in the population raises the chances of compatible mating between individuals. The frequency ratio between recessive and dominant S alleles reflects a dynamic balance between reproductive assurance (favoured by recessive alleles) and avoidance of selfing (favoured by dominant alleles).{{cite journal | vauthors = Ockendon DJ |title=Distribution of self-incompatibility alleles and breeding structure of open-pollinated cultivars of Brussels sprouts |journal=Heredity |volume=32 |issue=2 |pages=159–171 |year=1974 |doi=10.1038/hdy.1974.84|doi-access=free }}

As previously mentioned, the SI phenotype of the pollen is determined by the diploid genotype of the anther. In Brassica, the pollen coat, derived from the anther's tapetum tissue, carries the translation products of the two S alleles. These are small, cysteine-rich proteins. The male determinant is termed SCR or SP11, and is expressed in the anther tapetum as well as in the microspore and pollen (i.e. sporophytically).{{cite journal | vauthors = Schopfer CR, Nasrallah ME, Nasrallah JB | title = The male determinant of self-incompatibility in Brassica | journal = Science | volume = 286 | issue = 5445 | pages = 1697–1700 | date = November 1999 | pmid = 10576728 | doi = 10.1126/science.286.5445.1697 }}{{cite journal | vauthors = Takayama S, Shiba H, Iwano M, Shimosato H, Che FS, Kai N, Watanabe M, Suzuki G, Hinata K, Isogai A | display-authors = 6 | title = The pollen determinant of self-incompatibility in Brassica campestris | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 97 | issue = 4 | pages = 1920–1925 | date = February 2000 | pmid = 10677556 | pmc = 26537 | doi = 10.1073/pnas.040556397 | doi-access = free | bibcode = 2000PNAS...97.1920T }} There are possibly up to 100 polymorphs of the S-haplotype in Brassica, and within these there is a dominance hierarchy. {{cn|date=July 2024}}

The female determinant of the SI response in Brassica, is a transmembrane protein termed SRK, which has an intracellular kinase domain, and a variable extracellular domain.{{cite journal | vauthors = Stein JC, Howlett B, Boyes DC, Nasrallah ME, Nasrallah JB | title = Molecular cloning of a putative receptor protein kinase gene encoded at the self-incompatibility locus of Brassica oleracea | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 88 | issue = 19 | pages = 8816–8820 | date = October 1991 | pmid = 1681543 | pmc = 52601 | doi = 10.1073/pnas.88.19.8816 | doi-access = free | bibcode = 1991PNAS...88.8816S }}: .{{cite journal | vauthors= Nasrallah JB, Nasrallah ME |title=Pollen–stigma signalling in the sporophytic self-incompatibility response |journal=Plant Cell |volume=5 |issue=10 |pages=1325–35 |year=1993 |doi=10.2307/3869785|jstor=3869785 |bibcode=1993PlanC...5.1325N }} SRK is expressed in the stigma, and probably functions as a receptor for the SCR/SP11 protein in the pollen coat. Another stigmatic protein, termed SLG, is highly similar in sequence to the SRK protein, and seems to function as a co-receptor for the male determinant, amplifying the SI response.{{cite journal | vauthors = Takasaki T, Hatakeyama K, Suzuki G, Watanabe M, Isogai A, Hinata K | title = The S receptor kinase determines self-incompatibility in Brassica stigma | journal = Nature | volume = 403 | issue = 6772 | pages = 913–916 | date = February 2000 | pmid = 10706292 | doi = 10.1038/35002628 | s2cid = 4361474 | bibcode = 2000Natur.403..913T }}

The interaction between the SRK and SCR/SP11 proteins results in autophosphorylation of the intracellular kinase domain of SRK,{{cite journal | vauthors = Schopfer CR, Nasrallah JB | title = Self-incompatibility. Prospects for a novel putative peptide-signaling molecule | journal = Plant Physiology | volume = 124 | issue = 3 | pages = 935–940 | date = November 2000 | pmid = 11080271 | pmc = 1539289 | doi = 10.1104/pp.124.3.935 | name-list-style = amp }}{{cite journal | vauthors = Takayama S, Shimosato H, Shiba H, Funato M, Che FS, Watanabe M, Iwano M, Isogai A | display-authors = 6 | title = Direct ligand-receptor complex interaction controls Brassica self-incompatibility | journal = Nature | volume = 413 | issue = 6855 | pages = 534–538 | date = October 2001 | pmid = 11586363 | doi = 10.1038/35097104 | s2cid = 4419954 | bibcode = 2001Natur.413..534T }} and a signal is transmitted into the papilla cell of the stigma. Another protein essential for the SI response is MLPK, a serine-threonine kinase, which is anchored to the plasma membrane from its intracellular side.{{cite journal | vauthors = Murase K, Shiba H, Iwano M, Che FS, Watanabe M, Isogai A, Takayama S | title = A membrane-anchored protein kinase involved in Brassica self-incompatibility signaling | journal = Science | volume = 303 | issue = 5663 | pages = 1516–1519 | date = March 2004 | pmid = 15001779 | doi = 10.1126/science.1093586 | s2cid = 29677122 | bibcode = 2004Sci...303.1516M }} A downstream signaling cascade leads to proteasomal degradation that produces an SI response.{{citation |journal = Nature Plants |volume = 1 |issue = 12 |doi=10.1038/nplants.2015.185 |title=Degradation of glyoxalase I in Brassica napus stigma leads to self-incompatibility response |author=Subramanian Sankaranarayanan, Muhammad Jamshed, and Marcus A. Samuel |date = 2015 |page = 15185 |pmid = 27251720 |bibcode = 2015NatPl...115185S }}

These mechanisms have received only limited attention in scientific research. Therefore, they are still poorly understood.

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.{{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 }} 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. 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.{{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 }}{{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 }}{{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 }}

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.{{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 }}

Many heterostylous 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, 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.{{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 }}{{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 }}

Tristylous plants generally 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.{{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 }}

Cryptic self-incompatibility (CSI) exists in a limited number of taxa (for example, there is evidence for CSI in Silene vulgaris, CaryophyllaceaeGlaettli, M. (2004). Mechanisms involved in the maintenance of inbreeding depression in gynodioecious Silene vulgaris (Caryophyllaceae): an experimental investigation. PhD dissertation, University of Lausanne.). 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.{{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 }} 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. {{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.{{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 }}

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.{{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 }}{{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 }} LSI can be pre-zygotic (e.g. deterioration of the embryo sac prior to pollen tube entry, as in Narcissus triandrus{{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 }}) or post-zygotic (malformation of the zygote or embryo, as in certain species of Asclepias and in Spathodea campanulata{{cite journal | vauthors = Sage TL, Williams EG |title=Self-incompatibility in Asclepias |journal=Plant Cell Incomp. Newsl. |volume=23 |pages=55–57 |year=1991 }}{{cite journal | vauthors = Sparrow FK, Pearson NL |title=Pollen compatibility in Asclepias syriaca |journal=J. Agric. Res. |volume=77 |pages=187–199 |year=1948 }}{{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 }}{{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 }}).

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).{{cite book | vauthors = Klekowski EJ | date = 1988 | title = Mutation, Developmental Selection, and Plant Evolution. | publisher = Columbia University Press | location = New York }}{{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 }}{{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 }} Supporters, on the other hand, argue for the existence of several basic criteria, which differentiate certain cases of LSI from the inbreeding depression phenomenon.

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. Approximately one half of angiosperm species are SI, the remainder being SC. Mutations that disable SI (resulting in SC) may become common or entirely dominate in natural populations. Pollinator decline, variability in pollinator service, the so-called "automatic advantage" of self-fertilisation, among other factors, may favor the loss of SI. {{cn|date=July 2024}}

Many cultivated plants are SC, although there are notable exceptions, such as apples and Brassica oleracea. Human-mediated artificial selection through selective breeding is often responsible for SC among these agricultural crops. SC enables more efficient breeding techniques to be employed for crop improvement. 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.{{cite journal | vauthors = Egea J, Burgos L |title=Detecting cross-incompatibility of three North American apricot cultivars and establishing the first incompatibility group in apricot |journal=Journal of the American Society for Horticultural Science |date=November 1996 |volume=121 |issue=6 |pages=1002–1005 |doi=10.21273/JASHS.121.6.1002 |url=https://www.researchgate.net/publication/200158820 |access-date=25 December 2020|doi-access=free }}{{cite journal |vauthors=Gómez EM, Dicenta F, Batlle I, Romero A, Ortega E |title=Cross-incompatibility in the cultivated almond (Prunus dulcis): Updating, revision and correction |journal=Scientia Horticulturae |date=19 February 2019 |volume=245 |pages=218–223 |doi=10.1016/j.scienta.2018.09.054 |bibcode=2019ScHor.245..218G |url=https://www.sciencedirect.com/science/article/abs/pii/S0304423818306642 |access-date=25 December 2020 |hdl=20.500.12327/55 |s2cid=92428859 |hdl-access=free |archive-date=19 March 2022 |archive-url=https://web.archive.org/web/20220319083847/https://www.sciencedirect.com/science/article/abs/pii/S0304423818306642 |url-status=live }} In this rare, intraspecific, cross-incompatible mechanism, individuals have more reproductive success when self-pollinated rather than when cross-pollinated with other individuals of the same species. In wild populations, intraspecific cross-incompatibility has been observed in Nothoscordum bivalve.{{cite journal |vauthors=Weiherer DS, Eckardt K, Bernhardt P |title=Comparative floral ecology and breeding systems between sympatric populations of Nothoscordum bivalve and Allium stellatum (Amaryllidaceae) |journal=Journal of Pollination Ecology |date=July 2020 |volume=26 |issue=3 |pages=16–31 |doi=10.26786/1920-7603(2020)585 |s2cid=225237548 |url=https://pollinationecology.org/index.php?journal=jpe&page=article&op=view&path%5B%5D=585 |access-date=25 December 2020 |doi-access=free |archive-date=29 July 2024 |archive-url=https://web.archive.org/web/20240729180617/https://pollinationecology.org/index.php/jpe |url-status=live }}

• {{annotated link|Allogamy}}
• {{annotated link|Dichogamy}}
• {{annotated link|Dimorphous flower}}
• {{annotated link|Dioecy}}
• {{annotated link|Heterosis}}
• {{annotated link|Monocotyledon reproduction}}
• {{annotated link|Outcrossing}}
• {{annotated link|Plant sexuality}}
• {{annotated link|Pollination}}
• {{annotated link|Protandry}}

{{reflist|2}}

{{refbegin}}

• {{cite journal | vauthors = Lan XG, Yu XM, Li YH | title = [Progress in study on signal transduction of gametophytic self-incompatibility] | language = Chinese | journal = Yi Chuan = Hereditas | volume = 27 | issue = 4 | pages = 677–685 | date = July 2005 | pmid = 16120598 }}
• {{cite journal | vauthors = Boavida LC, Vieira AM, Becker JD, Feijó JA | title = Gametophyte interaction and sexual reproduction: how plants make a zygote | journal = The International Journal of Developmental Biology | volume = 49 | issue = 5–6 | pages = 615–632 | year = 2005 | pmid = 16096969 | doi = 10.1387/ijdb.052023lb | doi-access = free | hdl = 10400.7/71 | hdl-access = free }}

{{refend}}

• {{cite web | url = https://www.biology-pages.info/S/SelfIncompatibilty.html | title = Self-Incompatibility: How Plants Avoid Inbreeding | date = 4 February 2019 | work = Kimball's Biology Pages | vauthors = Kimball JW }}
• {{cite web | url = http://www-biol.paisley.ac.uk/bioref/Genetics/Primula_heterostyly.html | title = Heterostyly in cowslip | publisher = Biological Sciences, University of Paisley | vauthors = Silverside AJ | date = January 2002 | archive-url = https://web.archive.org/web/20020306213959/http://www-biol.paisley.ac.uk/bioref/Genetics/Primula_heterostyly.html | archive-date = 2002-03-06 }}

Category:Pollination

Category:Plant reproduction

Category:Population genetics

Category:Plant sexuality