Self-incompatibility in plants
Encyclopedia
Self-incompatibility is a general name for several genetic mechanisms in angiosperms, which prevent self-fertilization and thus encourage outcrossing
. In plants with SI, when a pollen
grain produced in a plant reaches a stigma of the same plant or another plant with a similar genotype, the process of pollen germination
, pollen tube growth, ovule
fertilization, and embryo
development is halted at one of its stages, and consequently no seed
s are produced. SI is one of the most important means to prevent selfing
and promote the generation of new genotype
s in plant
s, and it is considered as one of the causes for the spread and success of the angiosperms on the earth.
-protein interactions, each mechanism being controlled by a single locus
termed S, which has many different allele
s in the species
population. Despite their similar morphological and genetic manifestations, these mechanisms have evolved independently, and are based on different cellular components; therefore, each mechanism has its own, unique S-gene
s.
The S-locus contains two basic protein coding region
s - one expressed in the pistil, and the other in the anther and/or pollen (referred to as the female and male determinants, respectively). Because of their physical proximity, these are genetically linked
, and are inherited as a unit. The units are called S-haplotype
s. 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.
Following is a detailed description of the different known mechanisms of SI in plants.
of the pollen is determined by its own gametophytic
haploid
genotype. This is the more common type of SI, existing in the families: Solanaceae
, Rosaceae
, Plantaginaceae
, Fabaceae
, Onagraceae
, Campanulaceae
, Papaveraceae
and Poaceae
. Two different mechanisms of GSI have been described in detail at the molecular level, and their description follows.
was found in 1989. Proteins in the same family were subsequently discovered in the Rosaceae
and Plantaginaceae
. Despite some early doubts about the common ancestry of GSI in these distantly related families, phylogenetic studies and the finding of shared male determinants (F-box proteins) clearly established homology. Consequently, this mechanism arose approximately 90 million years ago, and is the inferred ancestral state for approximately 50% of all plants.
In this mechanism, pollen tube elongation is halted when it has proceeded approximately one third of the way through the style. The female component ribonuclease
, termed S-RNase 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.
The male component was only recently putatively identified as a member of the "F-box" protein family. Despite some fairly convincing evidence that it may be the male component, several features also make it an unlikely candidate.
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 ion
s which exists inside the pollen tube, essential for its elongation. 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 cytosol
ic protein p26, a pyrophosphatase
, is inhibited by phosphorylation
, 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
. 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, and finally (at 10–14 hours), the cell dies apoptotically
.
) in which it was created. This form of SI was identified in the families: Brassicaceae
, Asteraceae
, Convolvulaceae
, Betulaceae
, Caryophyllaceae
, Sterculiaceae
and Polemoniaceae
. Up to this day, only one mechanism of SSI has been described in detail at the molecular level, in Brassica
(Brassicaceae).
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.
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 reproduction assurance (favoured by recessive alleles) and avoidance of selfing (favoured by dominant alleles).
, 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 (i.e. sporophytically), as well as in the microspore
and pollen (i.e. gametophytically). There are possibly up to 100 polymorphs of the S-haplotype in Brassica, and within these there is a dominance hierarchy.
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. 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.
The interaction between the SRK and SCR/SP11 proteins results in autophosphorylation
of the intracellular kinase domain of SRK, 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. The downstream cellular and molecular events, leading eventually to pollen inhibition, are poorly described.
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.
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 stamen
s; 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.
, 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.
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.
, variability in pollinator service, and life history traits that are associated with weediness, and the so-called "automatic advantage" of self-fertilisation, among other factors, may favor the loss of SI. As a result, mutations that break down SI (result in SC) may become common or entirely dominate in natural populations. Similarly, human-mediated artificial selection through selective breeding
may be responsible for the commonly observed SC in cultivated plants. SC enables more efficient breeding techniques to be employed for crop improvement.
Outcrossing
Outcrossing is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of all individuals being subject to disease or reducing genetic abnormalities...
. In plants with SI, when a pollen
Pollen
Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...
grain produced in a plant reaches a stigma of the same plant or another plant with a similar genotype, the process of pollen germination
Germination
Germination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the...
, pollen tube growth, ovule
Ovule
Ovule means "small egg". In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument forming its outer layer, the nucellus , and the megaspore-derived female gametophyte in its center...
fertilization, and embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
development is halted at one of its stages, and consequently no seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
s are produced. SI is one of the most important means to prevent selfing
Inbreeding
Inbreeding is the reproduction from the mating of two genetically related parents. Inbreeding results in increased homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is...
and promote the generation of new genotype
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
s in plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s, and it is considered as one of the causes for the spread and success of the angiosperms on the earth.
Mechanisms of 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 proteinProtein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
-protein interactions, each mechanism being controlled by a single locus
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
termed S, which has many different allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s in the species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
population. Despite their similar morphological and genetic manifestations, these mechanisms have evolved independently, and are based on different cellular components; therefore, each mechanism has its own, unique S-gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s.
The S-locus contains two basic protein coding region
Coding region
The coding region of a gene, also known as the coding sequence or CDS, is that portion of a gene's DNA or RNA, composed of exons, that codes for protein. The region is bounded nearer the 5' end by a start codon and nearer the 3' end with a stop codon...
s - one expressed in the pistil, and the other in the anther and/or pollen (referred to as the female and male determinants, respectively). Because of their physical proximity, these are genetically linked
Genetic linkage
Genetic linkage is the tendency of certain loci or alleles to be inherited together. Genetic loci that are physically close to one another on the same chromosome tend to stay together during meiosis, and are thus genetically linked.-Background:...
, and are inherited as a unit. The units are called S-haplotype
Haplotype
A haplotype in genetics is a combination of alleles at adjacent locations on the chromosome that are transmitted together...
s. 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.
Following is a detailed description of the different known mechanisms of SI in plants.
Gametophytic self-incompatibility (GSI)
In gametophytic self-incompatibility (GSI), the SI phenotypePhenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
of the pollen is determined by its own gametophytic
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....
haploid
Ploidy
Ploidy is the number of sets of chromosomes in a biological cell.Human sex cells have one complete set of chromosomes from the male or female parent. Sex cells, also called gametes, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number is...
genotype. This is the more common type of SI, existing in the families: Solanaceae
Solanaceae
Solanaceae are a family of flowering plants that include a number of important agricultural crops as well as many toxic plants. The name of the family comes from the Latin Solanum "the nightshade plant", but the further etymology of that word is unclear...
, Rosaceae
Rosaceae
Rosaceae are a medium-sized family of flowering plants, including about 2830 species in 95 genera. The name is derived from the type genus Rosa. Among the largest genera are Alchemilla , Sorbus , Crataegus , Cotoneaster , and Rubus...
, Plantaginaceae
Plantaginaceae
Plantaginaceae Juss. or plantain family, are a family of flowering plants in the order Lamiales. The type genus is Plantago L..In older classifications it used to be the only family of the order Plantaginales, but numerous phylogenetic studies, summarized by the Angiosperm Phylogeny Group, have...
, Fabaceae
Fabaceae
The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...
, Onagraceae
Onagraceae
Onagraceae, also known as the Willowherb family or Evening Primrose family, are a family of flowering plants. The family includes about 640-650 species of herbs, shrubs, and trees in 20-24 genera...
, Campanulaceae
Campanulaceae
The family Campanulaceae , of the order Asterales, contains about 2000 species in 70 genera of herbaceous plants, shrubs, and rarely small trees, often with milky non-toxic sap...
, Papaveraceae
Papaveraceae
Papaveraceae, informally known as the poppy family, are an economically important family of 44 genera and approximately 770 species of flowering plants in the order Ranunculales. The family is cosmopolitan, occurring in temperate and subtropical climates, but almost unknown in the tropics...
and Poaceae
Poaceae
The Poaceae is a large and nearly ubiquitous family of flowering plants. Members of this family are commonly called grasses, although the term "grass" is also applied to plants that are not in the Poaceae lineage, including the rushes and sedges...
. Two different mechanisms of GSI have been described in detail at the molecular level, and their description follows.
The RNase mechanism
The female component of GSI in the SolanaceaeSolanaceae
Solanaceae are a family of flowering plants that include a number of important agricultural crops as well as many toxic plants. The name of the family comes from the Latin Solanum "the nightshade plant", but the further etymology of that word is unclear...
was found in 1989. Proteins in the same family were subsequently discovered in the Rosaceae
Rosaceae
Rosaceae are a medium-sized family of flowering plants, including about 2830 species in 95 genera. The name is derived from the type genus Rosa. Among the largest genera are Alchemilla , Sorbus , Crataegus , Cotoneaster , and Rubus...
and Plantaginaceae
Plantaginaceae
Plantaginaceae Juss. or plantain family, are a family of flowering plants in the order Lamiales. The type genus is Plantago L..In older classifications it used to be the only family of the order Plantaginales, but numerous phylogenetic studies, summarized by the Angiosperm Phylogeny Group, have...
. Despite some early doubts about the common ancestry of GSI in these distantly related families, phylogenetic studies and the finding of shared male determinants (F-box proteins) clearly established homology. Consequently, this mechanism arose approximately 90 million years ago, and is the inferred ancestral state for approximately 50% of all plants.
In this mechanism, pollen tube elongation is halted when it has proceeded approximately one third of the way through the style. The female component ribonuclease
Ribonuclease
Ribonuclease is a type of nuclease that catalyzes the degradation of RNA into smaller components. Ribonucleases can be divided into endoribonucleases and exoribonucleases, and comprise several sub-classes within the EC 2.7 and 3.1 classes of enzymes.-Function:All organisms studied contain...
, termed S-RNase probably causes degradation of the ribosomal
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
(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.
The male component was only recently putatively identified as a member of the "F-box" protein family. Despite some fairly convincing evidence that it may be the male component, several features also make it an unlikely candidate.
The S-glycoprotein mechanism
The following mechanism was described in detail in Papaver rhoeas. In this mechanism, pollen growth is inhibited within minutes of its placement on the stigma.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
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
receptor
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
. The interaction between male and female determinants transmits a cellular signal
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
into the pollen tube, resulting in strong influx of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
cations; this interferes with the intracellular concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
gradient of calcium ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s which exists inside the pollen tube, essential for its elongation. 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 cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
ic protein p26, a pyrophosphatase
Pyrophosphatase
Pyrophosphatase are acid anhydride hydrolases that act upon diphosphate bonds.Examples include:* Inorganic pyrophosphatase* Thiamine pyrophosphatase...
, is inhibited by phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
, possibly resulting in arrest of synthesis
Biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
of molecular building blocks, required for tube elongation. There is depolymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
and reorganization of actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
filaments, within the pollen cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
. 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
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
begins, and finally (at 10–14 hours), the cell dies apoptotically
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
.
Sporophytic self-incompatibility (SSI)
In sporophytic self-incompatibility (SSI), the SI phenotype of the pollen is determined by the diploid genotype of the anther (the sporophyteSporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
) in which it was created. This form of SI was identified in the families: Brassicaceae
Brassicaceae
Brassicaceae, a medium sized and economically important family of flowering plants , are informally known as the mustards, mustard flowers, the crucifers or the cabbage family....
, Asteraceae
Asteraceae
The Asteraceae or Compositae , is an exceedingly large and widespread family of vascular plants. The group has more than 22,750 currently accepted species, spread across 1620 genera and 12 subfamilies...
, Convolvulaceae
Convolvulaceae
Convolvulaceae, known commonly as the bindweed or morning glory family, are a group of about 60 genera and more than 1,650 species of mostly herbaceous vines, but also trees, shrubs and herbs.- Description :...
, Betulaceae
Betulaceae
Betulaceae, or the Birch Family, includes six genera of deciduous nut-bearing trees and shrubs, including the birches, alders, hazels, hornbeams and hop-hornbeams, numbering about 130 species...
, Caryophyllaceae
Caryophyllaceae
The Caryophyllaceae, commonly called the pink family or carnation family, is a family of flowering plants. It is included in the dicotyledon order Caryophyllales in the APG III system, alongside 33 other families, including Amaranthaceae, Cactaceae and Polygonaceae...
, Sterculiaceae
Sterculiaceae
Sterculiaceae is a botanical name for a group of flowering plants at the rank of family, which is now considered obsolete. As is true for any botanical name, the circumscription, status and placement of the taxon has varied with taxonomic point of view...
and Polemoniaceae
Polemoniaceae
Polemoniaceae are a family of about 25 genera with 270-400 species of annual and perennial plants, native to the Northern Hemisphere and South America, with the center of diversity in western North America, especially in California.Only one genus is found in Europe, and two in Asia, where they...
. Up to this day, only one mechanism of SSI has been described in detail at the molecular level, in Brassica
Brassica
Brassica is a genus of plants in the mustard family . The members of the genus may be collectively known either as cabbages, or as mustards...
(Brassicaceae).
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
Dominance
Dominance may refer to:* Dominance , an aspect of virtual inheritance in the C++ programming language* Dominance , in economics, the degree of inequality in market share distribution...
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
Zygosity
Zygosity refers to the similarity of alleles for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait...
for a recessive
Recessive
In genetics, the term "recessive gene" refers to an allele that causes a phenotype that is only seen in a homozygous genotype and never in a heterozygous genotype. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father...
S allele.
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 reproduction assurance (favoured by recessive alleles) and avoidance of selfing (favoured by dominant alleles).
The SI mechanism in Brassica
As previously mentioned, the SI phenotype of the pollen is determined by the diploid genotype of the anther. In BrassicaBrassica
Brassica is a genus of plants in the mustard family . The members of the genus may be collectively known either as cabbages, or as mustards...
, the pollen coat, derived from the anther's tapetum
Tapetum
Tapetum can refer to:* Tapetum * Tapetum lucidum* Tapetum of corpus callosum...
tissue
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
, carries the translation products of the two S alleles. These are small, cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
-rich proteins. The male determinant is termed SCR or SP11, and is expressed in the anther tapetum (i.e. sporophytically), as well as in the microspore
Microspore
In biology, a microspore is a small spore as contrasted to the larger megaspore.In botany, microspores develop into male gametophytes, whereas megaspores develop into female gametophytes. The combination of megaspores and microspores is found only in heterosporous organisms...
and pollen (i.e. gametophytically). There are possibly up to 100 polymorphs of the S-haplotype in Brassica, and within these there is a dominance hierarchy.
The female determinant of the SI response in Brassica, is a transmembrane protein termed SRK, which has an intracellular kinase
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...
domain, and a variable extracellular domain. 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
Sequence (biology)
A sequence in biology is the one-dimensional ordering of monomers, covalently linked within in a biopolymer; it is also referred to as the primary structure of the biological macromolecule.-See also:* Protein sequence* DNA sequence...
to the SRK protein, and seems to function as a co-receptor
Co-receptor
A co-receptor is a cell surface receptor that binds a signalling molecule in addition to a primary receptor in order to facilitate ligand recognition and initiate biological processes, such as entry of a pathogen into a host cell.-Co-receptor Properties:...
for the male determinant, amplifying the SI response.
The interaction between the SRK and SCR/SP11 proteins results in autophosphorylation
Autophosphorylation
In biochemistry, autophosphorylation is the process in which a protein kinase attaches a phosphate group to itself. This usually lead to kinase activation or regulation, and phosphorylation of other kinase substrates....
of the intracellular kinase domain of SRK, and a signal is transmitted into the papilla cell of the stigma. Another protein essential for the SI response is MLPK, a serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
-threonine
Threonine
Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...
kinase, which is anchored to the plasma membrane from its intracellular side. The downstream cellular and molecular events, leading eventually to pollen inhibition, are poorly described.
Other mechanisms of self-incompatibility
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 heterostylousHeterostyly
Heterostyly is a unique form of polymorphism and herkogamy in flowers. In a heterostylous species, two or three different morphological types of flowers, termed morphs, exist in the population. On each individual plant, all flowers share the same morph. The flower morphs differ in the lengths of...
flowers, termed heteromorphic self-incompatibility. This mechanism is probably not evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
arily related to the more familiar mechanisms, which are differentially defined as homomorphic self-incompatibility.
Almost all heterostylous taxa
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
feature SI to some extent. The loci responsible for SI in heterostylous flowers, are strongly linked to the loci responsible for flower polymorphism
Polymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
, 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
Stamen
The stamen is the pollen producing reproductive organ of a flower...
s; 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 vulgarisSilene vulgaris
Silene vulgaris, Silene cucubalus or Bladder Campion is a plant species of the genus Silene of the Pink Family . It is native to Europe, where in some parts it is eaten, but is widespread in North America where it is considered a weed..-Gastronomy:In Spain, the young shoots and the leaves are used...
, Caryophyllaceae
Caryophyllaceae
The Caryophyllaceae, commonly called the pink family or carnation family, is a family of flowering plants. It is included in the dicotyledon order Caryophyllales in the APG III system, alongside 33 other families, including Amaranthaceae, Cactaceae and Polygonaceae...
). 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 ovarianOvary
The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...
self-incompatibility (OSI). In this mechanism, self pollen germinates and reaches the ovules, but no fruit
Fruit
In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
is set. LSI can be pre-zygotic
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
(e.g. deterioration of the embryo sac prior to pollen tube entry, as in Narcissus triandrus
Narcissus triandrus
Narcissus triandrus is a species of bulbous plant native to southwestern Europe. The common name is "Angel's Tears", and the plant has pendulant cream to yellowish flowers.-Source:* Germplasm Resources Information Network, , accessed on 2008-05-08....
) or post-zygotic (malformation of the zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
or embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
, as in certain species of Asclepias
Asclepias
Asclepias L. , the milkweeds, is a genus of herbaceous perennial, dicotyledonous plants that contains over 140 known species...
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
Inbreeding depression
Inbreeding depression is the reduced fitness in a given population as a result of breeding of related individuals. It is often the result of a population bottleneck...
). 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)
SI is not universal in flowering plants. Indeed, a great many species are self-compatible (SC). The best estimates indicate that approximately one half of angiosperm species are SI. Pollinator declinePollinator decline
The term pollinator decline refers to the reduction in abundance of pollinators in many ecosystems worldwide during the end of the twentieth century....
, variability in pollinator service, and life history traits that are associated with weediness, and the so-called "automatic advantage" of self-fertilisation, among other factors, may favor the loss of SI. As a result, mutations that break down SI (result in SC) may become common or entirely dominate in natural populations. Similarly, human-mediated artificial selection through selective breeding
Selective breeding
Selective breeding is the process of breeding plants and animals for particular genetic traits. Typically, strains that are selectively bred are domesticated, and the breeding is sometimes done by a professional breeder. Bred animals are known as breeds, while bred plants are known as varieties,...
may be responsible for the commonly observed SC in cultivated plants. SC enables more efficient breeding techniques to be employed for crop improvement.