Everything about Self-incompatibility In Plants totally explained
Self-incompatibility (
SI) 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
seeds are produced. SI is one of the most important means to prevent
selfing and promote the generation of new
genotypes in
plants, and it's considered as one of the causes for the spread and success of the angiosperms on our planet.
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
protein-protein interactions, each mechanism being 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; therefore, each mechanism has its own, unique S-locus.
The S-locus contains two basic SI
genes - 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 genes are genetically
linked, and are inherited as a unit. Variants of the S-locus are called S-haplotypes. The
translation products of the two genes 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 allele, 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-locus contains more than two genes.
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
phenotype 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,
Scrophulariaceae,
Fabaceae,
Onagraceae,
Campanulaceae,
Papaveraceae and
Poaceae. 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
Solanaceae was found in 1989. Proteins in the same family were subsequently discovered in the
Rosaceae and
Scrophulariaceae. 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 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 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. 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.
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
sporophyte) 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, for example 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 al S alleles are
co-dominant, the presence of dominance relationships in the population, raises the chances of compatible mating between individuals.
The SI mechanism in Brassica
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 gene encoding these proteins is termed
SCR or
SP11, and is expressed in the anther tapetum (for example sporophytically), as well as in the
microspore and pollen (for example gametophytically).
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 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.
Other mechanisms of self-incompatibility
These mechanisms are less abundant and have received only limited attention in scientific research. Therefore, they're still poorly understood.
Heteromorphic self-incompatibility
A distinct SI mechanism exists in
heterostylous flowers, termed
heteromorphic self-incompatibility. This mechanism is probably not
evolutionarily related to the more familiar mechanisms, which are differentially defined as
homomorphic self-incompatibility.
Almost all heterostylous
taxa feature SI to some extent. The loci responsible for SI in heterostylous flowers, are strongly linked to the loci responsible for flower
polymorphism, and these traits are inherited together.
Distyly is determined by a single locus, which has two alleles;
tristyly is determined by two loci, each with two alleles. Heteromorphic SI is sporophytic, for example 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. Tristylous plants contain, in addition to the S locus, the M locus, also with two alleles.
Cryptic self-incompatibility (CSI)
Cryptic self-incompatibility (CSI) exists in a limited number of taxa (for example, there's evidence for CSI in
Silene vulgaris,
Caryophyllaceae). In this mechanism, the simultaneous presence of cross and self pollen on the same stigma, results in higher seed set from cross pollen, relative to self pollen. However, as opposed to 'complete' or 'absolute' SI, in CSI, self-pollination without the presence of competing cross pollen, results in successive fertilization and seed set;
Late-acting self-incompatibility (LSI)
Late-acting self-incompatibility (LSI) is also termed
ovarian self-incompatibility (OSI). In this mechanism, self pollen germinates and reaches the ovules, but no
fruit is set. LSI can be pre-
zygotic (for example 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.
Pollinator decline, variability in pollinator service, and life history traits that are associated with weediness, 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 self-compatibility in cultivated plants. SC enables more efficient breeding techniques to be employed for crop improvement.
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