Incompatibility mechanisms, Biology tutorial

Self-incompatibility in plants:

Self-incompatibility (SI) is the general name for some genetic methods in the angiosperms that prevent self-fertilization and therefore encourage out crossing. In plants with SI, if a pollen grain generated in a plant reaches a stigma of the similar plant or the other plant having a similar genotype, the procedure of pollen germination, pollen tube growth, ovule fertilization and embryo growth is halted at one of its phases and as a result no seeds are produced. SI is one of the most significant means to prevent the selfing and promote the generation of new genotypes in plants and it is considered as one of the causes for the spread and accomplishment of the angiosperms on the earth.

Mechanisms of self-incompatibility:

The mechanisms are mainly based on protein-protein interactions, each mechanism or method being controlled through a single locus termed S, which consists of numerous different alleles in the species population. In spite of their similar morphological and genetic manifestations, such methods or mechanisms have evolved independently and are based on various cellular components; thus, each mechanism consists of its own, unique S-genes.

The S-locus includes two basic protein coding regions - one deduced in the pistil and the other in the anther and/or pollen (termed to as the male and female determinants, correspondingly). Due to their physical proximity, these are genetically linked and are inherited as a unit. The units are termed S-haplotypes. The translation products of the two areas of the S-locus are two proteins which, by interacting with one other, lead to the arrest of pollen germination and/or pollen tube elongation and thus produce an SI response, preventing the fertilization.

Though, if a female determinant interacts by a male determinant of a different haplotype, no SI is made and fertilization makes sure. This is a simplistic explanation of the general mechanism or method of SI, that is more complex and in some species the S-haplotype includes more than two protein coding regions.

Gametophytic self-incompatibility (GSI):

In gametophytic self-incompatibility (GSI), the SI phenotype of the pollen is found out by its own gametophytichaploid genotype. This is the more general type of SI, existing in the families: Solanaceae, Plantaginaceae, Rosaceae, Fabaceae, Onagraeae, Papaveraceae, Campanulaceae and Poaceae. Two different method of GSI have been explained in detail at the molecular level and their explanation follows.

1) The RNase mechanism:

The female component of GSI in the Solanaceae was found in the year 1989. Proteins in the similar family were afterward discovered in the Rosaceae and Plantaginaceae. In spite of some early doubts regarding the common ancestry of GSI in these distantly associated families, phylogenetic studies and the finding of shared male determinants (or F-box proteins) clearly established the homology. As a result, this method arose around 90 million years ago and is the inferred ancestral state for around 50% of all plants.

In this method, pollen tube elongation is halted when it has preceded around one third of the way via the style. The female component ribonuclease, known S-RNase probably causes the degradation of the ribosomal RNA (rRNA) inside the pollen tube, in the case of similar male and female S alleles, and thus pollen tube elongation is arrested, and the pollen grain dies. 

The male component was only newly putatively recognized as a member of the 'F-box' protein family. In spite of some fairly convincing evidence which it might be the male component, quite a few features as well make it an unlikely candidate.

2) The S-glycoprotein mechanism:

In this mechanism or method, pollen growth is inhibited in minutes of its placement on the stigma. The female determinant is a small, extracellular molecule, deduced in the stigma; identity of the male determinant remains elusive, however it is probably some cell membrane receptor. The interaction among female and male determinants transmits a cellular signal into the pollen tube, resultant in strong influx of calciumcations; this interferes by the intracellular concentration gradient of calcium ions that exists within the pollen tube, necessary for its elongation. The influx of calcium ions arrests tube elongation in 1 to 2 minutes. At this phase, pollen inhibition is still reversible and elongation can be resumed by applying some manipulations, resultant in ovule fertilization.

Afterward, the cytosolic protein p26, a pyrophosphatase, is inhibited through phosphorylation, perhaps resultant in arrest of synthesis of molecular building blocks, needed for tube elongation. There is depolymerization and reorganization of actin filaments, in the pollen cytoskeleton. In 10 minutes from the placement on the stigma, the pollen is committed to a procedure that ends in its death. At 3 to 4 hours past pollination, fragmentation of pollen DNA starts and finally (at 10 to14 hours), the cell dies apoptotically.  

3) Sporophytic self-incompatibility (SSI):

In sporophytic self-incompatibility (SSI), the SI phenotype of the pollen is found out by the diploid genotype of the anther (that is, the sporophyte) in which it was made. This form of SI was recognized in the families: Brassicaceae, Convolvulaceae, Asteraceae, Betulaceae, Sterculiaceae, Caryophyllaceae and Polemoniaceae.

As SSI is found out through a diploid genotype, the pollen and pistil each state the translation products of two different alleles, that is, two female and male determinants. Dominance relationships frequently exist among pairs of alleles, resultant in complicated patterns of compatibility or self-incompatibility. This dominance relationship as well let the generation of individuals homozygous for a recessive S allele. 

4) The SI mechanism in Brassica:

The SI phenotype of the pollen is found out by the diploid genotype of the anther. In Brassica, the pollen coat, derived from the anther's tapetumtissue, takes the translation products of the two S alleles. These are small, cysteine-rich proteins. The male determinant is known as SCR or SP11, and is expressed in the anther tapetum (that is, sporophytically), and also in the microspore and pollen (that is, gametophytically).There are maybe up to 100 polymorphs of the S-haplotype in Brassica, and in these there is a dominance hierarchy.

The female determinant of the SI response in Brassica is a trans-membrane protein known SRK, which comprises of an intracellular kinase domain and a variable extracellular domain. SRK is deduced in the stigma, and probably functions as the receptor for the SCR/SP11 protein in the pollen coat. The other stigmatic protein, known SLG, is highly identical in series to the SRK protein and seems to function as a co-receptor for the male determinant, amplifying the SI response.

The interaction among the SRK and SCR/SP11 proteins outcomes in autophosphorylation of the intracellular kinase domain of SRK and a signal is transmitted to the papilla cell of the stigma. The other protein necessary for the SI response is MLPK, a serine-threonine kinase that is anchored to the plasma membrane from its intracellular side. The downstream cellular and molecular events, leading ultimately to pollen inhibition, are poorly explained.

Heteromorphic self-incompatibility:

A dissimilar SI mechanism exists in heterostylous flowers, known as Heteromorphic self-incompatibility. This method is most likely not evolutionarily associated to the more familiar mechanisms that are differentially stated as homomorphic self-incompatibility.  Almost all the heterostylous taxa feature SI to certain extent. The loci responsible for SI in heterostylous flowers are strongly connected to the loci responsible for the flower polymorphism and such traits or characteristics are inherited altogether. Distyly is found out by a single locus that consists of two alleles; tristyly is found out by two loci, each having two alleles. Heteromorphic SI is sporophytic, that is, both alleles in the male plant, find out the SI response in the pollen. SI loci for all time have only two alleles in the population, one of which is dominant over the other, in both pollen and pistil.

Variance in the SI alleles parallels the variance in flower morphs; therefore pollen from one morph can fertilize just pistils from the other morph. In tristylous flowers, each flower includes two kinds of stamens; each stamen generates pollen capable of fertilizing just one flower morph, out of the three existing morphs.

A population of a distylous plant includes just two SI genotypes: ss and Ss. Fertilization is probable only among genotypes; each genotype can't fertilize itself. This restriction maintains a 1:1 ratio among the two genotypes in the population; genotypes are generally arbitrarily scattered in space. Tristylous plants have, in addition to the S locus, the M locus, as well by two alleles. The number of possible genotypes is more here, however a 1:1 ratio exists among individuals of each SI type.

Cryptic self-incompatibility (CSI):

Cryptic self-incompatibility (CSI) exists in the limited number of taxa (for illustration, there is a proof for CSI in Silene vulgaris, Caryophyllaceae. In this method, the concurrent presence of cross and self pollen on the similar stigma, outcomes in higher seed set from cross pollen, relative to the self pollen. Though, as opposed to 'complete' or 'absolute' SI, in CSI, self-pollination devoid of the presence of competing cross pollen, outcomes in successive fertilization and seed set; in this manner, reproduction is assured, even in the absence of the cross-pollination. CSI acts, at least in certain species, at the phase of pollen tube elongation and leads to faster elongation of cross pollen tubes, relative to the self pollen tubes. The cellular and molecular methods of CSI have not been explained.

The strength of a CSI response can be stated, as the ratio of crossed to selfed ovules, made when equivalent amounts of cross and self pollen, are placed on the stigma.

Late-acting self-incompatibility (LSI):

Late-acting self-incompatibility (LSI) is as well known as ovarian self-incompatibility (OSI). In this method, self pollen germinates and reaches the ovules, however no fruit is set. LSI can be pre-zygotic (example: deterioration of the embryo sac prior to pollen tube entry, as in the Narcissus triandrus) or post-zygotic (that is, malformation of the zygote or embryo, as in some species of Asclepias and in Spathodeacampanulata.

The existence of LSI method among various taxa and in general is subject for scientific debate. Criticizers claim, that absence of fruit set is due to genetic defects (that is, homozygosity for lethal recessive alleles), which are the direct outcome of self-fertilization (that is, inbreeding depression).Supporters, on the other hand, argue for the survival of some fundamental criteria, that distinguish some cases of LSI from the inbreeding depression phenomenon.

Self-compatibility (SC):

SI is not universal in the flowering plants. Certainly, a great many species are self-compatible (SC). The best estimates point out that around one half of the angiosperm species are SI. Pollinator decline, variability in the pollinator service, and life history characteristic or traits which are related having weediness and the so-called 'automatic benefit' of self-fertilization, among other factors, might favor the loss of SI. As an outcome, mutations that break down SI (result in SC) might become common or wholly dominate in natural populations. Likewise, human-mediated artificial selection via selective breeding might be responsible for the generally observed SC in the cultivated plants. SC facilitates more efficient breeding methods to be employed for crop enhancement.

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