Product Code Database
Self-incompatibility ( SI) is a general name for several genetic mechanisms that prevent self-fertilization in sexually reproducing organisms, and thus encourage and allogamy. It is contrasted with separation of sexes among individuals (dioecy), and their various modes of spatial (herkogamy) and temporal (dichogamy) separation.
SI is best-studied and particularly common in flowering plants, although it is present in other groups, including ascidians and fungi. In plants with SI, when a pollen grain produced in a plant reaches a stigma of the same plant or another plant with a matching allele or genotype, the process of pollen germination, pollen-tube growth, ovule fertilization, or embryo development is inhibited, and consequently no are produced. SI is one of the most important means of preventing inbreeding and promoting the generation of new in and it is considered one of the causes of the spread and success of on Earth.
The S-locus contains two basic protein – 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 genetic linkage, and are inherited as a unit. The units are called 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.
By 2000, proteins involved in GSI 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 and the finding of shared male determinants ( F-box proteins) 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.
Predictions about the wide distribution of this mechanism of SI were confirmed in the early 21st century, 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, Rutaceae,, Cactaceae,, and Primulaceae. 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 (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).
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 ; this interferes with the intracellular concentration gradient of calcium 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 protein p26, a pyrophosphatase, is inhibited by phosphorylation, possibly resulting in arrest of Biosynthesis of molecular building blocks, required for tube elongation. There is polymerization 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 apoptosis.
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 Zygosity for a recessive S allele.
Compared to a population in which all S alleles are co-dominance, 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).
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. A downstream signaling cascade leads to proteasomal degradation that produces an SI response.
Many heterostylous taxon 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 ; 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 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.
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.
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.
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. 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.
|
|
