Sexual Incompatibility and its types: MSC BOTANY
- 1. 1 Self-incompatibility in plants (NOTES) Sexual incompatibility If a pistil, carrying functional female gametes, fails to set seeds after pollination with viable and fertile pollens, the two are called incompatible. This phenomenon is called as sexual incompatibility. Inter-specific Incompatibility Intra-specific Incompatibility (Self Incompatibility) Between individuals of different species Between individuals of the same species Inter-specific Incompatibility It is heterogenic incompatibility controlled by more than one genes at different loci on the chromosome It prevents cross pollination It creates new races and species CAUSES: (Inter-specific incompatibility) 1. Fertilization (fusion of male & female gametes) does not occur 2. Hybrid embryo is aborted 3. Inadequate development of endosperm 4. Endosperm-embryo incompatibility Intra-specific Incompatibility (Self Incompatibility or SI) Many species of flowering plant are bisexual, suggesting that they generally set the seeds by self-pollination. However, a number of mechanisms exist to prevent the self- pollination. Self-incompatibility (SI) is the mechanism by which plants prevent self-fertilization and maintain genetic diversity. Self-incompatible plants are not able to produce seeds when
- 2. 2 its flowers are pollinated from its own flowers or flowers from plants that are genetically the same. SI is an active process where self-incompatible pollen tubes are killed in the female flower-parts to prevent fertilization. In plants with SI, when a pollen grain, produced by a plant, reaches the stigma of the same plant or of 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 due to the following reasons: 1. Pollen grain fails to germinate on the stigma. 2. Pollen grain germinates, but pollen tube fails to enter into stigma and style. 3. Pollen tube enters into the style but its growth is very slow. 4. Pollen tube enters the ovule but there is no fertilization due to degeneration of egg cells. 5. Fertilization is affected but embryo is degenerated at very early stage. FACTS: SI is estimated to occur in 30–50% of flowering plant species It is one of the most important means of preventing self-fertilization and promoting the generation of new genotypes in plants. It is considered as one of the causes for the spread and success of angiosperms on the earth. Lewis (1954) has suggested the following classification of self-incompatibility in plants: 1. Heteromorphic System (different morphology of flowers) 2. Homomorphic System (Two kinds) o Gametophytic Control o Sporophytic Control
- 3. 3 Heteromorphic System: In Heteromorphic System, flowers of different incompatibility groups have different morphology. For example, relative length of styles and stamens in the flowers of the same species, e.g. Distyly (Primula species) and Tristyly (Oenothera) has been reported as Heteromorphic Systems. Diagrammatic representation of heteromorphic incompatibility Distyly Tristyly DISTYLY Primula has two types of flowers, viz. Pin and Thrum; this situation is referred as Distyly. Pin flowers: bear long styles and short stamens Thrum flowers: bear short styles and long stamens. In the case of Distyly, the only compatible mating is possible between Pin and Thrum flowers. The characteristic is governed by single S-gene (self-incompatibility gene); Ss produce Thrum flowers and ss produce Pin flowers. Mating Phenotype Mating Genotype Genotype Phenotype Pin × Pin ss× ss Incompatible mating - Thrum × Thrum Ss × Ss Incompatible mating - Pin × Thrum ss × Ss 1Ss : 1 ss 1 Thrum : 1 Pin Thrum × Pin Ss× ss 1Ss : 1 ss 1 Thrum : 1 Pin
- 4. 4 The incompatibility reaction of pollen is controlled by the genotype (genetic makeup) of the plant producing them. Allele S dominates over allele s. Incompatibility system, thus, represents Heteromorphic-Sporophytic type. Such type of system has been reported in plants like sweet potato and buckwheat. In Tristyly, the style of a flower may be of short, long or of medium length. It is known in some plant species, e.g. Lythrum. This characteristic is also governed by a single S-gene with alleles Ss and ss. Allele Ss produces Thrum, while ss allele produces Pin flowers. The incompatibility reaction of pollen is determined by the genotype of the plant producing them. Allele S is dominant over s. Here also, the incompatibility system is Heteromorphic-Sporophytic. Homomorphic System In the Homomorphic System, incompatibility is not associated with morphological differences among flowers. The incompatibility reaction of pollen may be controlled by: 1. The genotype of the sporophyte (the plant on which it is produced) → Sporophytic control or 2. The own genotype of male gamete/pollen grain → Gametophytic control.
- 5. 5 In Homomorphic flowers, all flowers have exactly the same structure. Avoidance of self- fertilization depends on genetic/biochemical mechanisms. There are two quite different types of self-incompatibility. o Sporophytic self-incompatibility (SSI) o Gametophytic self-incompatibility (GSI) Sporophytic self-incompatibility (SSI) Gametophytic self-incompatibility (GSI) Incompatibility reaction between the pollen-tube and the style is determined by the genotype of the sporophyte (diploid tissue). S-locus products are synthesized before completion of meiosis. Growth of the pollen tube is arrested at the surface of the stigma. Incompatibility reaction between the pollen tube and the style is determined by the genotype of the pollen (gamete). S-locus products are synthesized after completion of meiosis. Growth of the pollen tube is arrested in the style. Sporophytic Self-incompatibility (SSI) In the Sporophytic System, the stigma is papillate and dry, and is covered with a hydrated layer of proteins known as ‘pellicle’. Within few minutes of reaching the stigmatic surface, the pollen releases an exine-exudate, which is either protein or glycoprotein in nature. This exudate induces immediately the callose formation in the papillae of incompatible stigma. Thus, in Sporophytic System, stigma is the site of incompatibility reaction; once the pollen tube crosses the stigmatic barrier, there is not further inhibition of the pollen tube growth. This form of self-incompatibility has been studied intensively in members of the mustard family (Brassicaceae), including turnips, rape-seed, cabbage, broccoli, and cauliflower.
- 6. 6 Sporophytic SI: Stigmatic papilla in cross-pollination (A) and self-pollination (B). Note the deposition of callose at the tip of pollen tube and in the papilla in self-pollinated stigma The S-locus In dicots, it is the single gene of self-incompatibility on the chromosome called as S- locus. In contrast, grasses (monocots) have two (unlinked) S-loci. While a species may have as many as 50 or 60 different S-alleles, each plant has only two of them, one inherited from each parent. Pollen grains are haploid; they contain only a single set of chromosomes and, thus, each pollen grain contains only one of the two S-alleles of the parent plant. Pistil (style) is diploid; it has two sets of chromosomes (one from each parent) and, therefore, pistil has both S-alleles of the parent plant. Molecular dissection of the S-locus in several plant species has revealed that: The S-locus consists of multiple, tightly-linked genes, encoding male and female compatibility determinants. Divergent mechanisms of self-incompatibility are encoded by the S-loci of different plant species.
- 7. 7 Individual plants carry a pair of different S-loci (e. g. S1 and S2). However, dozens of different S-alleles may be present in a population of the species; that is, the S-locus in the species is extremely polymorphic (having multiple alleles). Homorphic-Sporophytic SI (Figure) Rules for Homomorphic SSI: 1. Pollen will not germinate on the stigma (diploid tissue) of a flower that contains either of the two alleles present in the pollen-producing sporophyte-parent (Case 1). 2. Pollen will also not germinate if the haploid pollen-grain contains only one of these alleles. (Case 2). 3. Pollen will germinate successfully on the stigma of a flower that contains none of the two alleles present in the pollen- producing sporophyte-parent. (Case 3). In Homographic-Sporophytic Self-incompatibility (SSI), recognition as well as rejection reactions for pollen-tube growth take place on the stigma. The rejection of self-pollen (with the same genotype as that of stigma-bearing plant) is controlled by the diploid genotype of the sporophyte generation (stigma). Biochemical Mechanisms of Sporophytic Self Incompatibility (SSI) The control lies in the "S-locus", which is actually a cluster of three tightly-linked loci. The three genes of the S-locus, produce the following three proteins, respectively: SLG (S-Locus Glycoprotein). It encodes the part of a receptor-protein present in the cell wall of the stigma. SCR (S-locus Cysteine-rich Protein). It encodes a soluble ligand for the same receptor-protein, which is secreted by the pollen.
- 8. 8 SRK (S-Receptor Kinase). It encodes the kinase domain of the receptor-protein. SRK is transmembrane protein embedded in the plasma membrane of the stigma cell. S1 S2 Biochemical Mechanism of SSI The sporophyte, producing S1S2 pollen, synthesizes both SCR1 and SCR2 for incorporation in (and later release from) both S1 and S2 pollen grains. If either of the SCR molecule (SCR1 or SCR2) binds to either receptor-protein (SLG1 or SLG2) on the pistil, the enzyme Kinase attaches the phosphate group to other proteins, which trigger a series of events that lead to failure of the stigma to support germination of the pollen grain. If this path is not triggered (e.g., when pollen from an S1S2 lands on an S3S4 stigma), the pollen germinates successfully.
- 9. 9 Sporophytic Self-Incompatibility (SSI) Gametophytic Self-Incompatibility (GSI) (SSI) (GSI) (1) Fully Incompatible (S1 S2 × S1 S2) (2) Fully Incompatible (S1 S2 × S1 S3) (3) Fully Compatible (S1 S2 × S3 S4) (1) Fully Incompatible (S1 S2 × S1 S2) (2) Partially Compatible (S1 S2 × S2 S3) (3) Fully Compatible (S1 S2 × S3 S4) GSI rules: 1. In Homorphic-Gametophytic Self-incompatibility (GSI), recognition as well as rejection reactions occur inside the pistil. 2. The rejection of self-pollen is controlled by the haploid genotype of the Gametophytic generation (Pollen). 3. The S-loci (as those present in SSI plants) are extremely polymorphic; that is, there is an abundance of multiple alleles in the population. 4. Incompatibility is controlled by the single S-allele present in the haploid pollen grain. 5. Thus, in GSI system, a pollen grain will grow into any pistil that does not contain the same allele (as that of pollen) in contrast to what happens in SSI system, where a pollen grain will not grow into a pistil that contains any one of the S-allele of the pollen grain.
- 10. 10 Biochemical Mechanisms of Gametophytic Self Incompatibility (GSI) The best studied mechanisms of self incompatibility (SI) are the following: 1. inhibiting the germination of pollen on the stigmas 2. Inhibiting 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 S-gene”, termed as S-locus, 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- genes. The S-locus contains two basic protein-coding regions: 1. Expressed in the pistil (Female Determinant) 2. Expressed in the anther and/or in pollen (male determinant) Because of their physical closeness, these S-loci are genetically linked, and are inherited together as single unit. These units are called S-haplotypes. As a result of interaction of the proteins encoded by the two regions of the S-locus of the same haplotype, the pollen germination and/or pollen-tube elongation is arrested, preventing fertilization. However, when a female determinant interacts with a male determinant of a different haplotype, no self-incompatibility occurs, and fertilization ensues. This is a simplistic description of the general mechanism of self-incompatibility. Following is the detailed description of the different known mechanisms of self-incompatibility in plants. In Gametophytic Self-incompatibility (GSI), the self-incompatibility phenotype of the pollen is determined by its own Gametophytic haploid genotype. This is the more common type of self-incompatibility, existing in the families such as Solanaceae, Rosaceae and Papaveraceae.
- 11. 11 Two different mechanisms of GSI have been described in detail at the molecular level, and their description follows. (1) The RNase mechanism The RNase mechanism was discovered in Petunia (Solanaceae). In this mechanism, pollen tube elongation is stopped when it has moved approximately one third of the way through the style. The female component ribonuclease, termed as S-RNase, causes degradation of the ribosomal RNA (rRNA) inside the pollen tube, in the case of identical male and female S-alleles. Consequently, pollen tube elongation is arrested and the pollen grain dies. STEPS: All pollen grains — incompatible as well as compatible — germinate on stigma, forming pollen tubes that begin to grow down the style. However, growth of incompatible pollen tubes stops in the style; while, compatible pollen tubes grows normally and fertilizes the egg inside the ovary.
- 12. 12 The growth-blockage within incompatible pollen tubes is created by a ribonuclease (S- RNase) encoded by an S-locus. The S-RNase It is synthesized within the style It enters the pollen tube It destroys the RNA molecules in the pollen tube As a result, the pollen tube growth is arrested. Self-Pollen Mechanism: In the self-incompatible pollen-tubes, the interaction of the SLF (male determinant) with the S-RNase (Female determinant) blocks the degradation of S-RNas (in proteasomes; a barrel-shaped cell organelle for protein breakdown); so, the RNAs of the pollen tube are destroyed and pollen tube growth is halted. Cross-Pollen Mechanism: In compatible pollen-tubes, a protein-complex (Ubiquitin- Ligase Complex), designated as “SCF(SLF)”, triggers the degradation of the S-RNase thus permitting RNAs in the pollen tube to survive and growth to continue.
- 13. 13 (2) The S-glycoprotein mechanism (discovered in Papaver rhoeas) 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 male determinant is probably some cell membrane receptor-protein. 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 pollen-tube elongation. The influx of calcium ions arrests tube elongation within 1–2 minutes. Within 10 minutes from the placement of pollen-grain on the stigma, the pollen is committed to a physiological-process, which leads to its death. At 3–4 hours past pollination, fragmentation of pollen DNA begins, and finally (at 10–14 hours), the cell dies apoptotically. Significance of Self Incompatibility 1. Nature has got balanced inbreeding and outbreeding regulated by inter-specific and intra-specific incompatibilities. 2. Extensive selfing leads to highly homozygous individuals that carry very low survival value. 3. Inter-specific incompatibility brings about reproduction-isolation, which is responsible for emergence of new races. 4. Self-incompatibility stops free flow of genes, enabling fairly large differences in populations. 5. Self-incompatibility prevents self-pollination effectively. As a result, it has a profound effect on plant-breeding approaches and objectives as given below:
- 14. 14 6. Self-incompatibility may be used in hybrid seed-production. For that, two self- incompatible but cross-compatible lines are to be interpolated; seeds obtained from both the lines would be hybrid seed. 7. Self-incompatibility provides a way for hybrid seed-production without emasculation and without resorting to genetic or cytoplasmic male sterility. 8. Self-incompatibility system permits combining of desirable genes in a single genotype from two or more different sources through natural cross pollination, which is not possible in self compatible species. 9. In case of pineapple, commercial clones are self-incompatible. As a result, their fruits develop parthenocarpically and are seedless. 10. In self-incompatible fruit trees, it is necessary to grow two cross-compatible varieties to ensure fruitfulness (formation of fruits). <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>