Among the results obtained from the numerous hybridization experiments of the late 18th and the beginning 19th century was the discovery of self-incompatibility. Self-incompatibility means that the pollen of a plant is unable to develop a pollen tube at the stigma of the same plant, while pollen of another plant of the same species does fertilize the plant. C. DARWIN performed extensive series of experiments and concluded that self-incompatibility safeguards cross-fertilization and is a precondition of the evolution of monoecious plants (species where a single plant carries both sexes). Self-incompatibility is a barrier against inbreeding and the homozygosis caused by it. It displays its effect during the so-called progamic stage of development, i.e. before fertilization occurs, so that the chances of the egg-cell to be fertilized by a foreign pollen remain unimpaired.
It was possible to search for the genetic basis of self-incompatibility after the Mendelian laws of heredity had been rediscovered. C. CORRENS (1913) performed first studies on this topic. But the breakthrough was achieved by E. M. EAST and A. J. MANGELSDORF with their studies on Nicotiana (1925).
Their results show that self-incompatibility is caused by a gene (SI) with numerous alleles: SI1, SI2, SI3, SI4....SIn. Incompatibility occurs whenever the two plants to be crossed carry the same alleles. Since pollen is haploid is just one of the SI alleles expressed at its surface, while the diploid stigma surface bears the products of two alleles. Ferilization occurs only, if the allele product of the pollen and those of the stigma are unlike.
The case just mentioned is a gametophytic self-incompatibility (GSI). It is opposed by sporophytic self-incompatibility (SSI) where the components of the pollen exine produced by the tapetal cells are responsible for the stigma's repulsion of the pollen.
The situation of GSI is rarely as simple as in the case of Nicotiana (and numerous other Solanaceous species). A. LUNDQUIST (1956) and D. L. HAYMAN (1956) discovered independent of each other a second self-incompatibility locus (Z) in grasses. In such cases is the pollen rejected only, if both plants contain the same alleles in both loci (SI and Z). Such bifactorial systems were later on found in numerous monocot and dicot families. Finally discovered A. LUNDQUIST and his collaborators even plant species with three or more incompatibility loci (Ranunculus acris 3, Beta vulgaris 4) each of which may have numerous alleles
All this aggravates a genetic analysis since it is impossible to identify all components of the combination matrix individually. The situation becomes completely confused in case of polyploidy. Accordingly have the models proposed by LUNDQUIST been questioned (D. L. MULCAHY, G. BERGAMINI-MULCAHY, 1983). Probability calculus allows a simple, but far-reaching prediction: the frequency of incompatible combinations decreases rapidly with the increasing number of gene loci.
Systems with several genes are therefore weaker self-incompatibility systems than those with just one gene.
It seems as if angiosperm evolution was at first developing towards monoecious plants. The parallel perfecting of the SI-system made monoecism superior to dioecism. After monoecism had been established and polyploidy became more and more important did the SI-system loose importance with increasing complexity. Polyploid species do not need the SI-system any more. They can withdraw to self-pollination and inbreeding which renders them an advantage hard to make up especially in disturbed habitats.
BREWBAKER's Correlations. J. L. BREWBAKER (University of Hawaii) discovered in 1957 several remarkable correlations between SI-system and pollen features.
Gametophytic self-incompatibility occurs nearly always in pollen with two nuclei. The stigma of the respective species is usually wet.
Sporophytic self-incompatibility occurs in pollen with three nuclei. Such pollen germinates badly under natural conditions and its life expectancy is short. The respective stigmata are dry.
BREWBAKER noticed, too, that these correlations are not absolute but are opened by several spectacular cases. They are, for example, invalid in heteromorphic self-incompatibility. The Primula-example discussed earlier is such a case. The correlations do also not apply to grasses with gametophytic self-incompatibility and pollen with three nuclei. Their stigmas are dry but, in contrast to other plant families, very hairy.
It was repeatedly possible to identify glycoproteins at the surfaces of pollen and stigma. The addition of ConA reduces pollen adhesion in Galanthus nivalis drastically, an indication that lectin-lectin receptor interactions play a substantial part in the stabilization of the cells' linkages.
In contrast to many other systems like the just mentioned lectin-lectin receptor interaction, the antigen-antibody reaction or the enzyme-substrate interactions that are key-keyhole analogues, is it much more difficult to understand the mechanism of the SI system where the interacting components are alike. This was proven by an experiment performed by H. F. LINSKENS (Botanical Institute of the University of Nijmwegen, 1960) who produced antibodies against the SI-antigens of petunia pollen that reacted, too, with antigen determinants of the surface of non-compatible stigmas. R. B. KNOX and his collaborators found further proof. The proteins of the stigma surface are subject to a strong turn-over. A large amount of new material is produced after the pollen grain has been bound. The identity of the surface molecules of the two reactants is not enough to explain an incompatibility. The recognition seems only to be an elicitor of the stigma tissue's actual defensive reaction. The repulsion may occur directly at the stigma surface itself or in the transfusion tissue. It is usually accompanied by an overproduction of callose by the tissue of stigma or style.
During the last years have the genes and gene products of tobacco and petunia involved in self-incompatibility been examined closer. A. CLARKE and collaborators (University of Melbourne) cloned numerous SI-alleles and detected that the produced glycoproteins display RNase activity. Plants with sporophytic self-incompatibility like Brassica-species have glycoproteins, too, but they belong to other protein families than that of the SI products of gametophytic self-incompatibility and they lack RNase activity. The significance of the RNase activity remains unknown. The analyzed gene products occur at the stigma surface in large quantities, but they have not been identified at pollen surfaces. This seems paradox since self-incompatibility is based on the simultaneous expression of the same gene product at the surface of both pollen and stigma. In plants with sporophytic self-incompatibility was it possible to identify the respective gene products in the tapetal cells, too.
Incongruent reactions. Compatibility is not enough to explain the penetration of the stigma and style tissue by the pollen tube. Numerous enzymes of the pollen tube surface are activated and begin to disintegrate the transfusion tissue. The orientation of the directed growth is maintained in every stage of the process. The sequence of reactions of pollen tube and style tissue has to be co-ordinated. This co-ordination is impossible with pollen of foreign species. The consequence is a rejection called incongruent reaction. Usually causes the lack of one or several steps in a chain of events that starts with the encounter of pollen and style cells the incongruent reaction. Gymnosperms display no self-incompatibility, presumably because they have no style. Gymnosperms do show incongruent reactions.
Two possibilities to avoid fertilization barriers exist:
The stigma is removed and the pollen is transferred directly onto the transfusion tissue of the style. This procedure circumvents the SI system (M. KROH, 1955).
The protoplasts of incompatible plants are fused. It is tried to stimulate the fusion product to regenerate, an attempt that fails with most monocots. Such an experiment requires protoplasts of haploid plants that are difficult to obtain. More about this can be found in the section about
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de