You might wonder why I named this article “Fertilize or not? It’s up to you”. It’s because plants have the ability not to fertilize even though the mature pollen falls onto mature stigma. Self-fertilization may occur in hermaphrodite (bisexual) plants and self-fertility might be beneficial for plants. However, plants also have developed mechanisms to prevent self-fertilization, which can lead to inbreeding depression. Higher plants which are mostly hermaphrodite can assure reproduction through cross-fertilization. Self-incompatible plants increase outbreeding and retain genetic variety, which is thought to be crucial in the evolution of flowering plants.
Self-incompatibility is essentially a carefully regulated genetic system that offers a highly selective detection and rejection system for pollen that is genetically similar to the pollen of the same plant. Darwin initially identified heteromorphic Self-incompatible systems, which use blooms with diverse morphs (e.g. distyly in Primula), but they have not been adequately studied and defined at the molecular level. Self-incompatibility provides a clever genetic mechanism for avoiding self-fertilization. S-determinant genes, which allow self-recognition and rejection, determine self-incompatibility. Several S-determinant genes have been discovered in various plants. A highly polymorphic, multiallelic S-locus is responsible for homomorphic Self incompatibility. It has been calculated that natural populations of a single species may have up to 30 to 60 alleles. As it is maintained, this polymorphism has been the topic of several populations of genetics research in the modern world.
There are mainly two types of self-incompatibility in plants. They are gametophytic self-incompatibility (GSI) and Sporophytic self-incompatibility (SSI). In Gametophytic self-incompatibility when the solitary S allele contained in the haploid pollen grain matches either of the S alleles present in the diploid tissues of the pistil, pollen is rejected. This is due to the detection of some proteins in pollens by the receptors in the stigma of the flowers. Some examples of plants that show gametophytic self-incompatibility are ornamental tobacco (Nicotiana alata), petunia (Petunia inflata and Petunia hybrida), potato (Solanum tuberosum and Solanum chacoense), and wild tomato (Lycopersicon peruvianum). In Sporophytic self-incompatibility systems, rejection is controlled by the interaction of the pistil’s self-incompatibility genotype with the genotype of the pollen parent, rather than the pollen’s haploid genotype, as in the gametophytic system. In plants with sporophytic self-incompatibility, each pollen grain contains the products of two S alleles, and rejection occurs when either of these alleles matches either of the S alleles expressed in the pistil; complex dominant or codominant interactions between S alleles frequently occur, influencing the outcome of specific crosses.

This amazing mechanism in plants has saved plants from genetic disorders and many other genetic disadvantages. Thus, self-incompatibility has provided a great evolutionary advantage to plants.

References:
Matton, D. P., Nass, N., Clarke, A. E., & Newbigin, E. (1994). Self-incompatibility: How plants avoid illegitimate offspring. Proceedings of the National Academy of Sciences of the United States of America, 91(6), 1992–1997. https://doi.org/10.1073/pnas.91.6.1992

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