Q.What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?

Self-incompatibility is a genetic mechanism that prevents self-pollen from fertilizing ovules of the same flower, ensuring cross-pollination and maintaining genetic diversity in plant populations. It acts as a natural barrier against inbreeding.

The mechanism works through recognition systems where the pollen and pistil carry specific incompatibility genes. When pollen lands on a stigma of the same plant, these genes trigger a rejection response. The pistil recognizes the pollen as "self" and either prevents pollen germination on the stigma or inhibits pollen tube growth down the style before it can reach the ovule.

Because fertilization requires the male gamete from the pollen tube to fuse with the egg cell in the ovule, this blockage means no fertilization occurs. Without fertilization, the ovule cannot develop into a seed. The plant essentially rejects its own pollen while accepting pollen from genetically different individuals of the same species. …

Self-incompatibility is one of nature's elegant solutions to a fundamental problem: how to prevent inbreeding in organisms that cannot move. Flowering plants, rooted in place, face the constant risk of their own pollen landing on their own stigma—a situation that would lead to generation after generation of self-fertilization, reducing genetic variation and vigor. Self-incompatibility acts as a recognition system, a kind of molecular identity check that allows the pistil to distinguish "self" pollen from "non-self" pollen and reject the former.

The mechanism operates through a genetic system controlled by multiple alleles of one or more genes, most commonly the S-locus (S for sterility or self-incompatibility). Each plant carries two S-alleles, and the critical rule is this: if the pollen carries an S-allele that matches either of the two S-alleles present in the pistil, fertilization is blocked. The pistil essentially recognizes the pollen as coming from the same genetic individual or a close relative and refuses to allow the pollen tube to grow down to the ovary.

When self-pollination occurs in a self-incompatible species, the pollen does land on the stigma and may even germinate. But the recognition system kicks in almost immediately. The pollen tube either fails to penetrate the stigma, grows abnormally slowly, or is actively inhibited from reaching the ovule. The biochemical basis involves signaling between proteins produced by the S-alleles in the pollen and corresponding proteins in the pistil tissue. When a match is detected, a cascade of cellular responses halts pollen tube growth—the tube may burst, stop elongating, or be chemically blocked. Without a functional pollen tube reaching the ovule, the male gamete cannot fuse with the egg cell, and no seed forms.

This system has profound evolutionary advantages. By enforcing outcrossing, self-incompatibility maintains genetic diversity within populations, combines beneficial mutations from different lineages, and masks deleterious recessive alleles. Many commercially important crops and wild species rely on this mechanism. …