selfincompatible

Self-incompatibility is a genetic mechanism in many flowering plants that prevents self-fertilization and promotes outcrossing. It is controlled by a highly polymorphic S-locus, a genomic region with multiple S-alleles. The interaction between the pistil and pollen determines compatibility: if the pollen carries an S-allele that matches one of the pistil’s S-alleles, the pollen is rejected; if there is no match, fertilization can proceed. There are two main types of self-incompatibility: gametophytic self-incompatibility (GSI) and sporophytic self-incompatibility (SSI).

Gametophytic self-incompatibility is determined by the haploid genotype of the pollen. Pollen tubes carrying an S-allele that matches any of the pistil’s active S-alleles are inhibited or degraded as they grow through the stigma and style. The mechanism is well documented in families such as Solanaceae, Rosaceae, and Plantaginaceae, with S-RNases playing a key role in pollen rejection.

Sporophytic self-incompatibility is controlled by the diploid genotype of the parent plant, and the pollen's surface characteristics reflect this parental genotype rather than the pollen’s own haploid genotype. In SSI systems, receptor-ligand interactions between the pistil (often involving SRK receptors) and pollen (such as SCR/SP11 ligands) trigger the rejection of self-pollen. Brassicaceae and related groups are classic examples.

Self-incompatibility helps maintain genetic diversity but can be lost or reduced in some lineages, leading to self-compatibility. This shift can influence mating systems, breeding, and population structure.