Gametophytic Self-Incompatibility: Biochemical, Molecular Genetic, and Evolutionary Aspects

This chapter focuses on the gametophytic self-incompatibility—a prezygotic barrier to self-fertilization in plants that otherwise produce fully functional gametes. Two types of reproductive barriers operate in plants: interspecific and intraspecific. Interspecific reproductive barriers ensure the stability of each species, whereas intraspecific barriers permit a reasonable degree of variability within the species. Interspecific reproductive barriers are a consequence of evolutionary divergence among species, and the failure to cross-fertilize lies in the inability of pistil tissue to sustain the growth of pollen from other taxa, a phenomenon described as “incongruity.” Self-incompatibility is an intraspecific reproductive barrier. It is a system of cell-to-cell recognition allowing the pistil to recognize and reject pollen from genetically related individuals, thereby promoting outbreeding and heterozygosity in the population. Two main types of self-incompatibility systems exist: heteromorphic and homomorphic. New S alleles that appear in a population have a reproductive advantage over the extant S alleles because pollen bearing the new allele is less likely to land on stigma carrying the same allele and, thus, more likely to escape rejection.