Molecular Genetics of Self-incompatibility in Nicotiana alata

Self-incompatibility is a genetically controlled mechanism which prevents inbreeding in plants (de Nettancourt, 1977). In many, but not all cases, it is controlled by a multi-allelic, single gene, the S-gene. The system operates to enhance outcrossing and to ensure that a plant is fertilized by a genetically distinct individual of the same species. There are two major types of self-incompatibility. The most widespread is gametophytic self-incompatibility which involves interaction of a product of the haploid genome of the male gametophyte (carried within the pollen grain) and a product of the diploid genome of the female tissue of the sporophyte, the pistil. In incompatible matings, as is the case when the S-allele carried by the haploid pollen matches either of the S-alleles present in the diploid style, pollen tube growth is arrested within the transmitting tract (Figure 1).