Gametophytic Self-Incompatibility: A Mechanism for Self/Nonself Discrimination during Sexual Reproduction.

Various mechanisms in flowering plants have evolved to prevent the tendency of self-fertilization created by close proximity of male and female reproductive organs in a perfect flower. One such mechanism, called self-incompatibility (SI), allows the pistil of a plant to reject self pollen or pollen from genetically related individuals, thus preventing inbreeding and promoting outcrosses. Genetic studies camed out early in this century led to the identification of two different types of SI: gametophytic self-incompatibility (GSI) and sporophytic self-incompatibility (SSI). GSI was first studied in the Solanaceae family, and SSI was first studied in the Cruciferae family. To date, these two families still remain the best characterized for their respective SI systems. In both families, SI is controlled by a multiallelic locus termed the S-locus. For GSI, the SI behavior of the pollen is determined by the genotype of the pollen grain itself, whereas for SSI, the SI behavior of the pollen is determined by the genotype of the pollen parent. This difference most likely reflects the difference in the site of expression of the pollen S-allele: microspores for GSI and tapetal tissue for SSI (de Nettancourt, 1977). In the case of SSI, depending on the S-allele combination, the two S-allele specificities displayed by the pollen may be co-dominant or may exhibit other relationships such as dominance or mutual weakening (de Nettancourt, 1977). Figure 1 illustrates the dlfference in the SI behavior of SSI and GSI, assuming co-dominance of S-alleles in the pollen for SSI. Crosses between an S1S2 plant (female) and an s1s3 plant (male) will be compatible if the species displays GSI and will be incompatible if the species displays SSI. The reasons are as follows. In the case of GSI, the SI phenotypes of SI and S3 pollen produced by the S1S3 plant are S1 and S3, respectively. The pollen bearing Sl-allele is rejected by the pistil of the S1S2 plant because of the matching of the S1allele, whereas the pollen bearing S3-allele, an allele different from those camed by the pistil, is able to effect fertilization to produce SlS3 or S2S3 progeny. The rejection of incompatible pollen usually occurs after the pollen has germinated and