It is not uncommon for plants to fertilize themselves. However, several species of Brassica reject their own pollen and are self-incompatible. Self-incompatibility is controlled by the highly polymorphic S locus, which contains two genes that encode the S receptor kinase (SRK) and S locus glycoprotein (SLG). Takasaki et al. transfected self-incompatible plants (bearing the S60 or S52/60 haplotypes) with SRK or SLG (bearing the S28 haplotype) separately and determined that the expression of SRK28 alone conferred incompatibility to S28 pollen. Although expression of SLG28 alone was incapable of promoting self-incompatibility, coexpression of SLG28 with SRK28 resulted in greater self-incompatibility than that observed with SRK28 alone. Therefore, expression of SRK is necessary and sufficient to control self-incompatibility, whereas SLG can augment the response. By having a mechanism that prevents inbreeding, plants increase their robustness through hybrid vigor.
Takasaki, T., Hatakeyama, K., Suzuki, G., Watanabe, M., Isogai, A., and Hinata, K. (2000) The S receptor kinase determines self-incompatibility in Brassica stigma. Nature 403: 913-916. [Online Journal]