is one of the mechanisms that have evolved to encourage outbreeding in flowering plants and is defined as “the inability of a fertile hermaphrodite seed plant to produce zygotes after self-pollination” (de Nettancourt, 1977). The effectiveness of SI in promoting outbreeding is believed to be one of the most important factors that ensured the evolu- tionary success of flowering plants (Whitehouse, 1951). It is a genetically controlled phenomenon, and in many cases, the control is by a single locus (known as the S locus) with a large number of alleles, up to severa1 hundred in some species (Ockendon, 1974; de Nettancourt, 1977). SI has been a favor- ite topic for botanists and geneticists since Darwin (1877) first discussed the phenomenon and suggested the idea of its cen- tral significance during the evolution of flowering plants. During the century or more of work on the subject, there have been a number of key reviews, the most significant