Self-incompatibility is an inherited ability of a flower to reject its own pollen. This trait effectively prevents inbreeding and promotes outcrossing. In the absence of self-incompatibility, inbreeding would be favored in many species, as the flowers often have male and female organs within close proximity on the same plant and even within the same flower. Higher plants do not have the option of moving about and choosing a mating partner, so the mechanisms that prevent self-pollination are of crucial importance for maintaining genetic variability within plant populations.

The various systems of self-incompatibility have fascinated plant biologists since Darwin’s time, and there is a rich literature describing the phenomena for a range of species in terms of classical genetics and microscopic observation. Much of this early literature has been considered by de Nettancourt (1977) in his classic monograph “Incompatibility in Angiosperms;’which is a key reference for any student of self-incompatibility. Since 1985, there has been a surge of interest in the field, as the techniques of molecular biology have been applied to dissecting the nature and control of the S (self-incompatibility) genes. This work has been reviewed on many occasions (Nasrallah and Nasrallah, 1986; Bernatzky et al., 1988; Cornish et al., 1988a; Bacic and Clarke, 1987). This present review is intended to give a perspective of the field, and draws on information presented in recent reviews.