Recognizing Self in the Self-Incompatibility Response

Francois Jacob once facetiously lamented the ar- rangement in the human body whereby reproduc- tion is the only function for which an individual is equipped with only one-half of the necessary organs, thus entailing the expenditure of a substan- tial amount of time and energy into finding the other half (10). The spatial separation of the sexual partners (or organs) is indeed an obligatory feature of sexual reproduction, but curiously, organisms that are her- maphroditic and hence have the potential for self- fertilization are frequently seen to be indulging in the social facet of sexuality, i.e. the search for a mate. Plants, being sessile organisms, cannot actively search for mates but have contrived a variety of genetic and nongenetic mechanisms to hinder self- pollination and promote cross-pollination. The time and energy spent on facilitating cross-pollination is amply compensated for by an increased vigor of the organism as demonstrated by Darwin (4). Self- incompatibility (SI) is an example of a genetic barrier to self-fertilization and represents the most common antiselfing mechanism among the angiosperms. CLASSICAL VIEW OF THE SI RESPONSE