Sporophyte Versus Gametophyte: A Note on the Origin of Self-Incompatibility in Flowering Plants

The evolution of gametophytic and sporophytic self-incompatibility systems in flowering plants may be the result of a conflict created by dissimilar reproductive requirements of the gametophytic and sporophytic generations. Stig- matic recognition and rejection of self-pollen is proposed as an evolutionary ad- vancement by the sporophyte in this conflict. An increased number of incompatibility groups and flower heteromorphisms are breeding system characteristics that prob- ably evolved to increase mating efficiency. Although the genetic benefits of outcrossing must be the ultimate force behind the evolution of self-incompatibility in flowering plants, still un- resolved are the selective factors that have promoted the development of two common, yet distinctively different incompatibility mechanisms. The cytological and physiological derivation of sporophytic self-incom- patibility (SSI) from gametophytic self-incompatibility (GSI) has been discussed (Brewbaker 1957; Pandey 1958, 1960; Heslop-Harrison 1978) and Heslop-Harrison (1978) has pointed out that SSI may be more met- abolically e-fficient than GSI, but the selective basis for the two incom- patibility mechanisms remains unknown. The selective forces responsible for the evolution of these two incom-. patibility systems is clarified by considering the reproductive biology of the male gametophyte. After emphasizing the evolutionary identity of the gametophytic generation, we will discuss the reproductive goals of the angiosperm male gametophyte, point out that these goals are dissim- ilar to those of the sporophyte, and propose that gametophytic and spo- rophytic incompatibility mechanisms are adaptations of the sporophytic generation in response to this conflict of reproductive requirements. We assume GSI is the primitive and fundamental type of self-incompatibility system in the angiosperms and SSI is the derived system (Arasu 1968;