The Role of Self-Incompatibility and Sexual Selection in the Gymnosperm-Angiosperm Transition: A Hypothesis

Abstract

Self-incompatibility is viewed as a mechanism by which the female can evaluate the quality of the male gametophyte. This does not preclude its function as an outbreeding mechanism, but rather expands its role as a means to mediate the quality of the offspring by the female. The early angiosperm self-incompatibility system is envisioned as consisting of stigmatic recognition, with an incompatible combination resulting in carpel abortion. The morphological transition from the gymnospermous to the angiospermous condition (i.e., the placement of sterile sporophytic tissue between ovules and pollen) occurred concomitantly with a change in the timing of mate assessment (postzygotic to prezygotic). The selective impetus for the evolutionary development of the closed carpel is hypothesized to be the origin of self-incompatibility rather than solely a means of protection. This shift in timing prevents mechanisms that were effective for reducing male competition in gymnosperms and intensifies male competition in angiosperms (Mulcahy 1979, 1981). Thus, strong selective pressure was exerted on the male to develop alternative mechanisms for increasing its chances of acceptance by the female in angiosperms. The evolutionary development of accessory floral parts and the morphological modification of these parts to influence insect behavior might be viewed as male mechanisms to better ensure more-efficient transfer of pollen to a receptive female, thus improving its chances of acceptance by the female (cf. Willson and Burley 1983). The evolution of various insect-pollination syndromes, especially those involving faithful pollinators, is considered one of the most important events to have influenced the morphological diversity subsequently observed in angiosperms in the Cretaceous (Crepet 1979, 1983, 1984, and references therein; Mulcahy 1979).