Incompatibility in Flowering Plants: Adaptation of an Ancient Response.

Abstract

Angiosperms differ in their ability to fertilize themselves. Differ? ences in self-compatibility (SC) presumably reflect ancient events because both self-compatible and self-incompatible plants occur throughout the angiosperms. It has often been assumed that the first angiosperms were self-compatible and that self-incompatibility (Sl) is a derived condition, but this has not been proven definitively. Another possibility is that Sl is the ancient state and SC the derived condition. That this may be the case grows out of a consideration of the requirements for the development of angiospermy itself. Although the principal characteristic of the angiosperms is usu? ally regarded as a closed carpel, the existence of a small number of species in which the carpel is open (e.g., the monocotyledon genus Butomus and the dicotyledon genus Reseda) shows that closure of the carpel is not essential for angiospermous reproduction. What does appear to define the angiosperms is the ability of the male gametophyte, repre? sented by the pollen tube, to grow through intact sporophytic tissue (commonly the style and nucellus), a process termed penetrative siphonogamy. The angiosperms are the only group of land plants in which the pollen tube carrying the male ga? metes is known to maintain intimate cell-cell contact with the