S-Gene action and the significance of characters in the heterostylous syndrome

Abstract

Heteromorphic characters of stigmas and pollen grains participate in the physiological mechanism of incompatibility. This points to incompatibility as a major factor in the evolutionary moulding of pollen and stigmas. A first reconstruction of S-gene action shows that it controls normal growth and differentiation of floral parts. The S gene operates through a stepwise progression of events. The independently mutable subunits of the S gene revealed by breakdowns of the heterostylous syndrome control stages in the chain of activities of the gene. Inception of pollen size heteromorphism during early meiosis indicates that sporophytic incompatibility factors may be derived not only from the tapetum, but from the pollen mother cells as well. It is suggested that the differential growth of styles and stamens in the morphs mediates the production of chemical recognition specificities and that metabolism related to wall extension plays an important role in incompatibility. The physiological basis of differential growth control in the morphs and the significance of pollen size heteromorphism are discussed.