a Morphological Hypothesis for the Origin of Heterostyly in the Rubiaceae

• Anderson, William R.

Abstract

Summary Dimorphic heterostyly is common in the Rubiaceae, distributed throughout the family in a way that suggests that it has evolved repeatedly. In its essentials this heterostyly suggests protandry in which some plants have normal elongation of the styles and others have precocious maturation of the stigmas at a stage of arrested elongation of the styles. It is suggested that the initiation of heterostyly in this family may lie in a modification of the common protandry. High reproductive fitness of the self-pollinating short-styled plants would begin a shift in the population from predominant outcrossing to predominant autogamy, even though the resulting homozygosity might be disadvantageous. This shift might be halted by the addition of diallelic self-incompatibility of the kind regularly found in heterostylous plants. The population would then achieve the stable polymorphism characteristic of dimorphic heterostyly. Additional morphological refinements might accrue in time.