AN UNNATURAL BREEDING SYSTEM POLYMORPHISM IN CACAO (THEOBROMA-CACAO, STERCULIACEAE) IN TRINIDAD

The evolution of selfing vs. outbreeding has been of major interest to plant population biology. The independent historic introductions of s elf-compatible and self-incompatible genotypes of cacao in Trinidad ha ve allowed us to study the selection acting upon an unnatural breeding system polymorphism. Field observations of an abandoned cacao plantat ion indicate that the self-incompatible phenotype has slightly increas ed in frequency within a single generation. The self-compatible trees produced significantly less flowers but still set more pods than did t he self-incompatible trees, although compatibility types did not diffe r in tree size or mature seed production. Greenhouse observations sugg est that the apparent failure of self-compatibility to increase in the population is related to inbreeding depression resulting from selfing , expressed as reduced seedling establishment.