Temporal Dioecism: Systematic Breadth, Associated Traits, and Temporal Patterns

Abstract

Temporal dioecism, the temporal alternation of pistillate and staminate functions within a plant, is a relatively common mating system among flowering plants. It occurs in at least 37 angiosperm families and is common in Apiaceae, Araceae, Arecaceae, Juglandaceae, Sparganiaceae, Typhaceae, and hermaphroditic species in Aceraceae and Lauraceae. It is more common in the primitive subclasses, e.g., Magnoliidae, Hamamelidae, and Alismatidae, and less common in the more advanced subclasses, e.g., Asteridae and Liliidae. Because it occurs in more families than does heterostyly (37 vs. 24) and relatively few genera have been studied in those families whose mating systems are characterized by temporal dioecism, I suggest that it is more common than heterostyly. Temporal dioecism is strongly associated with monoecism, self-compatibility, many simultaneously receptive stigmas, and less efficient pollen vectors, e.g., wind, beetles, and flies. In such species, temporal dioecism should increase pollinator efficiency, minimize geitonogamy, and facilitate xenogamy. Because it occurs primarily in self-compatible species and minimizes self-fertilization, it may be an evolutionary alternative to self-incompatibility. There are numerous temporal phenotypes that include many of the possible combinations of protogyny and/or protandry, sequential and/or synchronized flowering, and single or multiple alternations of sexual phases. The simplest phenotype, which results from the sequential opening of the flowers within an inflorescence in monoecious species, is uncommon. The relative lengths of the pistillate and staminate phases result in sex-phase ratios at the population level. Four general associations involving sex-phase ratios, lengths of the two phases, and pollen vectors are evident: (1) wind pollination and relatively long sexual phases with slightly pistillate- or staminate-biased sex-phase ratios; (2) nonspecialized insect vectors and strongly staminate-biased ratios; (3) specialized vectors (beetles, moths, and wasps), short phases, and unbiased ratios; and (4) generalist insect vectors, short and repeated sexual phases, and unbiased ratios. The evolution of facultatively autogamous and facultatively xenogamous species from temporally dioecious relatives is associated with changes in the temporal phenotype that produce an overlap of sexual function, either within and/or between inflorescences.