Floral Variation and Sexual Differentiation in Muntingia calabura (Elaeocarpaceae), a Species with Hermaphrodite Flowers

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Abstract
Muntingia calabura is a small tree of early successional habitats in lowland Central America. In Costa Rica individual trees flower throughout the year producing a few flowers each day in small fascicles. M calabura is self-compatible, and the 1-day flowers are bee-pollinated. Each tree produces a wide and continuous range of floral forms: at one extreme flowers have a large pistil and as few as 10 stamens, and at the other extreme there are flowers with a very reduced pistil and over 100 stamens. Although this variation in floral form is continuous, flowers with 15-25, 40-60, c. 100 stamens predominate. All flowers produce pollen and retain the potential to produce fruit, but the probability of a flower producing fruit is proportional to pistil size. Fruits contain thousands of tiny seeds. Floral sexuality is strongly related to the order in which flowers open within fascicles; those which open earlier have fewer stamens and larger pistils. The slight, but significant, differences in ovary size and stamen number found between trees are greatly exceeded by the variation among fascicle positions within individual trees. The form of sexual differentiation in flower morphology found in M. calabura has not been described in detail before, although we suggest that similar patterns may remain undetected in other hermaphroditic species. The continuous nature of the variation in floral form distingishes the sexual system of M. calabura from the discontinuous variation in flower morphology which characterizes monoecism, gynomonoecism and andromonoecism. In addition, although the sexual system bears close resemblance to andromonoecism, it differs in that even those flowers with the greatest bias toward male function, may produce fruit if earlier flowers within the fascicle are aborted; it differs from monoecism and gynomonoecism in that flowers with very well developed pistils retain at least a few anthers which may be necessary for pollinator attraction or for self-pollination in order to fertilize the numerous ovules. The continuous variability in floral form, combined with continuous flowering, means that plants can respond more flexibly to the different selective pressures acting on the male and female components of fitness when allocating resources to stamens, pistils, and fruit maturation. As all flowers retain at least the potential to perform maternal and paternal functions, individual fascicles can respond to unpredictable changes, such as the loss of earlier flowers, by altering the sexuality of later opening flowers.

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