Dioécie cryptique chez Geniostoma borbonica, espèce pionnière endémique de La Réunion
In the general context of conservation of insular habitats, the floral biology, natural fecundity, and breeding system of Geniostoma borbonica (Lam.) Spreng. was studied in natural populations. The species, which was initially assumed to be gynodioecious, presents two kinds of individuals, both with morphological hermaphroditic but functionally unisexual flowers. Some individuals produce pollen grains, but neither ovules nor fruit, whereas other fructiferous individuals have anthers without pollen grains. Geniostoma borbonica is the first species in this genus to be reported as dioecious; its floral morphology illustrates an example of cryptic dioecy. This species combines an apomictic breeding system, assuring a fast installation without reproductive constraints, and a reproductive breeding system assuring maintenance of genetic variability in the population. These two breeding systems allow adaptation to evolution of the habitat during plant succession. This point is discussed in the colonization context of pioneer habitats, where herma- phro ditic species are recognized to be advantageous, as well as in the context of the evolution of dioecy in this genus.Key words: cryptic dioecy, pioneer habitat, oceanic island, Geniostoma.