Cryptic dioecy and leaky dioecy in endemic species of Dombeya (Sterculiaceae) on La Reunion

The high frequency of dioecy on oceanic islands such as Hawaii and New Zeal and has attracted a great deal of attention from plant evolutionary biologi sts. One reason suggested for the high prevalence of dioecy on oceanic isla nds is that taxa considered truly dioecious may have occasional hermaphrodi te flowers. i.e., show leaky dioecy. In this study, we quantified the prese nce and distribution of leaky dioecy in a group of congeneric endemic speci es of the genus Dombeya (Sterculiaceae) on La Reunion island (Indian Ocean) . All eight species show cryptic dioecy. Five species show strict dioecy an d three species show leaky dioecy due to the presence of male trees that se t fruit. Species with strict dioecy and large populations tend to occur in mid- to high-altitude moist tropical cloud forest, whereas species in small er populations at lower altitude and in semidry tropical forest tend to sho w leaky dioecy. Two reasons for this differential distribution of strict di oecy and leaky dioecy are discussed. First, environmental variation along t he altitudinal gradient, biotic and/or abiotic, may influence the breeding system. Second, leaky dioecy may be favored in lowland populations due to t he small size and disturbed nature of such populations.