REPRODUCTIVE TRAIT VARIATION IN THE FUNCTIONALLY DIOECIOUS AND MORPHOLOGICALLY HETEROSTYLOUS ISLAND ENDEMIC CHASSALIA CORALLIOIDES (RUBIACEAE)

Documenting the floral biology and breeding system of species througho ut the Rubiaceac. family provides data on the number of times heterost yly and dioecy may have evolved in this large family. The objectives o f this paper ale to quantify (a) whether Chassalia corallioides, a sma ll tree endemic to La Reunion Island in the Indian Ocean, is another e xample of the evolution of dioecy from distyly and (b) whether reprodu ctive traits linked to male and female function vary over the ecologic al distribution of this species. Quantification of pollen production a nd fruit set following controlled and natural pollinations demonstrate that this species is dioecious. Male flowers have longer corolla tube s than female flowers. Female flowers have long styles with stigmas pl aced above the anthers whereas males have short styles with stigmas pl aced below the anthers. Stigmas and anthers are reciprocally placed in each morph, illustrating that the species is morphologically heterost ylous. Both fecundity and flower size are negatively correlated with a ltitude. In male plants, corollas are shorter and wider and anthers ar e placed closer to the mouth of the corolla tube with increasing altit ude. Male plants flowered more often than female plants, the likely ca use of the male biased sex ratio in each of the two years studied. The evolution of dioecy in relation to the island biogeography of the reg ion and the diversification of the genus Chassalia is discussed. (C) 1 998 The Linnean Society of London.