Reproductive trait variation in the functionally dioecious and morphologically heterostylous island endemic Chassalia corallioides (Rubiaceae)

Abstract

Documenting the floral biology and breeding system of species throughout the Rubiaceae family provides data on the number of times heterostyly and dioecy may have evolved in this large family. The objectives of this paper are to quantify (a) whether Chassalia corallioides, a small tree endemic to La Reunion Island in the Indian Ocean, is another example of the evolution of dioecy from distyly and (b) whether reproductive traits linked to male and female function vary over the ecological distribution of this species. Quantification of pollen production and fruit set following controlled and natural pollinations demonstrate that this species is dioecious. Male flowers have longer corolla tubes than female flowers. Female flowers have long styles with stigmas placed above the anthers whereas males have short styles with stigmas placed below the anthers. Stigmas and anthers are reciprocally placed in each morph, illustrating that the species is morphologically heterostylous. Both fecundity and flower size are negatively correlated with altitude. In male plants, corollas are shorter and wider and anthers are placed closer to the mouth of the corolla tube with increasing altitude. Male plants flowered more often than female plants, the likely cause 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 region and the diversification of the genus Chassalia is discussed.