A monograph of Exostema (Rubiaceae)

The genus Exostema (Rubiaceae) was described in 1807 to include species separated from the Cinchona on the basis of exserted stamen. Like Cinchona, the source of quinine, some Exostema species have anti-malarial properties. Taxonomically the genus Exostema has been poorly known, with many uncertain species, due to its early economic potential and to the highly endemic pattern of its Antillean distribution. The last revision of the entire genus was in 1830 (De Candolle) and has little relevance to present taxonomy. This investigation is a systematic revision and evolutionary analysis of Exostema throughout its range. It is based on a review of taxonomic literature, herbarium study of over 3000 specimens, field observations for 16 Antillean Exostema species, light and scanning electron microscopy of leaf anatomy and pollen, and an examination of ribosomal DNA ITS sequences for ten species. Parsimony analyses were performed on morphological, anatomical and molecular characters to elucidate patterns of evolution and provide a basis for a natural infrageneric classification. The results of these investigations presented here include: a discussion of taxonomic history and nomenclatural problems; descriptions of morphology, anatomy, ecology, floral biology and economic botany; a new infrageneric classification, dichotomous keys and descriptions for all species and sections; and phylogenetic analyses of morphological, molecular and combined data sets, with discussions of character evolution and biogeography in Exostema. Twenty-five species of Exostema are recognized, ranging in distribution from western South America to the Bahamas, with 17 species endemic to Cuba and Hispaniola. Three sections are described, E. sect. Exostema, E. sect. Pitonia D.C., and E. sect. Parviflora T. McDowell ined., based on inflorescence position and flower length. These natural groups are also supported by anatomical and molecular characters. The major evolutionary patterns in Exostema involve the colonization of drier habitats and specialized serpentinic or rheophytic sites, and a shift from lepidopteran to bee pollination. The proliferation of species in Cuba and Hispaniola reflects the complex geological history of the Caribbean region. The biogeographical analysis indicates a South American origin for Exostema, with independent migrations to the Greater Antilles occurring in each section of the genus.