The genus Exostema (Rubiaceae) comprises 25 species of trees and shrubs with narrow tubular flowers, dehiscent capsular fruits and small, usually winged seeds. Twenty-two species are endemic to the Greater Antilles, mostly in Cuba and Hispaniola. Middle America, the Andes, western Amazonia and the Lesser Antilles each have one endemic species. Phylogenetic analyses of morphological and ITS (nuclear DNA) sequences have placed the South American species basal to the Caribbean Exostemas, but exact relationships among mainland and island species were uncertain. To clarify infrageneric and outgroup relationships, new rbcL (chloroplast DNA) sequences have been obtained for ten Exostema species and several closely related taxa, and extra ITS sequences added for several taxa. Both ITS and rbcL DNA data indicate that Exostema is polyphyletic with respect to various neotropical genera with funnelform flowers and capsular fruit such as Coutarea (mainland neotropics) and Cubanola (Greater Antilles). These molecular data also suggest that Exostema is paraphyletic with respect to the genera Chiococca (widespread neotropical) and Erithalis (circum-Caribbean & Antilles), which have indehiscent fruit quite unlike Exostemas'. Certain morphological characters remain consistent in all species of these disparate genera: basifixed, linear anthers and spinulose, tricolporate pollen. Thus, the broadened ITS and rbcL analyses challenge the generic limits of Exostema and necessitate reexamination of relationships among various long-recognized genera. Despite these conflicts, an unambiguous biogeographic pattern is clearly supported for the clade comprising the short-flowered species of section Parviflorum. The middle American E. mexicanum is sister to the four Cuban and Hispaniolan species, and the Andean endemic E. corymbosum is probably most basal among this group. Overall biogeographic trends in Exostema s.l. include extreme morphological divergence in both vegetative and reproductive features for island species, with species radiations in Cuba and Hispaniola on both serpentine and calcareous soils.

Key words: Antilles, biogeography, Exostema, ITS, phylogeny, rbcL