MCDOWELL, TIMOTHY D. Dept. of Biological Sciences, Box 70703, East Tennessee State University, Johnson City, TN 37614. - Exostema biogeography in light of molecular phylogenies and possible polyphyly.
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