New delimitations and phylogenetic relationships of Sabiceeae (Ixoroideae, Rubiaceae) and revision of the Neotropical species of Sabicea Aubl.
The pantropical tribe Sabiceeae (Ixoroideae s.l., Rubiaceae) has been variously circumscribed previously with strong conflicts. The monophyly of Sabiceeae including its all established genera or the monophyly of any non-monotypic genus traditionally included in this tribe has never been examined using molecular data sets. Sabicea, the type and most species rich genus of the tribe has not been revised to any extent after its first monographic treatment (Wernham, 1914), mostly based on typological concept. The present study reveals for the first time that neither the tribe Virectarieae, currently established as the associated tribe of Sabiceeae, nor the subtribe Virectariinae is monophyletic; rather, the tribe Sabiceeae including the genera of Virectarieae or Virectariinae is strongly resolved as a monophyletic group in trnT-F and combined ITS-trnT-F trees. This study suggests the broad circumscription of the tribe Sabiceeae (Sabiceeae s.l.) consisting of eight genera: Ecpoma, Hekistocarpa, Pseudosabicea, Sabicea, Schizostigma, Stipularia, Tamridaea, and Virectaria. Additionally, this study suggests the placement of both Pentaloncha and Temnopteryx, in subfamily Rubioideae. The monophyly of the newly delimited Sabiceeae s.l. is entirely based on molecular data. This study reveals that the genera Ecpoma, Pseudosabicea, Sabicea, Schizostigma, and Stipularia, traditionally associated with the tribe Sabiceeae, together form a monophyletic group and their morphological distinctions are not clear-cut. Therefore, all of these genera are merged under Sabicea s.l., and finally the tribe Sabiceeae s.l. is recognized with four genera: Hekistocarpa, Sabicea s.l., Tamridaea, and Virectaria. The resolved clades of combined ITS-trnT-F tree (Chapter 6.1, Fig. 3) indicate that the São Tomean, Malagasy, and all Neotropical Sabicea presumably originated from African ancestors. They reached São Tomé & Principe, Madagascar, and the Neotropics independently via four dispersal events. This study also indicates that most probably the major diversification of Sabicea s.l. started in mainland Africa and the second major radiation occurred in the Neotropics. The presence of only one species– Sabicea ceylanica in Sri Lanka indicates the unsuccessful diversification of the genus in Asia. The results of combined ITS-rpoC1-trnT-F analysis (Chapter 6.2, Fig. 1) suggest that the monotypic genus Hekistocarpa is closely related with the genus Virectaria, the monotypic Tamridaea and the Pantropical Sabicea s.l. The clade of all Virectaria accessions strongly resolved in the ITS-rpoC1-trnT-F tree supports the monophyly of the genus Virectaria. The combined ETS-ITS-rpoC-trnT-F tree (Chapter 6.2, Fig. 2) exhibits the close relationships within the genus under two highly resolved groups, and the monophyly of the six sampled species of Virectaria in correspondence to their morphological characters. The resolved clades of the combined ITS-rpoC1-trnT-F tree suggest the tropical African, possibly Guineo-Congolian, origin for the whole Sabiceeae as Hekistocarpa, constantly resolved as sister to the clade of Tamridaea, Virectaria and Sabicea s.l., is Lower-Guinean. The high number of both molecular and morphological autapomorphies of Tamridaea testifies its long isolated evolution. The resolved clades of Virectaria (Chapter 6.2, Fig. 2) indicate the floral exchange between Lower- and Upper-Guinean, Guineo-Congolian and Zambezian regions. The ETS-ITS-rpoC-trnT-F tree indicates the presence of at least three (V. herbacoursi and V. multiflora, V. angustifolia, and V. procumbens, V. major and V. belingana) vicariant couples within the Guineo-Congolian and Zambezian regional center of endemism. The taxonomic revision of Neotropical Sabicea, based on a large bulk of specimens, recognizes 37 species from the previously described 54 species and merges the remaining 17 with other well-defined species of the genus, as no reason was found for their recognition. Additionally six new species (S. boyacana, S. chiapensis, S. cochabambensis, S. liedeae, S. noelii, and S. tayloriae) are reported. Finally, this revisionary study provides a comprehensive taxonomic treatment of 43 species, 37 from South America and five from Mesoamerica, with distribution maps and 30 illustrations. All descriptions are originally generated from DELTA (Dallwitz & al., 1999) using 620 vegetative and reproductive characters. A detailed indented key to all of 43 species is provided. Lectotypes are designated for six species. This study reveals that the highest degree of endemism of the genus in the Neotropics occurs in Brazil, Bolivia, Venezuela, and Colombia with four species each. Mexico harbors three endemic species, Peru two and Jamaica and Surinam one each. Lastly, this study is an excellent base for the completion of the reclassification within the tribe Sabiceeae, especially within the genera Virectaria and Sabicea s.l. in Africa.
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