Monophyly and relationships of the tribe <i>Exaceae</i> (Gentianaceae) inferred from nuclear ribosomal and chloroplast DNA sequences

Both chloroplast <i>trn</i>L (UAA) intron and nuclear ribosomal ITSsequences highly confirmed the monophyly of the tribes of the Gentianaceae defined by the recent classification, and revealed the tribe <i>Exaceae</i> as a basal clade just next to the basal-most lineage, the tribe <i>Saccifolieae</i>. Within the tribe <i>Exaceae</i>, <i>Sebaea</i> (except <i>Sebaea madagascariensis</i>) appeared as the most basal clade as the sister group to the rest of the tribe. The Madagascan endemic genera <i>Gentianothamnus</i> and <i>Tachiadenus</i> were very closely related to each other, together standing as sister to a clade comprising <i>Sebaea madagascariensis</i>, <i>Ornichia</i>, and <i>Exacum</i>. The saprophytic genus <i>Cotylanthera</i> nested deeply inside <i>Exacum</i>. <i>Sebaea madagascariensis</i> was shown closer to the Madagascan endemic genus <i>Ornichia</i> than to any other sampled <i>Sebaea</i> species. <i>Exacum</i> appeared as the most derived taxon within this tribe. The topology of the phylogenetic trees conform with the Gondwana vicariance hypothesis regarding the biogeography of <i>Exaceae</i>. However, no evidence for matching the older relationships within the family to the tectonic history could be corroborated with various divergence time analyses. Divergence dating estimated a post-Gondwana diverging of the Gentianaceae about 50 million years ago (MYA), and the tribe <i>Exaceae</i> as about 40 MYA. The Mozambique Channel land-bridge could have played an important role in the biogeographic history of the tribe <i>Exaceae</i>.