Begonia is one of the largest angiosperm genera, with 1400 species currently recognised. These were placed into 63 sections in the most recent taxonomic treatment. However, there is considerable uncertainty in both section inter-relationships and sectional composition, and there is no formalised phylogenetic hypothesis for the genus. Using the nuclear internal transcribed spacers (ITS) and partial large subunit (26S) sequences of ribosomal DNA, we have produced phylogenetic trees to form the basis of a cladistic framework for the interpretation of the evolution and sectional level systematics of Begonia. Cladograms were produced for 35 Begonia, one Symbegonia and two Datisca species, for partial 26S and ITS sequence data. Analyses suggest that African taxa are basal in Begonia. The New Guinea genus Symbegonia is nested deeply within Begonia. A far larger data set was constructed by sequencing the ITS region for 153 species (two Datisca, three Symbegonia, one Hillebrandia and 147 Begonia). In the resulting tree, the African species of Begonia resolve as paraphyletic, with both the Asian species (including Socotra) and the American species (sister to southern African species) monophyletic. Using a combined morphology - ITS analysis, the fit of individual morphological characters to a tree was examined. Some of the characters which have traditionally been considered important in Begonia taxonomy (e.g. number of placental branches per locule) proved highly homoplastic. The correlation between phylogenetic relationships implied by the ITS tree and the geographical distributions of species was explored. As a general rule, related species are geographically proximal, suggesting limited dispersal of lineages. In contrast, morphologically similar but geographically separated species tend to be phylogenetically disparate.

Key words: 26S, Begonia, Begoniaceae, ITS, Symbegonia