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