Systematics and Biogeography of the Didymopanax group of Schefflera (Araliaceae)

Schefflera is the largest genus in the angiosperm family Araliaceae, with about 900 species, of which c. 300 belong to five subgeneric groups in the Neotropical region. Previous phylogenetic studies of Schefflera have been limited to a small number of species from this region, and very little is know about phylogenetic relationships in the Brazilian-centered Didymopanax group of this genus. Therefore, to gain a better understanding of the diversity and evolution of the Didymopanax group of Schefflera, I investigated the systematics of these plants in the broader context of the entire Neotropical clade. The main goals were (1) to investigate pollen diversity in Neotropical species of Schefflera; (2) to test the monophyly of these species; (3) to provide a taxonomic revision for species of the Didymopanax group of Schefflera; and (4) to investigate evolutionary relationships within the Didymopanax group. Pollen morphology exhibits an uneven variability across Neotropical Schefflera. For example, pollen characters support the distinctiveness of the Didymopanax group from all remaining groups. Moreover, S. tremula has a distinctive pollen morphology compared to remaining species of the Sciodaphyllum group. However, among the remaining groups of Neotropical Schefflera, pollen characters are less distinctive. Molecular phylogenetic analyses have confirmed the monophyly of the Neotropical species of Schefflera, and helped to identify four major clades. One of these clades includes subclades representing the Didymopanax and Crepinella groups, while another clade includes all species from groups Cotylanthes and Sciodaphyllum, excluding Schefflera tremula, a finding that corroborates pollen data. In the formal taxonomic revision of Didymopanax, 37 species are recognized, together with three insufficiently known species. The revision also includes updated species circumscriptions and nomenclatural adjustments for 26 names. Phylogenetic analyses among Didymopanax species recovered four morphologically and geographically coherent clades (Atlantic Forest, Imeri, Five-carpellate and Savannic clades), but their phylogenetic inter-relationships were generally weakly supported. Poorly resolved relationships in the Savannic clade suggests a rapid diversification in the campos rupestres vegetation, which accounts for the greatest species richness in the group. The presence of multiple Didymopanax lineages in the Amazonian and Atlantic forests corroborates that these regions may be composite biogeographic areas