African Continent a Likely Origin of Family Combretaceae (Myrtales). A Biogeographical View

Aim: The aim of this study was to estimate divergence ages and reconstruct ancestral areas for the clades within Combretaceae.

Methods: We utilized a comprehensive dataset of 144 species of Combretaceae with a worldwide sampling to reconstruct a dated phylogeny based on a Bayesian analysis of five gene regions (ITS, rbcL, matK, psaA-yf3 and trnH-psbA). Bayesian phylogenetic tree was generated using a Bayesian MCMC approach implemented in BEAST v.1.7.5 to generate lineage dating. Two fossils Dilcherocarpon Combretoides (93.5-112 mya) and Terminalioxylon (28 mya) were used for calibration. S-DIVA and DEC model analysis were used to estimate ancestral area ranges.

Results: Our results indicate that the earliest diversification of Combretaceae occured ca. 110 mya. This was followed by the splitting of the family into two subfamilies, Combretoideae and Strephonematoideae during the Late Cretaceous period. This event followed the radiation of Combretoideae, ca 105.6 mya to give rise to two tribes, Combretaeae and Laguncularieae which diverged around 60.9 mya and 52.9 mya, respectively. The two main subtribes Combretineae and Terminaliinae, radiated at ca. 48.3 and 46.4 mya respectively. African continent is inferred as the origin of Combretaceae, with dispersal as the major event responsible for the intercontinental disjunct distribution observed in the tropical and subtropical regions.

Main Conclusions: Our results revealed that the crown age of Combretaceae is ca.110 mya, a time hypothesised to be marked by high angiosperm diversification. Two largest subtribes Combretineae and Terminaliinae, split occurred in the Late Cretaceous period with divergence estimated at the commencement of Eocene epoch. The African continent is hypothesised to have emerged from the split of the super continent Gondwana. Long distance dispersal is postulated as the major modeller, with vicariance and extinction playing marginal roles in shaping the current intercontinental disjunct distribution of Combretaceae in the tropical and subtropical regions of the world.