Generic delimitation and historical biogeography in the early-divergent 'ambavioid' lineage of Annonaceae: Cananga, Cyathocalyx and Drepananthus

Abstract

Generic delimitation of Cyathocalyx and Drepananthus has been controversial, with some authors recognizing them as distinct genera, and others recognizing a more broadly defined Cyathocalyx, inclusive of Drepananthus. Some doubt also exists regarding the relationships between these taxa and Cananga. Molecular phylogenetic analyses are presented based on combined psbA-trnH spacer, trnL-F, matK and rbcL sequences. Results indicate that Cananga, Cyathocalyx s.str. and Drepananthus form three generally well-supported clades, although with inadequate resolution of relationships among the three clades. Morphological variation is re-evaluated, and the narrower delimitation of Cyathocalyx proposed, necessitating 21 new nomenclatural combinations following the recognition of Drepananthus as a distinct genus. Divergence times are estimated using an uncorrelated lognormal distributed (UCLD) relaxed molecular clock. Historical biogeographical analysis suggests that the ambavioid lineage originated in Africa, with subsequent dispersal into Asia. Alternative hypotheses for this dispersal, involving rafting on the Indian tectonic plate versus migration via the extensive boreotropical forests associated with the Eocene thermal maximum, are evaluated, and the latter route identified as the most consistent with the divergence age estimates and the geological and palaeoclimatic data.