Reevaluation of Species Delimitations in Dombeya Section Hilsenbergia (Dombeyaceae)

Dombeya section Hilsenbergia includes five species endemic to Madagascar and the Comoros that are characterized by pendulous inflorescences and long staminal tubes. Morphological data and previous molecular work support the inclusion of these species within Dombeya. Study of morphological characters previously used in the taxonomy of section Hilsenbergia found overlap in these characters among species and sometimes even significant variation within individuals or populations, leading to a broader circumscription of some species than that of previous workers. Incongruence between plastid and nuclear trees suggests the possibility of plastid introgression within section Hilsenbergia, and morphological data indicate potential hybridization, in areas of sympatry, among the three most widely-distributed species, D. baronii, D. cannabina, and D. hilsenbergii. Preliminary conservation assessments based on herbarium collections and field observations categorize D. wallichii as Critically Endangered and D. hafotsy as Endangered. This work summarizes the taxonomic history, morphology, nomenclature, and distribution of section Hilsenbergia and a dichotomous key to the species is provided.