Timing and tempo of evolutionary diversification in a biodiversity hotspot: Primulaceae on Indian Ocean islands

Aim: We examined phylogenetic relationships and spatio-temporal diversification in Indian Ocean Primulaceae, assessing correlations between speciation rates, geographical expansion and ecomorphological specialization. Location: Madagascar and Indian Ocean Islands. Methods: We conducted phylogenetic analyses with plastid and nuclear DNA sequences of Primulaceae using maximum likelihood and Bayesian algorithms, and estimated divergence times using a Bayesian relaxed molecular clock. Temporal changes in diversification rate and possible correlations with the biogeographical history of the group were examined. We performed parametric ancestral area reconstruction incorporating a stratified palaeogeographical model that reflects changes in terrestrial configuration and the presence of phytogeographical connections through time in the western Indian Ocean Basin. Shifts in diversification rate were compared with ancestral area assignments and divergence age estimates. Results: Indian Ocean Primulaceae were recovered as monophyletic with a sister relationship to Asian Ardisia. Oncostemum, a genus confined to Madagascar and the Comoros, was resolved as paraphyletic by the inclusion of a monophyletic Mascarene Badula group consisting of single-island endemics. We found evidence for diversification bursts early in the history of Indian Ocean Primulaceae that correspond closely to the sequence of dispersal and the appearance of newly formed Mascarene Islands. Age estimates suggest a dispersal to Rodrigues that is older than the estimated geological age of the island. Main conclusions: Results suggest a Madagascan origin of Indian Ocean Primulaceae with subsequent dispersal to the Mascarenes in the middle to late Miocene, with initial establishment on either Mauritius or Rodrigues and subsequent stepping-stone dispersal to the other two Mascarene islands within the last 2 Myr. Analyses suggest that diversification has slowed over time, with significant rate changes following dispersal to new geographical areas. Onset of diversification in species-rich Oncostemum appears to have been recent, with major cladogenesis commencing in the early Pliocene.