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The origin and evolution of Indomalesian, Australasian and Pacific island biotas: insights from Aglaieae (Meliaceae, Sapindales)

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Abstract Aim 

The role of long-distance dispersal in the Indomalesian, Australasian and Pacific flora is currently hotly debated. The lack of well-resolved phylogenetic trees for Pacific plants has been a major limitation for biogeographical analysis. Here, we present a well-resolved phylogenetic tree for the tribe Aglaieae in the mahogany family, Meliaceae, and use it to investigate the origin, evolution and dispersal history of biotas in this area. The subfamily Melioideae, including the tribe Aglaieae (Meliaceae, Sapindales), is a plant group with good representation in the region in terms of biomass and species numbers, wide ecological attributes and known animal vectors. The family has a good fossil record (especially from North America and Europe). Genera and species in the tribe Aglaieae therefore provide an excellent model group for addressing this debate. Location 

Indomalesia, Australasia, Pacific islands. Methods 

Results from nuclear internal transcribed spacer ribosomal DNA analyses of 82 taxa, based on sequence alignment guided by secondary structure models, were combined with evidence from fossils and distribution data. We used strict and relaxed molecular clock approaches to estimate divergence times within Aglaieae. Putative ancestral areas were investigated through area-based and event-based biogeographical approaches. Information on dispersal routes and their direction was inferred from the investigation of dispersal asymmetries between areas. Results 

Our study indicates that the crown group of Aglaieae dates back at least to the Late Eocene, with major divergence events occurring during the Oligocene and Miocene. It also suggests that dispersal routes existed during Miocene–Pliocene times from the area including Peninsular Malaysia, Sumatra and Borneo to Wallacea, India and Indochina, and from the area including New Guinea, New Ireland and New Britain further east to the Pacific islands at the peripheries of the distribution range. The origin of the Fijian species dates back to the Pliocene. Main conclusions 

Dispersal over oceanic water barriers has occurred during geological time and seems to have been a major driving force for divergence events in Aglaieae, with some old Gondwanan land masses (e.g. Australia) colonized only during recent times. Movement from the ancestral area was predominantly towards the east. Extant Fijian species of Aglaia are monophyletic and share morphological features rarely found in species of other areas, suggesting speciation within an endemic clade. Divergence of living taxa from their closest living relatives took place during both the Miocene and the Pliocene, and peaked in the Pliocene. The present-day distribution of many species in the tribe must therefore have arisen as a result of dispersal rather than vicariance events. Furthermore, colonization from Indomalesia to Australasia and the Pacific has frequently been followed by speciation.
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Keywords: Aglaia; Bayesian relaxed clock; Meliaceae; Sapindales; biogeography; dispersal; fossil constraints; internal transcribed spacer; molecular clock; vicariance

Document Type: Research Article

Affiliations: 1: Department of Plant Sciences, University of Oxford, South Parks Road, Oxford, UK 2: Division of Biology and Medicine, Brown University, Providence, RI, USA 3: Molecular Systematics Section, Jodrell Laboratory, Royal Botanic Gardens Kew, Richmond, Surrey, UK

Publication date: October 1, 2008

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