Dispersal pathways across the Pacific: the historical biogeography of Astelia s.l. (Asteliaceae, Asparagales)

Aim To investigate the origin and pathways of expansion of the austral genus Astelia s.l. (Asteliaceae, Asparagales) and to reconstruct the historical biogeography of the genus across the Pacific. Location Australia, New Zealand, New Caledonia, New Guinea, South America, Pacific archipelagos (Austral Islands, Fiji, Hawaii, Marquesas Islands, New Caledonia, Samoa, Society Islands and Vanuatu) and the Mascarene Islands. Methods Biogeographical patterns were determined using the Asteliaceae phylogeny, divergence dating, and ancestral area reconstruction techniques. All Asteliaceae genera, including 46 of 47 ingroup taxa, were sampled. Divergence times were estimated using a relaxed molecular clock model, calibrated according to Astelia s.l. fossil data from New Zealand. Ancestral areas were reconstructed using statistical dispersal–vicariance (S-DIVA) and dispersal–extinction–cladogenesis (DEC) models. Results Asteliaceae originated in Australia and Astelia s.l. (including Collospermum) underwent its greatest radiation in New Zealand. Within Astelia s.l., every node along the backbone of the phylogeny was assigned to New Zealand, with the earliest of these dated to 27.1 Ma (95% HPD: 17.6–40.0 Ma). Five range expansions from Australia and New Zealand to islands in the Pacific and Indian Oceans were identified for Astelia s.l. New Zealand origins are identified for all Pacific taxa, with the earliest divergence of a Pacific taxon, that of the New Caledonian A. neocaledonica, at 7.6 Ma (95% HPD: 1.1–15.3 Ma). There were six dispersals from New Zealand, including four to Pacific archipelagos, and independent dispersals to Australia and South America. Additionally, stepping-stone pathways from New Zealand included dispersal to the western Pacific (Fiji, Samoa and Vanuatu) and – independently – to the eastern/northern Pacific (Austral Islands, Hawaii and the Marquesas Islands). Main conclusions The Asteliaceae originated in Australia, but the largest genus, Astelia s.l., radiated most extensively in New Zealand, beginning at c. 27.1 Ma. Both western and eastern Pacific islands were colonized via direct and stepping-stone dispersal pathways from New Zealand.