West to east dispersal in a widespread animal-dispersed woody angiosperm genus (Aglaia, Meliaceae) across the Indo-Australian Archipelago

Aim Despite increasing scientific interest in the Indo-Australian Archipelago (IAA), this geographically complex region, hosting an outstanding level of marine and terrestrial biodiversity, remains only partly understood biogeographically. We present an updated and resolved phylogenetic tree for the woody genus Aglaia, and we investigate the origin and dispersal history of Aglaia. Location Australasia, Indo-Australian Archipelago, Indomalesia, Pacific islands. Methods Sequence data from the nuclear ribosomal internal transcribed spacers (ITS) were used to reconstruct the phylogenetic relationships of 182 taxa. The phylogeny was then used in combination with fossil data and secondary calibration information to estimate divergence times using a relaxed molecular clock approach. Biogeographical scenarios and potential ancestral areas were investigated using statistical dispersal-vicariance analysis (S-DIVA) and the dispersal-extinction-cladogenesis (DEC) method. Results Our study showed that the crown group of Aglaia dates back to the Oligocene/Miocene boundary (c. 23.0Ma), with major divergence events beginning in the middle Miocene (c. 13.8Ma). Sundaland was confirmed as the area of origin of modern Aglaia. The results further provided evidence that dispersal routes existed between the Sunda Shelf and Wallacea and further east to the Pacific islands, crossing both Wallace's and Lydekker's lines. Main conclusions The directionality of dispersal events to or across Wallacea was predominantly eastwards, following the expansion of wet tropical forest across Wallace's Line and beyond. However, several reversal dispersal events occurred as well. During the Pleistocene (0.1-2.6Ma) there were long-distance dispersal events from the western Pacific islands back to Sundaland or the Philippines, which might have been facilitated by birds. In summary, our study confirms that Aglaia had a Sundanian origin and started to disperse eastwards during the Miocene (5.3-23.0Ma), with limited dispersal across Lydekker's Line. The comparison of the age of clades with the proposed age of their areas of distribution based on independent geological evidence showed good temporal agreement.