Phylogenetics and biogeography of the olive family (Oleaceae).

Abstract

Progress in the systematic studies of the olive family (Oleaceae) during the last two decades provides the opportunity to update its backbone phylogeny and to investigate its historical biogeography. We additionally aimed to understand the factors underlying the disjunct distribution pattern between East Asia and both West Asia and Europe that is found more commonly in this family than in any other woody plant families. Using a sampling of 298 species out of ca. 750, the largest in a phylogenetic study of Oleaceae thus far, and a set of 36 plastid and nuclear markers, we reconstructed and dated a new phylogenetic tree based on maximum likelihood and Bayesian methods and checked for any reticulation events. We also assessed the relative support of four competing hypotheses [Qinghai-Tibet Plateau uplift (QTP-only hypothesis), climatic fluctuations (Climate-only hypothesis), combined effects of QTP uplift and climate (QTP-Climate hypothesis), and no effects (Null hypothesis)] in explaining these disjunct distributions. We recovered all tribes and subtribes within Oleaceae as monophyletic, but uncertainty in the position of tribe Forsythieae remains. Based on this dataset, no reticulation event was detected. Our biogeographic analyses support the QTP-Climate hypothesis as the likely main explanation for the East-West Eurasian disjunctions in Oleaceae. Our results also show an earlier origin of Oleaceae at ca. 86 Mya and the role of Tropical Asia as a main source of species dispersals. Our new family-wide and extensive phylogenetic tree highlights both the stable relationships within Oleaceae, including the polyphyly of the genus Chionanthus, and the need for further systematic studies within the family's largest and most under-sampled genera (Chionanthus and Jasminum). Increased sampling will also help to fine-tune biogeographic analyses across spatial scales and geological times.