Post-Boreotropical dispersals explain the pantropical disjunction in Paederia (Rubiaceae)

Abstract

Background and aims: Pantropical intercontinental disjunction is a common biogeographical pattern in flowering plants exhibiting a discontinuous distribution primarily in tropical Asia, Africa and the Americas. Only a few plant groups with this pattern have been investigated at the generic level with molecular phylogenetic and biogeographical methods. Paederia (Rubiaceae) is a pantropical genus of 31 species of woody lianas, with the greatest species diversity in continental Asia and Madagascar and only two species from tropical America. The aim of this study was to reconstruct the biogeographical history of Paederia based on phylogenetic analyses to explore how the genus attained its pantropical distribution.

Methods: Maximum parsimony and Bayesian inference were used for phylogenetic analyses using sequences of five plastid markers (the rbcL gene, rps16 intron, trnT-F region, atpB-rbcL spacer and psbA-trnH spacer). Biogeographical inferences were based on a Bayesian uncorrelated lognormal relaxed molecular clock together with both Bayesian and likelihood ancestral area reconstructions.

Key results: The data suggest an early diverged Asian lineage sister to the clade of the remaining species consisting of a predominantly Asian sub-clade and a primarily Malagasy sub-clade. Paederia is inferred to have originated in the Oligocene in tropical continental Asia. It then reached Africa in the early to middle Miocene, most probably via long-distance dispersal across the Indian Ocean. The two Neotropical species are inferred to have derived independently in the late Miocene from ancestors of Asia and East Africa, respectively.

Conclusions: The results demonstrate the importance of post-Boreotropical long-distance dispersals (across three major oceans) in shaping the global pantropical disjunction in some plants, such as Paederia, with small, winged diaspores adapted to long-distance dispersal by various agents including wind, ocean currents or birds. Overland migration is less likely to explain its palaeotropical disjunction between Asia and Africa.

Fig. 1.

The Bayesian consensus tree of Paederia and closely related taxa based on five plastid sequences (rbcL, rps16, trnT-F, atpB-rbcL and psbA-trnH). The Bayesian posterior probabilities are shown above the branches and the bootstrap values below.

Fig. 2.

Chronogram of Paederia and relatives based on four plastid sequences (rbcL, rps16, trnT-F and atpB-rbcL) as inferred from BEAST. Grey bars represent the 95 % highest posterior density intervals for node ages. Global temperature means are shown by the curve adapted from Zachos et al. (2001). C1–C5 are calibration points; 1–8 indicate nodes with interests (see Table 1 for details).

Fig. 3.

Bayesian CTMC ancestral area reconstruction from the BEAST tree (right) with branches coloured according to the most probable location state of their descendent nodes. The letter and colour coding for areas are indicated on the map (left upper). Left below are state posterior probability distributions for selected nodes as shown on the right-hand tree (see Fig. 2 and Table 1 for details).