Abstract
Aim: Gondwanan vicariance, boreotropical migration and long-distance dispersal have been posited as alternative hypotheses explaining the tropical distribution patterns and diversifications in many fern groups. Here, the historical biogeography of Diplazium is reconstructed to evaluate the impact of these biogeographical processes in shaping the modern tropical disjunctions.
Location: World-wide with a focus on tropical forest habitats.
Methods: Divergence times were estimated by analysing nucleotide sequences of seven plastid DNA regions (atpA, atpB, matK, rbcL, rps4, rps4–trnS and trnL–F) from 123 species of Diplazium and its allied genera, using a Bayesian relaxed clock method and three fossil calibrations. The ancestral areas were reconstructed using the likelihood dispersal–extinction–cladogenesis (DEC) approach.
Results: The crown group of Diplazium was estimated to have originated in Eurasia and undergone an initial diversification in the Northern Hemisphere around 41.7 Ma [95% highest posterior density (HPD): 34–49 Ma] during the Eocene. Two disjunct events between the Old and New World were identified: one in subgenus Diplazium around the Eocene–Oligocene boundary (31.2 Ma, 95% HPD: 25–38 Ma), and the other in subgenus Callipteris during the middle Miocene (12.6 Ma, 95% HPD: 15–23 Ma). Furthermore, Palaeotropical disjunctions in subgenus Callipteris are indicative of multiple dispersal events during the Miocene.
Main conclusions: The evolutionary history of Diplazium involves a variety of biogeographical scenarios. Early diversification of Diplazium in the Northern Hemisphere during the Eocene corresponds with the migration from Eurasia to North America over land bridges as a member of the boreotropical flora. The current tropical amphi-Pacific disjunctions in subgenus Diplazium can be better explained by the disruption of boreotropical belt, however, long-distance dispersal between Eurasia and tropical America cannot be ruled out. Island-hopping and trans-Pacific dispersals followed by speciation characterize the disjunctions and diversifications of subgenus Callipteris during the Neogene. Gondwanan vicariance is not supported by any of our results.
Wei, Ran