Biogeographic disjunction between eastern Asia and North America in the Adiantum pedatum complex (Pteridaceae)

Biogeographic analyses of ferns with an eastern Asian-North American disjunction are few. The Adiantum pedatum complex has such a disjunct distribution. The monophyly of the complex needs to be tested and diversification history of the four species needs to be reconstructed.

Plastid (atpA, atpB, rbcL, trnL-F, and rps4-trnS) sequences of 100 accessions representing the biogeographic diversity of Adiantum were analyzed with parsimony and Bayesian inference. Biogeography of the Adiantum pedatum complex was inferred using programs DIVA and LAGRANGE. Divergence times of clades were estimated with the program BEAST.

The A. pedatum complex is monophyletic and sister to the eastern Asian A. edentulum. Accessions of A. pedatum do not form a clade; instead three subgroups are recognizable. The clade of A. aleuticum and A. viridimontanum is nested within A. pedatum. The Asian A. myriosorum is sister to the A. pedatum-A. aleuticum clade. Both DIVA and LAGRANGE analyses suggest an eastern Asian origin of the A. pedatum complex. The age of the crown A. pedatum complex is dated to be at 4.27 (2.24-6.57) million years ago.

The currently recognized eastern Asian-North American disjunct species A. pedatum needs to be segregated into three species, corresponding to populations in eastern North America, China, and Japan. The eastern Asian-North American disjunction in the complex is inferred to be the result of two intercontinental migrations, one from eastern Asia into North America in the late Tertiary and the other from North America back to eastern Asia in the Pleistocene.