Evolution of the eastern Asian-North American biogeographic disjunctions in ferns and lycophytes

This paper reviews 31 groups in ferns and lycophytes hypothesized to show eastern Asian-North American disjunctions. Fourteen lineages have been supported by recent phylogenetic evidence: Lycopodium nikoense and Lycopodium sitchense; Isoetes asiatica and the clade of the North American species complex closely allied to I. maritima; Osmundastrum cinnamomeum; Osmunda claytoniana; the Adiantum pedatum complex; the Cryptogramma acrostichoides complex; Diplaziopsidaceae; Cystopteris chinensis and the Cystopteris bulbifera clade; Asplenium rhizophyllum and Asplenium ruprechtii; diploid Phegopteris; Onoclea sensibilis; the Polypodium appalachianum clade; and the Polypodium glycyrrhiza clade. Phylogenetic and/or cytological evidence did not support the biogeographic disjunctions in six cases: (1) Isoetes asiatica and I. truncata; (2) Botrychium ternatum; (3) Thelypteris beddomei and T. nipponicaThelypteris noveboracensis and T. nevadensis; (4) Thelypteris glanduligera and Thelypteris. japonicaT. simulata; (5) Woodwardia japonica and W. virginica; and (6) Woodwardia orientalis and Woodwardia fimbriata. Both vicariance and dispersal have been suggested to be the mechanisms for the formation of the disjunct pattern; and the Beringian region has been an active pathway for the migration of ferns and lycophytes between Asia and North America. Disjunctions of ferns and lycophytes reviewed here have been dated in the Tertiary, and are similar to the ages of eastern Asian-North American disjunctions in seed plants, supporting the close biogeographic co-diversification of ferns and seed plants. Future studies are needed to estimate divergence times and reconstruct biogeographic events in a broad phylogenetic framework, and to test the morphological stasis hypothesis in disjunct ferns and lycophytes.