Aim and location New Zealand began to separate from Gondwana c. 85 Ma, and has been isolated from the nearest large landmass, Australia, by some 2000 km of the Tasman Sea since c. 60 Ma. Given New Zealand's long geographical isolation, there has been considerable interest in explaining the origins of its different biotic elements. Here we investigate the biogeography of the fern genus Polystichum from temperate Australasia. Six species are found in New Zealand, four in Australia, and two on Lord Howe Island.
Methods The evolutionary relationships between the twelve Polystichum species found in temperate Australasia were inferred from phylogenetic analyses of two molecular data sets: DNA sequence from the chloroplast rps4–trnS spacer locus; and AFLP DNA-fingerprinting. The timing of the separation between Australian and New Zealand Polystichum was estimated by using the fossil record to temporally calibrate the rbcL sequence differentiation between representative species from these regions.
Results Species of Polystichum from New Zealand appear to comprise a monophyletic group. This suggests that Polystichum crossed the Tasman only once. Temporal calibration of the rbcL sequence differentiation between Australian and New Zealand Polystichum indicates that a vicariant explanation for their separation can be rejected in favour of trans-oceanic dispersal.
Main conclusions The extant diversity within New Zealand Polystichum appears to have been derived from a single, trans-oceanic dispersal event (within the last c. 20 Myr), followed by a relatively extensive in situ ecological radiation.