Genetic variation, breeding systems, and patterns of diversification in Hawaiian Polypodium (Polypodiaceae)

The Hawaiian endemic fern Polypodium pellucidum, named for its characterist ic translucent leaf veins, is a member of the monophyletic, primarily tempe rate species group known as the Polypodium vulgare complex. Since its origi nal description in 1824, P. pellucidum has been recognized as morphological ly diverse and several varieties have been named. The most commonly recogni zed are a rainforest epiphyte (var. pellucidum) a lava flow terrestrial (va r vulcanicum) and var. opacum, characterized by a lack of translucent veins . Electrophoretic analysis of isozymes was used to test available systemati c hypotheses and gain new perspectives on the breeding system and distribut ion of genetic variation within this species. These analyses showed that 1) levels of genetic variability were very high for an island endemic species , 2) based on fixation indices (F), outcrossing between genetically differe ntiated gametophytes was rare, 3) values of Nm were consistent with interpo pulational gene flow as a relatively rare event, 4) genetic identity (I) be tween populations within taxa averaged above 0.85 whereas that between two of the varieties averaged 0.785, and 5) although morphologically distinct, P. pellucidum var. opacum is isozymically identical to P. pellucidium var. vulcanicum. Assuming a single introduction with subsequent diversification, the high levels of genetic variation indicate that P. pellucidum has been a resident of the Hawaiian islands for a relatively long period of time, pe rhaps longer than the oldest of the current islands, and has migrated from island to island. Although reduced genetic identities between the two ecolo gically divergent varieties suggest that they may eventually become separat e species, the genetic similarity of P. pellucidum var opacum to P. pelluci dum var. vulcanicum indicates that it should not be recognized as a separat e variety.