RbcL sequences provide phylogenetic insights among sister species of the fern genus Polypodium

Opinions on the phylogeny and classification of the Polypodiaceae sensu stricto have been particularly divergent and recent DNA sequencing analyses have supported the hypothesis that this group should be allied to the higher indusiate ferns. Within the genus Polypodium, isozymic and restriction site analyses demonstrated that interspecific genetic identities are quite low, suggesting that sequences of the conservative rbcL gene would be appropriate for addressing open systematic questions. Average infrageneric sequence divergence (1.87%) was sufficient to generate reliable species differences, and the skewness test of randomly generated trees suggested that the data were non random and probably phylogenetically informative. A strict consensus of the eight most parsimonious trees supported the monophyly of the P. vulgare complex, and decay analysis showed that primary tree branches were well supported. Most of the phylogenetically informative nucleotides were at third codon positions. Neotropical Polypodinm species were chosen a priori as a close out-group, and emerged as a sister group to the P. vulgare complex. Polypodium amorphum + P. appalachianum formed a clade that was morphologically supported by the synapomorphic feature of sporangiasters in the sori. Although the indication that P. glycyrrhiza was related to the Asian species P. fauriei was not unexpected, the close association of these two species with P. scouleri was quite surprising, and provided insights on the possible origin of the latter species through adaptation to stressful habitats. Whereas the long held hypothesis of a close association between P. australe and P. macaronesicum was upheld, the rbcL sequence data could provide no insights on the phylogeny of the enigmatic Hawaiian endemic, P. pellucidum. Pleopeltis thyssanolepis, until recently classified as a Polypodium, was solidly allied to Pleopeltis macrocarpa var. complanata. It is anticipated that future molecular analyses will provide additional insights on the evolutionary history of the Polypodiaceae.