Phylogeny, morphology and distribution of the Hymenophyllum polyanthos (Sw.) Sw. species complex (Hymenophyllaceae, Polypodiidae) in Japan

The taxonomy of the filmy fern Hymenophyllum polyanthos species complex has been unclear for a long time. A recent study clarified the phylogenetic and morphological relationships among Taiwanese plants of the complex. This raised questions about which lineages occurred in the surrounding geographic regions. Hence, we investigated whether the Taiwanese phylotypes of H. polyanthos also occurred in Japan. To test this we gathered 63 new samples of H. polyanthos from Japan, and generated 126 new plastid DNA sequences that were phylogenetically compared with sequences available from previous studies. We also analyzed herbarium specimens, assessing the ranges of morphological and distribution variation of the groups identified in the phylogeny. As a result, we confirmed that subgenus Mecodium is represented in Japan by H. coreanum, H. mikawanum, and H. wrightii, plus three cryptic lineages of H. polyanthos, namely the 'fujisanense, 'parallelocarpum', and 'punctisorum' phylotypes. We prefer to use informal names for these three lineages because of unresolved taxonomic and nomenclatural issues. Among the H. polyanthos phylotypes, the 'punctisorum' form is very distinct from the other two, both in morphology and distribution. Although the 'fujisanense' and 'parallelocarpum' forms show considerable overlap in morphology and distribution, this study brings enough evidence to distinguish them with significant accuracy.