A multi-character analysis of Struthiopteris leads to the rescue of Spicantopsis (Blechnaceae, Polypodiopsida)

Abstract

The family Blechnaceae is a moderately sized leptosporangiate fern lineage, with 24 genera and around 250 species. Struthiopteris accommodates small to medium-sized, dimorphic, pinnate species. It is composed of six northern species: S. spicant is distributed in western parts of Europe and North America; S. fallax is endemic to Iceland; S. niponica, S. amabilis and S. castanea are endemic to Japan, and S. hancockii occurs in Japan and Taiwan. Due to the lack of a global review and to its highly interesting geographical distribution, this genus merits further study to clarify its taxonomy, species relationships, and distributional pattern. The present study aims to achieve the following goals: (a) identify and describe morphological characters supporting the taxonomy of Struthiopteris; (b) reconstruct a complete phylogeny for the genus; (c) study the biogeographical history of Struthiopteris at a global scale. The morphological study involved the observation of characters ranging from rhizome scales to spores over 164 individuals. Phylogenies were constructed applying ML and BI techniques over 51 newly produced sequences of three chloroplast markers (rbcL, trnL-trnF, psbA-trnH), using the species Blechnidium melanopus and Brainea insignis as closest relatives. For the molecular dating and historical biogeography analyses, we estimated and compared ancestral ranges under several models. Most of the morphological characters led us to discern two groups of species: the S. spicant group (S. spicant, S. fallax, and S. castanea) and the S. niponica group (the remaining species). In our molecular phylogeny, the supposed sister genus Blechnidium always appeared as nested within Struthiopteris, rendering this genus non-monophyletic. The two groups identified by the morphology appeared as monophyletic clades within Struthiopteris, with the clade S. spicant more closely related to Blechnidium than to the clade S. niponica. For all these reasons, we propose to rescue the now-disused genus Spicantopsis for the species belonging to the S. niponica group: indeed, this genus was created c. 100 years ago to reunite the same species S. amabilis, S. niponica, and S. hancockii. Our results suggest that all members of this group of genera (Blechnidium, Struthiopteris, Spicantopsis) emerged in East Asia about 85 mya, at a time when Japan was still part of the mainland. It appears that, for most of their history, the members of these genera have been confined to East Asia, with one dispersal to the Americas by an ancestor within Struthiopteris s.str., and additional dispersals to India and the Philippines by Blechnidium melanopus.