Synopsis of Osmunda (royal ferns; Osmundaceae): towards reconciliation of genetic and biogeographic patterns with morphologic variation

Abstract We present an overview of the morphology, biogeography, and ecology of the fern genus Osmunda (i.e. without previously included taxa of Claytosmunda, Osmundastrum, and Plenasium), with a focus on the American O. spectabilis Willd. and Old World O. regalis L. While genetic data supports the separation of these taxa, commonly used morphologic characters to distinguish between O. regalis and O. spectabilis (maximum frond size; pinnae sessile vs. stalked; pinnules opposite vs. alternate; general pinnule proportions) are not infallible. We recorded morphometric differences among and between O. regalis and O. spectabilis and correlated these with spatio-environmental gradients. Our work provides an updated taxonomic overview with full synonymy and diagnostic key of Osmunda. We preliminarily recognize six informal subtaxa in O. regalis (‘regalis’, ‘abyssinica’, ‘huegeliana’, ‘longifolia’, ‘transvaalensis’, ‘obtusifolia’) and three in O. spectabilis (‘spectabilis’, ‘palustris’, ‘piresii’) that separate in a morphological traits-based principal component analysis. The morphotaxa appeared geographically structured and their distribution showed significant correlations with elevation, mean annual precipitation, annual mean temperature, and temperature seasonality. The taxonomic rank and validity of the proposed subtaxa has to be tested through a large-scale sampling, e.g. with comparative cp/nr genomic and cytogenetic analyses. Such future analyses may help to ascertain whether the observed morphologic peculiarities are conditioned solely by abiotic factors (which would allow addressing them as forms) or are manifested in the genome.