Phylogeny and relationships of the neotropical Adiantum raddianum group (Pteridaceae)

With more than 200 species, the maidenhair fern genus Adiantum is among the top ten most diverse fern genera. Adiantum is pantropical in distribution and, due to the presence of a unique synapomorphy (sporangia borne on indusia rather than laminae), perhaps the most easily recognized fern genus. Many of its members, including numerous cultivars derived from A. raddianum, are grown as ornamentals. Because of its size, a comprehensive taxonomic study of Adiantum is difficult and the genus is perhaps better approached through a series of narrower studies. Here, we focus specifically on A. raddianum and putative allies. We find a newly defined A. raddianum group to be strongly supported as monophyletic and segregated from other maidenhair ferns on the basis of genetic as well as morphological characteristics. Bayesian inference and maximum likelihood analyses of plastid atpA, chlL, chlN, rbcL, and rpoA sequences support the A. raddianum clade as sister to A. poiretii and its allies. We identify round­reniform indusia to be a characteristic of the A. raddianum group (vs. lunate in the A. poiretii group). Additionally, we find species in the A. poiretii group to differ in having a unique 66 nucleotide deletion in our chlN gene alignment. The neotropical A. raddianum group comprises at least 17 species (14 studied here), some widely distributed; one was recently described (A. alan-smithii).