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A worldwide phylogeny of Adiantum (Pteridaceae) reveals remarkable convergent evolution in leaf blade architecture

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Adiantum is among the most distinctive and easily recognized leptosporangiate fern genera. Despite encompassing an astonishing range of leaf complexity, all species of Adiantum share a unique character state not observed in other ferns: sporangia borne directly on the reflexed leaf margin or "false indusium" (pseudoindusium). The over 200 species of Adiantum span six continents and are nearly all terrestrial. Here, we present one of the most comprehensive phylogenies for any large (200+ spp.) monophyletic, subcosmopolitan genus of ferns to date. We build upon previous datasets, providing new data from four plastid markers (rbcL, atpA, rpoA, chlN) for 146 taxa. All sampled taxa can be unequivocally assigned to one of nine robustly supported clades. Although some of these unite to form larger, well-supported lineages, the backbone of our phylogeny has several short branches and generally weak support, making it difficult to accurately assess deep relationships. Our maximum likelihood-based ancestral character state reconstructions of leaf blade architecture reveal remarkable convergent evolution across multiple clades for nearly all leaf forms. A single unique synapomorphy—leaves once-pinnate, usually with prolonged rooting tips—defines the philippense clade. Although a rare occurrence in Adiantum, simple leaves occur in three distinct clades (davidii, philippense, peruvianum). Most taxa have leaves that are more than once-pinnate, and only a few of these (in the formosum and pedatum clades) exhibit the distinct pseudopedate form. Distributional ranges for each of the terminal taxa show that most species (75%) are restricted to only one of six major biogeographical regions. Forty-eight of our sampled species (nearly one-third) are endemic to South America.
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Keywords: BIOGEOGRAPHY; LEAF MORPHOLOGY; MAIDENHAIR FERNS; NEOTROPICS; PHYLOGENY; VITTARIOIDS

Document Type: Research Article

Affiliations: 1: Department of Biology, Duke University, Durham, North Carolina 27708, U.S.A.;, Email: [email protected] 2: Boyce Thompson Institute, Ithaca, New York 14853, U.S.A., Section of Plant Biology, Cornell University, Ithaca, New York 14853, U.S.A. 3: Department of Biology, Duke University, Durham, North Carolina 27708, U.S.A. 4: Instituto de Botânica, C.P. 68041, 04045-972, São Paulo, SP, Brazil 5: University Herbarium, 1001 Valley Life Sciences Building #2465, University of California, Berkeley, California 94720-2465, U.S.A. 6: Department of Botany, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20013-7012, U.S.A.

Publication date: 01 June 2018

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