Article révisé par les pairs
Résumé : Rumohra is a fern genus comprising seven species, three in South America, three in Madagascar and one (R. adiantiformis) which is widely distributed across the Southern Hemisphere. Our goals were to assess species delimitation based on molecular data and to infer the biogeographical history that led to such contrasting distributions among species. We sampled all Rumohra spp., with 46 samples including 28 R. adiantiformis accessions from 14 regions, and sequenced eight plastid DNA regions: atpA, atpB, atpB-rbcL, rbcL, rps4-trnS, trnG-trnR, trnH-psbA and trnL/trnLtrnF. The resulting phylogenetic trees showed R. adiantiformis to be polyphyletic, with at least six lineages found in distinct geographical regions. Given the apparent absence of distinctive morphological characters among lineages, they are best understood as cryptic species. Such genetically distinct but morphologically similar populations may result from a recurrent history of hybridization, morphological convergence or (most probably) morphological stasis. Molecular dating and ancestral area estimations showed that Rumohra diverged from the genus Megalastrum c. 46.4 Mya in the Neotropics, and started to diversify 11.2 Mya. Its biogeographical history was probably shaped by seven long-distance dispersal (LDD) events including three initial events from the Neotropics to southern Africa, the Malagasy region and southern South America. The Australasian lineage resulted from a LDD from southern South America, and the three species endemic to Madagascar diversified in situ.

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