Oceanic islands as cradles for plant diversification: A global phylogeny and biogeography of the <i>Neottopteris</i> clade ( <i>Asplenium</i> ; Aspleniaceae)

BACKGROUND AND AIMS: The Neottopteris clade, comprising over 150 species distributed across pantropical and subtropical regions, represents a morphologically distinct and ecologically significant group within the largest fern genus Asplenium. Although earlier studies have established the monophyly of this clade, they were based on limited regional sampling, leaving its major phylogenetic relationships and biogeographic history unresolved. This study aims to reconstruct a comprehensive phylogeny of the Neottopteris clade, estimate its divergence times, infer its ancestral distribution, and evaluate the role of oceanic islands in its diversification. METHODS: We utilized DNA sequences from eight plastid markers obtained from 296 samples, representing 137 species (ca. 91% of the clade's diversity), collected from all major distribution centers. Phylogenetic analyses were conducted using maximum likelihood and Bayesian inference. Divergence times were estimated with fossil-calibrated molecular dating, and ancestral ranges were reconstructed in RASP using Statistical Dispersal-Vicariance Analysis (S-DIVA). In addition to a global-scale area coding scheme, we conducted a supplementary analysis using a refined Asia-Australasia/Pacific coding scheme to evaluate whether the inference of island-centered diversification remained robust when the broad Australasian region was subdivided. KEY RESULTS: Our analyses strongly support the monophyly of the Neottopteris clade and resolve 12 major subclades. The clade diverged from its sister group during the Paleocene (∼59.5 Ma), with crown-group diversification beginning in the Eocene (∼46.1 Ma). Ancestral area reconstruction under the global coding scheme indicated an origin in the Asia-Australasia region, followed by substantial diversification associated with Australasian and Pacific island systems. Under a refined coding scheme that subdivided the former broad Australasian region, the deeper backbone retained a strong Southeast Asian component, whereas several major subclades, especially Pacific-rich lineages such as clade IX, showed clearer island-centered diversification signals. African lineages originated from multiple dispersal events, while species on Indian Ocean islands exhibit ancestries derived from both Africa and the Asia-Australasia region. CONCLUSIONS: This study provides a resolved phylogenetic framework for the Neottopteris clade and supports an evolutionary pattern best described as continental origin followed by repeated island-associated diversification. The results underscore the important role of Pacific island systems in the diversification of tropical epiphytic ferns, while also indicating that deeper ancestral-area inferences remain sensitive to geographic coding scheme and should be interpreted with appropriate caution. Overall, our findings highlight the combined influence of long-distance dispersal, regional island diversification, and post-Eocene climatic change in shaping the pantropical distribution of the clade.