Origins of the fern genus Hymenophyllum (Hymenophyllaceae) in New Caledonia: Multiple independent colonizations from surrounding territories and limited in situ diversification
Authors: Rio Cédric Del , Hennequin Sabine , Rouhan Germinal , Ebihara Atsushi , Lowry II Porter P. , Dubuisson Jean-Yves , Gaudeul1 Myriam 
Published: October 2017

Based on a worldwide dataset of molecular sequence data from three plastid DNA markers (rbcL,rbcL‐accD‐accD,rps4‐trnS) obtained from 109 species ofHymenophyllum(Hymenophyllaceae), we investigated the systematics and bioge‐ographic origins of the New Caledonian (NC) members of this fern genus, which were thought to include 16 species with 10 endemics. Located in the southwest Pacific, NC is a continental Gondwanan island that was long supposed to harbour phylogenetic relicts. However, molecular and geological data suggest that the current flora and fauna are the result of recent colonizations that occurred after total submersion (during the Eocene) and subsequent re‐emersion of NC ca. 37 Ma. Our molecular phylogenetic results, complemented by morphological observations of herbarium specimens, show the existence of a new species that was as yet undescribed, and suggest that a previously recognized variety should be elevated to the rank of species to avoid species polyphyly. These two species, which are recognizable based on several morphological features, are endemic to NC: (i)Hymenophyllum soriemersumsp. nov. is sister to the other NC endemicH. dimidiatum; and (ii)H. neo‐caledonicumcomb. & stat. nov, previously described as a variety withinH. lyallii, was retrieved as a distinct lineage. We also confirmed the synonymy ofH. streptophyllumandH. subdimidiatum(both previously considered as endemic to NC) with the more widely distributedH. javanicumandH. holochilum, respectively. These synonymies were previously hypothesised based on morphology alone. New Caledonia thus has as total of 17Hymenophyllumspecies, 12 (71%) of which are endemics. Biogeographic inferences based on the results of our phylogenetic analyses suggest thatHymenophyllumoriginated in New Zealand ca. 49.8 Ma (95% HPD: 40.7–59.5 Ma). Its spatio‐temporal history may in part have been influenced by vicariance (with potential exchange via the Antarctica land bridge that persisted between Australia and South America until 34 Ma), but long‐distance dispersal (LDD) appears as the predominant process leading to the present‐day distribution of species. At least eight LDD events are needed to explain the presence of the 10 NC endemics analysed here (H. franciiandH. humboldtianumwere not included), each of which occurred < 22.1 Ma and originated from a surrounding territory: Southeast Asia was probably the dispersal source for the NC endemic clade comprisingH. soriemersum,H. dimidiatum, andH. rolandi‐principiswhile the ancestors of the other NC endemics most likely dispersed from New Zealand, Australia or other Pacific Islands. Such a regional pattern is congruent with the distribution of non‐endemic species, which are mostly shared among neighbouring territories. Lastly, we found evidence for only two cases of diversification within NC, yielding two and three species each (H. soriemersum,H. dimidiatumandH. rolandi‐principis; andH. lyalliiand the ancestor ofH. braithwaiteiandH. tomaniiviense). Such a low level of in situ diversification may be explained by the high dispersal abilities of ferns in general (via dust‐like spores), which slow down the differentiation process among populations and make a newly available niche likely to be filled by an immigrant before a new species might evolve locally.