Hymenophyllum chamaecyparicola T.C.Hsu & Z.X.Chang, a new filmy fern species (Hymenophyllaceae) has been described from Taiwan and illustrated based on morphological and phylogenetic evidence. Although the new species resembles members in the subgenus Mecodium, namely H. wrightii, our plastid phylogeny has revealed that it is genetically distant from H. wrightii and forms a clade nested within subg. Hymenophyllum. The most notable characteristic to differentiate H. chamaecyparicola from related species is the presence of minute spathulate hairs on the surface of the rachis and veins. Hymenophyllum chamaecyparicola is currently only known from a small area in northern Taiwan, and endemic to that country.

Filmy fern, Hymenophyllum, new species, Taiwan

Hymenophyllum is the largest subgenus among the ten subgenera in genus Hymenophyllum Sm., and includes at least 100 species (Ebihara et al. 2006; PPG I 2016). Generally, the members of this subgenus are distinguished by their long-creeping rhizomes, pinnate to tripinnate laminae, denticulation on the segment margins, bivalvate involucres, and included receptacles (Ebihara et al. 2006; Hennequin et al. 2010). However, subg. Hymenophyllum as a whole varies greatly in many aspects, including cytology and morphology (Hennequin et al. 2010). With a considerably large number of morphologically disparate species, members of subg. Hymenophyllum have been scattered among different groups or genera in Hymenophyllaceae (Pryer et al. 2001; Ebihara et al. 2002; Hennequin et al. 2003, 2006, 2010; Hennequin 2004) leaving many systematic issues in this subgenus unsettled. In Taiwan, 26 species are currently known to belong to the genus Hymenophyllum. Among them, eight were recognized in subg. Hymenophyllum (Hsu et al. 2019; TPG 2019, 2021), including H. barbatum (Bosch) Baker, H. blandum Racib., H. denticulatum Sw., H. devolii M.J.Lai, H. holochilum (Bosch) C.Chr., H. okadae Masam., H. oligosorum Makino and H. simonsianum Hook. Subgenus Hymenophyllum can be distinguished from its sister subgenus Mecodium by the presence of indumentum along the stipe, rachis, costae and veins vs. these surfaces glabrous in subgen. Mecodium.

In 2019, the first author discovered a Hymenophyllum species with an uncertain assignment in a subtropical montane cloud forest of northern Taiwan. After observing its dwarf habit, superficially glabrous laminae, entire segments, and bivalvate, subentire involucres, we initially considered it to be a member belonging to subg. Mecodium, and tentatively identified it as H. wrightii Bosch, a small species distributed across East Asia and North America (Liu et al. 2013; Lee et al. 2014; Duffy et al. 2015). However, morphological distinctions between the specimen and H. wrightii were observed after careful investigation. This species produces apically distributed sori and minute clavate hairs along the rachis and costae (Figs 1, 2), which are absent in the subg. Mecodium, especially H. wrightii. In this study, we provided not only morphological but also phylogenetic evidence to circumscribe this uncertain species. With no recording of a similar species from previous literature, we determined it as a new species and have described it here as Hymenophyllum chamaecyparicola T.C.Hsu & Z.X.Chang, currently known to be endemic to Taiwan.

Figure 1. 

Habitat and morphology of Hymenophyllum chamaecypericola, from Hsu 11888 (TAIF) A, B wild population growing on moss-covered basal trunk of a giant Chamaecyparis obtusa var. formosana C rhizome and young frond, showing the wingless and scarcely hairy stipe D–G fronds, adaxial views (D, E) and abaxial views (F–G) H, I laminae, adaxial view (F) and abaxial view (G), showing the minute yellow-brown clavate hairs on rachis and veins. J. Sori. Scale bars: 2 cm (B); 5 mm (C, J); 1 cm (D–G); 2 mm (H, I).

Figure 2. 

Line drawing of Hymenophyllum chamaecypericola T.C.Hsu & Z.X.Chang, sp. nov., based on the holotype Z.X. Chang ZXC01438 (TAIF) A rhizome and fronds B sterile frond C fertile frond D sori E a clavate hair.

The concatenated cpDNA dataset of rbcL (1365 bp) and rps4-trnS (1125 bp) contained a total of 2490 aligned sites. In our cpDNA phylogeny (Fig. 3), no conflicting relationship was found between ML and Bayesian trees. Our two samples of the uncertain Hymenophyllum (H. chamaecyparicola sp. nov.) possessed identical cpDNA sequences, and this species was found to be phylogenetically unrelated to H. wrightii in subg. Mecodium. Instead, it formed a strongly supported clade embedded in subg. Hymenophyllum and was well separated from all other species (Fig. 3). Interestingly, subg. Hymenophyllum as defined by Ebihara et al. (2006) was revealed to be non-monophyletic in our phylogenies because H. simosianum was found sister to subg. Mecodium with weak supporting values (BS/PP=73/0.6).

Figure 3. 

Maximum likelihood (ML) phylogeny of Hymenophyllum subg. Hymenophyllum and Hymenophyllum subg. Mecodium based on the chloroplast DNA dataset (rbcL + rps4-trnS). Branch support is indicated in ML bootstrap/ BI posterior probabilities.

Our phylogeny generally agrees with the “modern” circumscriptions of Hymenophyllum subg. Hymenophyllum and subg. Mecodium (Ebihara et al. 2006; Hennequin et al. 2010; Hsu et al. 2019; Vasques et al. 2019) with only one exception – H. simonsianum, which was placed within subg. Hymenophyllum based on morphology (Ebihara et al. 2006) but resolved here as an isolated lineage sister to subg. Mecodium (Fig. 3). Given that this unexpected position of H. simonsianum was weakly supported in our tree based on only two cpDNA regions, we consider that more sequence data is required to ascertain its systematic placement.

The phylogenetic position of Hymenophyllum chamaecyparicola, nested within subg. Hymenophyllum, was somewhat surprising in the beginning due to its superficial resemblance to H. wrightii in subg. Mecodium. However, after a detailed examination of the specimens, we concluded that its placement in subg. Hymenophyllum is also morphologically evident. Though hardly visible to the naked eye, H. chamaecyparicola bears clavate hairs on stipes and rachis, and such laminar trichomes are common in subg. Hymenophyllum but absent in subg. Mecodium (Ebihara et al. 2006; Hennequin et al. 2010; Hsu et al. 2019). Moreover, from our examination, two veinlets can be found at the bases of sori in members of subg. Mecodium sori but not in H. chamaecyparicola and other species of subg. Hymenophyllum as implied previously (Dubuisson et al. 2018).

Obviously, our sampling of Hymenophyllum subg. Hymenophyllum (11 species), with an estimate of more than 100 species (Ebihara et al. 2006), remains insufficient. Even so, this study still provides some insights about interspecific relationships and systematics within this subgenus. We revealed for the first time that sect. Pseudomecodium, mainly defined by the combination of abaxially hairy veins and entire segments (Iwatsuki 1984, 1985), is non-monophyletic. In our tree, the four sampled species with entire segments, including H. chamaecyparicola, H. exsertum, H. oligosorum and H. pachydermicum, were placed in three different lineages (Fig. 3). In addition, our data strongly supported that H. okadae, recently reinstated from a synonym of H. barbatum based on a few subtle morphological characters (Knapp and Hsu 2017), is also phylogenetically distinct.

In addition to H. chamaecyparicola, H. devolii is another subg. Hymenophyllum species endemic to Taiwan. Our study then revealed that H. devolii is affiliated, not only morphologically but also phylogenetically, with its sympatric relatives, H. okadae and H. barbatum, which are also distributed in other East Asian regions. It will be very worthy to further study the speciation pathways behind these endemic ferns in Taiwan. A comprehensive sampling in the subgenus, especially from Southeast Asia, and a dated phylogeny are ultimately necessary to clarify the evolutionary history of these Taiwan endemic ferns.

We are grateful to Ralf Knapp for his useful comments and enthusiastic participation in field trips. We thank Chih-Yun Sun for preparing line drawings; Alexandria Quinlan for English edits; the curators of K, L, MICH, NY, P, TAI, TAIF and US herbaria for access to their collections; Taiwan Pteridophyte Group (TPG) for maintaining DNA collections used in this study; Cheng-Wei Chen and Pei-Jung Xie for generating DNA sequences; Diego Tavares Vasques, Atsushi Ebihara, Joel Nitta, and one anonymous reviewer for their comments on the manuscript. This project was supported by MOST project (109-2621-B-007-001-MY3, 111-2628-B-007-006-MY3) in Taiwan, the Bioresource Conservation Research Center in College of Life Science from the Higher Education Sprout Project by MOE, Biodiversity Information Fund for Asia project (BIFA6_010).