A new fern species, Whittieria hengduanensis (Ophioglossaceae), from Sichuan, Xizang, and Yunnan, Southwest China (eastern Himalaya), is described and illustrated. This species is similar to W. engelmannii in the Americas in having a cylindrical rhizome and complex-reticulate venation. In addition, both species grow in open habitat on basic soil. However, the two species are distinguishable in root number per rhizome and the number of the larger areolae per trophophore. Our molecular study also supports that they are sister to each other but divergent at the molecular level. The discovery of W. hengduanensis shows that the genus is intercontinentally disjunct between the Himalaya and the Americas, a unique pattern not having been documented in the literature.

Adder’s tongues, fern biogeography, intercontinental disjunctions

Ophioglossaceae or Adder’s tongue ferns are known for their extremely simplified morphological characteristics (Clausen 1938; Wagner 1990; Zhang et al. 2020; Zhang and Zhang 2022) and the highest known chromosome numbers among known organisms (Ghatak 1977). Following the most recent phylogenetic analysis based on Sanger DNA sequence data, plastome sequences, and analyses of 34 morphological/ecological traits, this family was divided into four subfamilies and 15 genera (Zhang and Zhang 2022). Among the four subfamilies, Ophioglossoideae are the most difficult taxonomically because of their extremely simple morphology. Zhang and Zhang (2022) recognized seven genera in Ophioglossoideae: Cheiroglossa C. Presl, Goswamia Li Bing Zhang & Liang Zhang, Haukia Li Bing Zhang & Liang Zhang, Ophioderma (Blume) Endl., Ophioglossum L., Rhizoglossum C. Presl, and Whittieria Li Bing Zhang & Liang Zhang. Based on plastome and mitogenome data, Kuo et al. (2024) resolved the morphologically well-defined South African Rhizoglossum as sister to Whittieria and found the overall relationships to be consistent to those found by Zhang and Zhang (2022), albeit with lower support values, especially for the monophyly of Ophioglossum s.l. (31% maximum likelihood bootstrap value). Kuo et al.’s (2024) results are suggestive that the recognition of the seven genera in the subfamily is well founded.

Among the seven genera in Ophioglossoideae, Whittieria was believed to be monospecific and endemic to North to Central Americas. This genus is well defined morphologically, ecologically, and physiologically in Ophioglossoideae by having complex-reticulate venation (large areoles of the trophophore subdivided into smaller areoles; Wagner and Wagner 1994), growing in basic soil, and having relatively shorter spore germination time (70 days vs. 90–100 days of species in Ophioglossum s.s.; Whittier 1981).

During a field trip in western Sichuan and southeastern Xizang in 2021, we collected some materials of a species of Ophioglossoideae from several localities. This species has quite clear complex-reticulate venation, similar to the American Whittieria engelmannii, but about 8000 km away from the latter species’s distribution in air distance. We then conducted detailed morphological and phylogenetic analyses and found that this represents a new species of Whittieria.

Nine sequences of three samples were newly sequenced. The combined dataset of rbcL, trnL-F, and rps4-trnS contained 3,029 bp of which 462 sites were parsimoniously informative. In spite of only three markers and limited taxon sampling used to construct the phylogeny (Fig. 1), all four segregated genera of Ophioglossum s.l. recognized by Zhang and Zhang (2022) were recovered as monophyletic except the monospecific Haukia, of which only one accession was included. Goswamia was resolved as sister to the rest, and Whittieria was sister to a clade containing Ophioglossum s.s. and Haukia. The newly sampled three accessions of the southwestern Chinese (eastern Himalayan) species were resolved as monophyletic and were sister to W. engelmannii.

Figure 1. 

The maximum likelihood tree based on the combined plastid dataset of rbcL, trnL-F, and rps4-trnS. Maximum likelihood bootstrap support (MLBS) and Bayesian inference posterior probability (BIPP) are given above and below the branches, respectively. Voucher information is indicated behind the species name.

Consistent with previous findings (Zhang et al. 2020; Zhang and Zhang 2022; Kuo et al. 2024), Haukia is resolved as sister to Ophioglossum s.s. (Fig. 1). However, Goswamia is resolved as sister to the rest of the four genera, which differs from a resolution in which Goswamia was found to be sister to Whittieria (Zhang et al. 2020; Zhang and Zhang 2022). We consider this conflict as minor because our current resolution of Goswamia received low statistical support. This conflict might have been the result of different sampling sizes and characters included.

It is quite striking that the eastern Himalayan species (hereafter referred as Whittieria hengduanensis) is resolved as sister to the American endemic W. engelmannii. The genus Whittieria was believed to contain a single species in America (Zhang and Zhang 2022). With the second species discovered here, this genus now has an intercontinental disjunction between the Himalaya and the Americas. Wen (1999) listed 65 genera of seed plants displaying the eastern Asian-eastern North American disjunctions. Xiang et al. (2015) hypothesized that 31 groups of ferns and lycophytes showed eastern Asian-North American disjunctions. However, none of these taxa are endemic to the Himalaya and have their sister in the Americas (southern North America and Central America for Whittieria). Therefore, the intercontinental disjunction pattern in Whittieria has not yet been documented in any group of plants at any taxonomic ranks. The divergence time between Whittieria and the rest of Ophioglossoideae and that between two species in Whittieria are of great biogeographical significance. Where is the ancestral area of Whittieria then? How did the two species form today’s disjunct pattern? The discovery of W. hengduanensis is surely interesting.

Whittieria hengduanensis is indeed similar to W. engelmannii in having complex-reticulate venation, which is unique in Ophioglossaceae and in fact defines the genus Whittieria (Zhang and Zhang 2022). The morphological differences between the two species are minute (see below) because of the generally simple morphology in the subfamily. Both species grow in basic soil, which is unique in the family, too (Zhang and Zhang 2022). However, the two species are so widely displaced geographically (ca. 8000 km in air distance), with W. hengduanensis being in the eastern part of the Himalaya and W. engelmannii being endemic to North and Central America (Wagner and Wagner 1994; Wan et al. 2022). In addition, the two species occur in very different elevations with W. hengduanensis in elevations between 2500–4000 m with temperate and alpine climates and W. engelmannii in those between 200–2200 m with tropical, subtropical, and temperate climates.

For a long time, this species has been confused with Ophioglossum nudicaule L.f. They overlap in geographical distribution. However, O. nudicaule has no persistent old leaf stalks at the base of the rhizomes and fewer roots per rhizome, and the sporophore base is not slightly attached to the trophophore. Importantly, they have different venation patterns with W. hengduanensis with complex-reticulate venation and O. nudicaule with common reticulate venation, although Wagner and Wagner (1994) reported complex-reticulate venation for the North American O. nudicaule. It is unclear whether the materials from Africa (type locality), Asia, and the Americas of “O. nudicaule” represent the same species.

We thank Lisha Jiang and Qi Yu for help in the field. Andrew Murdock, Tom A. Ranker, and Liang Zhang are thanked for their helpful comments.

Taxon sampling, voucher information, and GenBank accession numbers used in this phylogenetic analysis.