Adiantum japonicum T. Zhao, Z. Y. Zuo, J. Wen & Jin Mei Lu 2021, sp. nov.

Adiantum japonicum T. Zhao, Z. Y. Zuo, J. Wen & Jin Mei Lu, sp. nov., Fig. 3.

Type:— JAPAN. Central Honshu: Kyoto Pref., Ukyo-ku, Nakagawa, ca 200 m alt., 2 June 1978, Y. Tateishi et J. Murata 4141 (holotype: KUN!; isotypes: TNS!, NCU!, WAG!)

Diagnosis: — Adiantum japonicum is similar to A. pedatum, but it is distinguished from the latter by the erect rhizome and clustered fronds.

Plants deciduous, 40–60 cm tall. Rhizome erect; scales bronzy deep yellow, broadly lanceolate, margins entire. Fronds lax-arching, closely spaced clustered; petiole 1–2 mm diam., castaneous, glabrous. Lamina pedately dichotomous, 15–32 × 15–35 cm, glabrous; pinnae 1-pinnate distally. Pinnae 3–8 pairs, oblong-lanceolate; inner pinnae up to 15–32 × 1.5–4.5 cm, outer pinnae slightly shorter; rachises castaneous, glabrous. Pinnules 17–45 pairs per pinna, alternate, obliquely spreading, herbaceous, green, both surfaces glabrous, oblong, ca. 3 times as long as broad; basal pinnules slightly smaller, flabellate or semi-orbicular, with longer petiolule; basiscopic margin straight; acroscopic margin lobed, lobes separated by 0.3–1 mm incisions, margins of lobes crenulate or crenate-denticulate; petiolule 0.6–1.8 mm long, castaneous, glabrous. Sori 4–6 per pinnule, horizontally attached in shallow sinuses; false indusia grayish green to dark brown, transversely oblong, ca. 2.2 (1.5–3.6) mm × 0.8 (0.5–1) mm, glabrous. Spores yellow or yellowish brown, tetrahedric, trilete, 40–50 µm diam., scabrate.

Habitat and distribution: — In forests: 100–1110 m. Hokkaido, Honshu, and Shikoku of Japan.

Additional specimens examined:— JAPAN. Hokkaido: Nakato N. 2162 (TNS!). Honshu: This new species is distributed throughout Honshu (see Appendix 1). Shikoku (rare): Ehime, Watanabe K. (TNS!).

Additional notes: — Adiantum japonicum has long been identified as A. pedatum. There are significant molecular differences between the Japanese lineage and other members of the A. pedatum complex. The Japanese samples were resolved as monophyletic and being sister to a clade formed by A. pedatum, A. aleuticum, and A. viridimontanum in our phylogenetic analyses. We checked the specimens and collected morphological data from the entire distribution area of the A. pedatum complex. The results showed that, compared with the North American A. pedatum, the Japanese samples have an erect rhizome (vs. creeping rhizome) and clustered leaves (vs. distant leaves). Therefore, we segregate the Japanese lineage from A. pedatum as a new species A. japonicum, and recognize A. pedatum as restricted to deciduous woodlands in eastern North America. So far, we have not detected significant morphological differences between the North American A. pedatum and Chinese and Himalayan “ A. pedatum ”. We will conduct further comparative morphological and phylogeographic analyses of the Chinese and Himalayan “ A. pedatum ” with the North American and Japanese counterparts in our future efforts.

Previous chromosomal counts showed that hybridization and polyploidy are frequent in the A. pedatum complex, and it has a base chromosome number of x = 29 and x = 30, and six cytotypes (two diploids 2 n = 58 and 2 n = 60, one triploid 2 n = 87, and three tetraploids 2 n = 116, 2 n = 118, and 2 n = 120) (Paris & Windham 1988, Nakato & Kato 2005). The Japanese populations have four cytotypes (2 n = 58, 2 n = 60, 2 n = 118, and 2 n = 120), of which diploids of 2 n = 60 are widely distributed in Japan, diploids of 2n=58 have been found only in eastern Hokkaido, and two tetraploids 2 n = 118 and 2 n = 120 are rare in Japan (Nakato & Kato 2005). Our phylogenetic analyses showed that the lineage into which the diploid 2 n = 60 (Nakato N. 2162 from Hokkaido, Japan) clustered is a sister group of the other lineages of this complex, and the rest of the Japanese cytotypes were clustered with Chinese lineages. The samples of A. japonicum were collected widely throughout Japan (e.g., Hokkaido, Nagano, Osaka, Shikoku, and Shizuoka), and we speculate that the newly described A. japonicum is a diploid of 2 n = 60. The present study implied that there may be more than one cryptic species remaining to be described in East Asia. We will carry out further studies on chromosomal survey and morphological and molecular systematic studies to resolve the species delimitations of this biogeographically significant species complex (Wen 2001; Wen et al. 2016; Zhou et al. 2020).