Plagiothecium schofieldii sp. nov. is described from the Aleutian Islands, Alaska, U.S.A. Some morphological features of this species correspond to P. lamprostachys (Southern Hemisphere species); however, Plagiothecium schofieldii is genetically and morphologically different from this and other common Northern Hemisphere species e.g., P. denticulatum, P. platyphyllum, or P. ruthei. The most important distinguishing morphological features differentiating this species are: the arrangement of the leaves on the stem; dimensions, concavity and symmetry of the leaves; dimensions of cells and their areolation; orientation of capsules. Additionally, due to the strong concavity of the leaves, they are very often badly damaged under the microscope. We present the results of DNA research of the analyzed samples, and a detailed description of the morphological features. The new species is illustrated, and its ecological preferences and currently known geographical distribution are presented. Additionally, the authors propose to add this species to Plagiothecium section, which is confirmed by morphological features and genetic analysis.

Bryophyta, Plagiotheciaceae, taxonomy, W. B. Schofield

Over the last several years, our perception has changed not only of Plagiothecium Schimp., but also of the whole family of Plagiotheciaceae M.Fleisch. (e.g., Pedersen and Hedenäs 2001, 2002; Wynns et al. 2018). The use of molecular methods has not only helped to understand many taxa previously considered problematic, but has also allowed for the description of a number of new taxa (e.g., Zuo et al. 2011; Wynns et al. 2018; Ignatova et al. 2019; Wolski and Nowicka-Krawczyk 2020). Nevertheless, for decades the taxonomic status of many species of this genus has been unclear and ambiguous, and those taxa currently require detailed morphological, genetic and taxonomic studies.

Although the Northern Hemisphere seems to be relatively well researched, there are still many areas (e.g., central Asia, Middle East) which remain as gaps on the world distribution map of Plagiothecium (Wolski et al. 2021). The results of taxonomic revisions conducted in recent years indicate the underestimation of the species richness of individual parts of the world. As a consequence of this research, many countries and regions have increased their number of ​known taxa of the described genus (e.g., Ellis et al. 2019a; Ellis et al. 2019b; Ellis et al. 2020, 2021; Müller and Wynns 2020; Wolski and Nowicka-Krawczyk 2020; Wolski 2020).

The Aleutian Islands, Alaska, U.S.A., are one of the many under-explored regions of the Northern Hemisphere. As a result of the taxonomic revision of Plagiothecium specimens from this area it was possible to describe a new species from this genus; the results are presented below.

Phylogenetic analyses based on the concatenated ITS-rpl16 matrix placed studied specimens within the branch of a Plagiothecium sect. Plagiothecium clade, or sister to it; however, the branch support was very low (BS = 49). The next branch down is to representatives of sect. Orthophyllum Jedl. and even more distant to sect. Leptophyllum Jedl. clade (Fig. 1). After branching off from the Orthophyllum clade, the internal division of sect. Plagiothecium was well supported by Bayesian inference (PP ≥ 0.98). Maximal support from both maximum likelihood and Bayesian Inference was recorded in the clade of Plagiothecium schofieldii, where the representatives create a monospecific clade (PP = 1).

Figure 1. 

Phylogenetic tree of Plagiothecium taxa with Isopterygiopsis pulchella as the outgroup based on concatenated nuclear (ITS1-5.8S-ITS2) and chloroplast (rpl16) DNA markers (total 1574 bp). The tree presents the position of Plagiothecium morphotypes from Alaska among the Plagiothecium group which is divided into individual sections. Numbers on branches indicate bootstrap values from ML followed by posterior probabilities from BI analysis. Asterisk (*) indicates 100 (ML) and 1.00 (BI), while minus (-) indicates values below 50 (ML) and 0.7 (BI). The topology of the tree was based on ML analysis.

The haplotype network (Fig. 2) also showed internal diversity in sect.Plagiothecium. At the center, the analysis grouped haplotypes from the Northern Hemisphere (P. denticulatum and P. ruthei). Three branches extending from the center apply to haplotypes from Central America and the Southern Hemisphere. The position of P. schofieldii haplogroup is fairly isolated and consists of two haplotypes: the first refers to three specimens from Simeonof Island, while the second to a representative from Adak Island (Table 1); however, as it grows in the Aleutian Islands, Alaska, the branch vector points out the same direction as haplotypes from the Northern Hemisphere.

Figure 2. 

Median-joining haplotype network of sections Plagiothecium and Orthophyllum of Plagiothecium constructed in PopART. Haplotypes are represented by circles with colors indicating geographic distribution. The number on the branches indicates the mutational steps.

The individual taxonomic features of Plagiothecium are related to a specific level of detail in our analyses, and for example: superficial layer of the stem (epidermis layer) of large, thin-walled cells; shortly pointed leaves; serration (if present) only at apex; absence of pseudoparaphyllia; leaves clearly decurrent at the base – distinguish this genus from other genera belonging to the Plagiotheciaceae. Within Plagiothecium, the shape of decurrent alar regions, and the shape of their cells distinguishes the species of individual sections, while the shape and dimensions of leaf cells are the most important features distinguishing species from each other (Iwatsuki 1970; Lewinsky 1974; Noguchi 1994; Smith 2001). Therefore, based on the fact that the analyzed specimens have decurrent alar cells that are rounded, inflated, and form distinct auricles, as well as the shape and size of the leaf cells of Plagiothecium schofieldii, we believe that this species belongs to Plagiotheciumsect. Plagiothecium. This is also confirmed by molecular and haplotype network analyses.

Species that are widespread in the Northern Hemisphere: Plagiothecium denticulatum (Hedw.) Schimp., P. platyphyllum Mönk., and P. ruthei Limpr., significantly differ in morphology from P. schofieldii, which, compared to the above-mentioned species, has erect stems, while the others are usually prostrate, or sometimes prostrate to ascending (Smith 2001; Li and Ireland 2008; Wynns 2015).

Leaves of P. schofieldii are julaceous and imbricate – very closely arranged on the stem, while in other species the leaves are strongly complanate, flaccid, and spreading on the stem. In the Northern Hemisphere only in P. denticulatum shoots are rarely julaceous (Lewinsky 1974; Smith 2001; Li and Ireland 2008; Wynns 2015). The appearance of the mats and the arrangement of the leaves on the stem are more similar to those features in P. cavifolium (Brid.) Z. Iwats. (which belongs to Plagiothecium sect. Orthophyllum).

Stem leaves of Plagiothecium schofieldii are very strongly concave, to such an extent that under the microscope they are clearly damaged and cracked from being flattened by the coverslip. The leaves of the closely related species are rather flat. Only in the case of P. denticulatum are the leaves more or less concave, but never to such an extreme (Lewinsky 1974; Smith 2001; Li and Ireland 2008). Plagiothecium schofieldii is characterized by symmetrical leaves, and from other members of sect. Plagiothecium only P. platyphyllum has more or less symmetrical leaves, but this is the only feature common to both species. Symmetrical leaves are typical, e.g., for species from sect. Orthophyllum (e.g., P. nemorale, P. cavifolium) (Lewinsky 1974; Smith 2001; Li and Ireland 2008; Wynns 2015; Wolski 2020). Also, leaves of P. schofieldii are clearly longer and wider than those of all the species mentioned above (Smith 2001; Li and Ireland 2008).

Plagiothecium schofieldii is clearly distinguished from P. denticulatum, P. platyphyllum and P. ruthei by the length and width of it laminal cells. The cells located in the central part of the leaf are long and very wide (88–190 × 13–29 μm), which makes the cell areolation very loose. None of the above-mentioned species has such long and broad cells, and thus their cell areolation is tighter (Lewinsky 1974; Smith 2001; Li and Ireland 2008).

Another feature that clearly distinguishes this newly described species from the previous species in sect. Plagiothecium is the orientation of the capsules. In the studied specimens of P. schofieldii, the capsules are orientated most often more or less vertically, i.e., erect, rarely inclined. Plagiothecium denticulatum, P. platyphyllum, and P. ruthei have inclined capsules (Lewinsky 1974; Smith 2001; Li and Ireland 2008; Wynns 2015).

On the other hand, in terms of morphology, P. schofieldii looks more like P. lamprostachys (Hampe) A. Jaeger – a Southern Hemisphere species (Ireland 1992; Wynns 2015) – than the common Northern Hemisphere species mentioned above. Both the morphological features and molecular analyses indicate the distinctiveness of the species (Figs 1–4).

Additionally, Wynns (2015) pointed out that P. lamprostachys forms a clade within P. denticulatum sensu lato, which is also confirmed by our research (Fig. 1). Phylogenetic analyses of concatenated nuclear and chloroplast markers placed P. schofieldii within sect. Plagiothecium next to P. denticulatum; however, the branch support was very low. After branching off from the Leptophyllum clade (BI = 0.70), Bayesian inference highly supported the phylogenetic relations within sect. Plagiothecium indicating the separateness of a P. schofieldii clade (as well as a sect. Orthophyllum clade). This separation was also visible in the haplotype network, where the analysis extended the Alaskan clade far from the center of the network grouping species of Plagiothecium from the Northern Hemisphere.

All the above morphological data, supported by molecular studies, warrant the recognition of the Aleutian samples as a new species.

Taxonomy

Plagiothecium schofieldii G.J.Wolski & W.R.Buck, sp. nov.

Type

U.S.A. Alaska, Shumagin Islands, Simeonof Island, mainly near saddle between Hill 1436 and 1265, wet cliff chimney, 54°55'N, 159°15'W, 19 July 1996, W.B. Schofield 106119, Holotype MO5135779.

Description

Plants small, light green to yellow, with a delicate metallic luster, forming very dense, often homogeneous mats. Stems erect, 1.5–3.0 cm long (Fig. 3), in cross-section rounded, with a diameter of 300–380 μm, the central strand developed, epidermal cells 10–25 × 6–12 μm, the parenchyma thin-walled, 20–40 × 15–32 μm; leaves julaceous, imbricate, very closely arranged on the stem, when dry not shrunken, very concave, therefore very often damaged under the microscope, symmetrical, ovate to elliptic, those from the middle of the stem 1.4–3.0 mm long, and the width measured at the widest point 0.9–1.9 mm; the apex obtuse and apiculate, often denticulate; costae two, thick and strong, extending usually to ½ of the leaf length, reaching 0.5–2.0 mm; laminal cells linear, rather symmetrical, in quite regular transverse rows, the length and width very variable but dependent on location: 66–178 × 14–33 μm at apex, 88–190 × 13–29 μm at midleaf, and 45–221 × 20–39 μm toward insertion, due to the very wide cells, cell areolation clearly loose; decurrencies of 4–5 rows of rounded, rounded-rectangular, inflated cells, 40–90 × 22–48 μm, forming distinct, quite long auricles, 300–750 μm long (Fig. 4). Autoicous. Sporophytes abundant; setae dark brown at base, yellowish-orange at apex, twisted when dry, 1.8–2.3 cm long; the capsules more or less erect, 700–950 × 280–350 μm; operculum short and rostellate; peristome double, well developed, 450–500 μm long; exostome teeth trabeculate at the ventral side.

Figure 3. 

Stems with the sporophytes of Plagiothecium schofieldii. Part of the turf of holotype (W.B. Schofield 106119, MO5135779). Scale bar: 1 cm.

Figure 4. 

The most important taxonomic features of Plagiothecium schofieldii. Dimensions of cells from the apex A the middle B and basal part of the leaf C leaves D–E leaf apex F auricles G. Scale bar: 100 µm (A–C, F–G); 500 µm (D–E). Photos from the holotype (W.B. Schofield 106119, MO5135779).

Etymology

The present species is named in honor of Wilfred “Wilf” Borden Schofield (1927–2008), who spent decades studying northern regions of North America, including the Aleutian Islands, and who on July 19, 1996, collected the specimen (No. 106119), chosen here as the holotype of Plagiothecium schofieldii. According to Stephen Talbot (pers. comm.), Schofield recognized this plant as distinct in the field.

Distribution and ecology

Plagiothecium schofieldii so far has only been recorded from Adak Island, Attu Island and Simeonof Island in Alaska. In this area it has been recorded on wetlands and hills, wet cliff chimney, open, moist crevice of a cliff, shaded face of hole on slope, shaded humid outcrop, along creek and adjacent slope, near saddle between hills and near base of mountain.

Additional specimens examined

U.S.A. Alaska: Adak Island, Finger Bay, along creek and adjacent slope, open, moist, crevice of cliff, 15–30 Jun 1975, D. K. Smith 3864 (NY02589541); Attu Island, near Jaemin Pass, slopes of Ribson Ridge, shaded face of hole on slope, 52°53'N, 173°10' W, 10 Aug 2000, W. B. Schofield & S. S. Talbot 115646, UBC ACC# B185126; Shumagin Islands, Simeonof Islands, near base of larger mountain, N. side, 54°55'N, 159°15' W, shaded humid outcrop, 17 Jul 1995, W. B. Schofield, S. S. Talbot & G. Argus 104056, ACC# B159650 (MO5140205); wetlands and Hill 624, 54°55'N, 159°15'W, seepy cliff chimney, 7 Jul 1996, W. B. Schofield 105769, ACC# B161483 (MO5148015).

We thank the late Dr. Judy Harpel for the opportunity to revise the Aleutian Plagiothecium species. The research was funded from a grant Genetic study on variability of selected taxa of the genus Plagiothecium NCN „Miniatura 4” – DEC-2020/04/X/NZ8/00420.