Plagiothecium talbotii sp. nov. is described from Attu Island, Alaska, U.S.A. The newly-described species is not similar in appearance to any Northern Hemisphere species; only the habit is similar to P. platyphyllum. However, it not only occupies a different habitat than that species, but genetically and morphologically, it is clearly distinct from it. The results of DNA sequencing, a detailed description of the morphological features, illustrations, ecological preferences and currently known geographical distribution of P. talbotii are presented. The most important distinguishing morphological features of this species are: the size of the plant; dimensions and symmetry of the leaves; dimensions of cells and their areolation; entire leaf apex; and long decurrencies with some inflated cells. Additionally, we propose to place P. talbotii in section Plagiothecium, which is confirmed by genetic analysis and morphological features.

Attu Island, Bryophyta, Plagiotheciaceae, S. S. Talbot, taxonomy

Herbarium collections are unquestionably a rich and very important source of data on the ecology and distribution of plants. Revisions based on herbarium material are the primary source for taxonomic research. Now, having a number of methods at our disposal (e.g. genetic analyses, mathematical modelling, SEM), integrative taxonomy sheds new light not only on the status or distribution of the taxa studied, but also on the relationships between them and not only at the species level (e.g. Huttunen et al. 2018; Guerra et al. 2019; Ignatov et al. 2020; Plášek and Ochyra 2020; Vigalondo et al. 2020; Wolski and Nowicka-Krawczyk 2020; Wolski et al. 2020; Melamed et al. 2021).

Despite the fact that the Northern Hemisphere is relatively well studied botanically, there are still many areas that are blank spots for this part of the world. The same is true for bryophytes as for vascular plants, but even more so. Many species, genera and even families require basic research related to their variability, distribution, ecology or taxonomic status (Anderson et al. 1990; Ignatov et al. 2006; Hodgetts 2015; Suzuki 2016; Hodgetts and Lockhart 2020; Wolski et al. 2021a, b).

The genus Plagiothecium Schimp. which currently has been divided by Wynns et al. (2018) into nine sections (Leptophyllum Jedl.; Lycambium Jedl.; Ortholimnobium (Dixon) J.T. Wynns; Orthophyllum Jedl.; Plagiothecium; Pseudo-Neckera (Kindb.) J.T. Wynns; Rectithecium (Hedenäs & Huttunen) J.T. Wynns; Saviczia (Abramova & I.I. Abramov) Z. Iwats.; Struckia (Müll. Hal.) J.T. Wynns) over the past decades may give the impression of being well-researched. However, the genus was described as fairly species-poor and represented in the Northern Hemisphere by only a dozen taxa (e.g. Ireland 1969, 1986; Iwatsuki 1970; Lewinsky 1974; Smith 2001). The understanding of Plagiothecium has changed relatively recently, when DNA-based research proved that many of the taxa have been too broadly circumscribed (Zuo et al. 2011; Wynns et al. 2018; Ignatova et al. 2019; Wolski and Nowicka-Krawczyk 2020) and that the bryoflora of North America, Europe and Asia is much richer in Plagiothecium than previously assumed (e.g. Wynns et al. 2018; Ignatova et al. 2019; Wolski, Jukoninė 2019; Wolski 2020a, b; Müller and Wynns 2020; Wolski and Nowicka-Krawczyk 2020; Wolski et al. 2021b).

However, despite extensive research now being carried out, the number of Northern Hemisphere species still seems to be underestimated. Alaska, including the Aleutian Islands, are a “blank spot” in our knowledge of mosses, including the genus Plagiothecium. As a result of the taxonomic revision of Plagiothecium from the Aleutians, we recently described a new species, Plagiothecium schofieldii (Wolski et al. 2021a). Examination of additional material resulted in yet another undescribed species being discovered.

Phylogenetic analyses using the concatenated ITS-rpl16 matrix placed this Alaskan specimen within the sister branch of the sect. Plagiothecium clade with high support from Bayesian Inference (PP = 0.99), but slightly lower support from Maximum Likelihood (BS = 87) (Fig. 1). Although the topology of the tree shows clear distinction of the examined material, the closest representative with maximum support from BI to P. talbotii is the specimen BONN: O.M. Afonina s.n. described as P. denticulatum.

Figure 1. 

Phylogenetic tree of Plagiothecium taxa (sect. Plagiothecium [upper] and Orthophyllum [lower clade] with Isopterygiopsis pulchella as the outgroup, based on concatenated nuclear (ITS1-5.8S-ITS2) and chloroplast (rpl16) DNA markers (total 1526 bp). The tree presents the position of the studied specimen from Alaska amongst Plagiothecium. Numbers on branches indicate bootstrap values from ML followed by posterior probabilities from BI analysis. An asterisk (*) indicates 100 (ML) and 1.00 (BI), while a minus sign (-) indicates values below 75 (ML) and 0.95 (BI). The topology of the tree was based on ML analysis.

The haplotype network (Fig. 2) also confirms a closer relationship of P. talbotii with sect. Plagiothecium than with representatives from sect. Orthophyllum; however, the position of P. talbotii is isolated. The lowest number of mutational steps to P. talbotii as mentioned earlier is the specimen BONN: O.M. Afonina s.n. of P. denticulatum. This material was collected from the Far East of Russia – from Chukotka (Table 1), a geographic region adjacent to Alaska.

Figure 2. 

Median-joining haplotype network of Plagiothecium [bottom-right] and Orthophyllum sections [top-left] of Plagiothecium constructed in PopART. Haplotypes are represented by circles with colours indicating geographic distribution. Numbers on branches indicate the mutational steps.

Although DNA analysis places the tested specimen as a sister clade to the clade represented by taxa from section Plagiothecium, it is morphologically different from most of the representatives of this section.

The shape of the decurrent angular cells is one of the most important features that divide the taxa of this genus into individual sections (Wynns et al. 2018; Wolski et al. 2021a, b). Almost all taxa from sect. Plagiothecium form distinct auricles, but not P. talbotii. The alar regions in this species are long and composed of sometimes inflated cells, a feature common to both P. talbotii and P. platyphyllum (Nyholm 1965; Lewinsky 1974; Smith 2001).

Having leaves shrunken in a dry condition and symmetrical make P. talbotii resemble, amongst the Northern Hemisphere of Plagiothecium, the previously-mentioned P. platyphyllum (Lewinsky 1974; Smith 2001). However, P. talbotii is distinguished from P. platyphyllum by the size of the leaves; lack of serration near leaf apex; lack of a group of eroded, thin-walled cells (nematocysts) and rhizoids near the leaf apex; dimensions of laminal cells; and habitat occupied by the species (Nyholm 1965; Lewinsky 1974; Smith 2001).

Amongst the Northern Hemisphere species belonging to sect. Plagiothecium at present, there is only one species characterised by a symmetrical leaf: P. schofieldii. However, for example, the size and appearance of the turf; the lack of shrinkage and leaf shape, i.e. their concavity and serration and the shape of the decurrent angular cells differentiates this species from P. talbotii (Wolski et al. 2021b).

Other species of this section are characterised by asymmetrical to slightly asymmetrical leaves and a completely different set of features when compared with P. talbotii (e.g. Lewinsky 1974; Smith 2001; Wolski et al. 2021b).

Taking into account the above facts, we believe that this species belongs to Plagiothecium sect. Plagiothecium. Thus, we consider that all the above morphological data, supported by molecular studies, warrant the recognition of the Aleutian samples as a new species.

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.