A new synonym from Hawaii and lectotypification of Plagiothecium longisetum (Plagiotheciaceae)

Abstract Plagiothecium mauiense was first described in 1927 by V.F. Brotherus, based on materials from Hawaii. It has, so far been, treated as a separate species. A detailed analysis of the original material housed in the New York Botanical Garden Herbarium (NY01256708) found the specimen to be characterised by a lack of metallic lustre; concave, asymmetrical, lanceolate to lanceolate-ovate leaves, shrunken in their dry condition; a straight, not denticulate, acute to apiculate apex; elongate-hexagonal cells in irregular transverse rows, 101–131 × 15–21 µm at mid-leaf; very lax areolation, with decurrencies composed of three rows of cells. These characteristics indicate that this species is identical to the original material of P. longisetum (e.g. H-SOL 1563 011; PC0132572). Hence, we propose that P. mauiense should be recognised as a new synonym of P. longisetum. In addition, a review of P. longisetum syntypes found one (H-SOL 1563 011) to have the same date of collection as the protologue, and to possess a quite abundant gametophyte turf with well-preserved sporophytes, indicating it to be fertile. Considering the above, we propose that specimen H-SOL 1563 011 be designated the lectotype of P. longisetum.


Introduction
In this paper we demonstrate that all the characteristics of the original material of P. mauiense Broth. are identical to those of the P. longisetum Lindb. type. Hence, we propose P. mauiense as a new synonym of P. longisetum. In addition, among the three syntypes of P. longisetum, we propose the specimen (H-SOL 1563 011) deposited at the Herbarium of the University of Helsinki (Finland) as the lectotype of this name. The aim of the work is to demonstrate that P. mauiense and P. longisetum are synonyms, and to propose a lectotype for the name of the latter.
The study was based on herbarium specimens analysed during research conducted at the NY Herbarium (The New York Botanical Garden, New York, U.S.A.) from November to December 2018 and November to December 2019, as well as at the PC Herbarium (Muséum National d'Histoire Naturelle, Paris, France) in January 2019; and on specimens loaned from the H Herbarium (The University of Helsinki, Helsinki, Finland). However, due to renovation ongoing at the S Herbarium (The Swedish Museum of Natural History, Stockholm, Sweden) and the temporary closure of some herbaria, some less important specimens could not be examined; despite this, they are cited in the manuscript to present the most complete dataset possible. Specimens that were analysed directly were marked with an exclamation mark.
During the revision of Plagiothecium nemorale sensu lato, the original materials collected by D.D. Baldwin from Hawaii were found in four herbaria: Harvard University Herbarium (FH00220142), New York Botanical Garden Herbarium (NY01256708), Miami University Herbarium (MU 000000546), and Yale University Herbarium (YU 233890). On the envelopes of two specimens, from the MU and NY Herbaria, notes indicating them to be isotypes of P. mauiense were also found. In 1967, a similar note was added to the specimen from the NY Herbarium (NY01256708) by H.A. Miller, who studied this material (Fig. 1). Since that time, this specimen has served as the "iso-type" (e.g. in the database of the Consortium of North American Bryophyte Herbaria, https://bryophyteportal.org/portal/ -access: May 2020).
However, given the above, and according to Article 9.6 of the Shenzhen Code (Turland et al. 2018) stating that "A syntype is any specimen cited in the proto- logue when there is no holotype, or any one of two or more specimens simultaneously designated in the protologue as types" as well as Article 40.2 (Note 1) stating that "When the type is indicated by reference to an entire gathering, or a part thereof, that consists of more than one specimen, those specimens are syntypes (see Art. 9.6.)", all the above-mentioned original specimens of P. mauiense should be regarded as syntypes.
Three years later, on April 3, 1970, following a study by H.A. Miller, Z. Iwatsuki analysed the same material (NY01256708) and attached a note (dated April 4, 1970) to the examined specimen, together with hand-drawn pictures of its gametophyte (Fig. 1). Based on the remaining notes, it appears that Iwatsuki characterised the material as follows: stems with a developed central strand; leaves rather asymmetrical; decurrencies composed of one row; the apex very slightly denticulate; thin-wall cells in irregular transverse rows, at mid-leaf 15-19 × 90-110 µm. Additionally, Iwatsuki left a note that P. mauiense is closely related to P. nemorale (Mitt.) A. Jaeger (Iwatsuki 1970 unpubl.). However, in an analysis of P. nemorale in a taxonomic revision of the genus Plagiothecium published the same year, Iwatsuki (1970) does not mention P. mauiense or its similarity to this species.
Over 40 years later (June 25, 2012), the specimen deposited at the New York Herbarium (NY01256708) was reviewed by J.T. Wynns, who added the note: "Insular form of Plagiothecium nemorale (Mitt.) A. Jaeger". In addition, in his revision of the genus Plagiothecium, he also added next to P. mauiense that the material did not differ from the type of Stereodon nemoralis Mitt. (being a basionym of P. nemorale) (Wynns 2015).
Despite the above-presented assumptions indicating that P. mauiense, recorded from Hawaii, is closely related or even identical to S. nemoralis, the two are still treated as separate species (see : Hoe 1974;Staples et al. 2004).
The features given in the diagnosis by Brotherus (1927), and indicated by Iwatsuki (1970 unpubl.) based on the analysis of the original material ( Fig. 1), clearly qualify the described collections as a taxon representing the genus Plagiothecium and belonging to the section Orthophyllum Jedl. They even classified it as belonging to P. nemorale sensu lato. However, as some of the features given by Brotherus and Iwatsuki contradict each other, particularly the most taxonomically significant one, i.e. the length of the cells of the central part of the leaves, it is impossible to clearly assess this material.
Its light green to yellow green turf colour, leaf asymmetry and narrow cell areolation and irregular arrangement of cells reported by Brotherus (1927) and Iwatsuki (1970 unpubl.) are all characteristic of P. longisetum; in addition, Iwatsuki (1970 unpubl.) described the presence of a denticulate apex, corresponding to that of P. nemorale. Most importantly, the two authors differ in their opinion of the cell length at mid-leaf, one of the most taxonomically important features of this genus: Brotherus (1927) reported the length to be 75-100 µm, which clearly matches P. nemorale, while Iwatsuki (1970 unpubl.) reported it as 90-110 µm, corresponding to P. longisetum. In addition to the leaf cells, another very important feature of the whole genus, which is characteristic of individual sections, is the nature of leaf decurrencies (e.g . Nyholm 1965;Smith 2001;Wynns et al. 2017). Iwatsuki (1970 unpubl.) reported the presence of a single row of decurrent cells, which is a feature shared with representatives of section Leptophyllum Jedl. rather than section Orthophyllum, which is characterised by 2-3 rows of decurrent cells (Figs 1, 2). The remaining set of features provided by both authors are characteristic of both species: a large plant with a thick turf; loosely arranged and complanate-foliate; large (2.25 mm long and 1.1 mm wide), concave, long-ovate leaves; two costae; an acute to acuminate apex; a developed central strand; thin-wall cells (Brotherus 1927;Iwatsuki 1970 unpubl.;Wolski 2017Wolski , 2018Wolski , 2020Wolski and Nowicka-Krawczyk 2020).
As only one syntype was examined (NY01256708), and due to the current inability to examine specimens from the other three herbaria (from FH, MU, YU), lectotypification of P. mauiense will be carried out later, once all original materials collected by D.D. Baldwin from Hawaii have been examined.
At the beginning of the 20 th century, Cardot (1912) indicated a relationship between P. longisetum and P. sylvaticum, writing that the former, similarly to P. nemorale, was just a form of P. sylvaticum. Wijk et al. (1967) indicated that P. longisetum was a synonym of P. sylvaticum, whereas Iwatsuki (1970) wrote that P. longisetum was a synonym of P. nemorale, adding that the former was only a habitat modification of the latter. After Iwatsuki (1970), for the next 50 years, this view spread throughout Europe and Asia (Lewinsky 1974;Iwatsuki 2004;Wynns 2015;Suzuki 2016). However, at the beginning of the 21 st Century, as a result of a taxonomic revision of P. nemorale sensu lato, Wolski and Nowicka-Krawczyk (2020) proposed the resurrection of P. longisetum, and for it to be treated as separate from P. nemorale, which also was distributed in Eurasia. Subsequent studies have revealed a number of differences between the two species in the micromorphology of their sporophyte; they also documented their presence in North America, thus extending their global range (Wolski 2020;. During this revision, the specimens on which Lindberg (1872) described P. longisetum were found in three herbaria: the University of Helsinki Herbarium (H-SOL1563011), the Herbarium of Swedish Museum of Natural History (S-B160017) and Muséum National d'Histoire Naturelle (PC0132572). The latter specimen is marked as the "isotype" (Fig. 4). Based on this information, and according to Article 9.6 and 40.2 Note 1 of the Shenzhen Code (Turland et al. 2018) cited above, all specimens listed above should be considered syntypes.
In addition to the description of gametophyte morphology given in the diagnosis of P. longisetum, Lindberg (1872) indicated that this material has sporophytes (i.e. is "fertile") and was collected on 16 June, 1863 ("16 Junii 1863") near Nikosan on Kyushu island ("ad Nikosan ins. Kiusiu") in Japan (Fig. 3). Addition- ally, Lindberg (1872) indicates, among others, that P. longisetum is characterised by a very long seta ("seta longissima") and a long operculum ("operculo longiore et acutiore"). The presence of such a long seta, i.e. up to 5 cm in length, and a long operculum, distinguish the sample from other species of Plagiothecium; these characteristics, combined with the gametophyte features, are unique to P. longisetum (Wolski and Nowicka-Krawczyk 2020).
The specimen deposited at the herbarium in Helsinki (H-SOL 1563 011) was awarded the same date of collection as in the prologue, and is characterised by a fairly large, well-preserved gametophyte turf with three sporophytes (Fig. 5). The material deposited in Stockholm (S-B160017) also has a full collection date; however, due to the ongoing renovation of this herbarium, the loan and subsequent analysis of this material is impossible. Unfortunately, the specimen housed in the herbarium in Paris (PC0132572) has an incomplete collection date ("1863"), and only three gametophyte stems, without sporophytes (Fig. 4).
The material deposited in Helsinki (H-SOL 1563 011) is distinguished from other analysed syntypes by the presence of a fairly large gametophyte turf and more importantly, well-preserved sporophytes (Fig. 5), which (as Lindberg described in the diagnosis) confirm that the specimen is prolific ("fertile"). Based on these facts, and according to Article 9.3 of the Shenzhen Code (Turland et al. 2018) cited above, we propose that specimen H-SOL1563011 should be designated as the lectotype of P. longisetum (Fig. 5). In addition, due to the fact that the specimen from Muséum National d'Histoire Naturelle (PC0132572) was examined at the beginning of 2019, and we did not have access to the specimen deposited at the Herbarium of the Swedish Museum of Natural History (S-B160017), a request was sent to both institutions to change the status of these specimens to the isolectotype.