Bucklandiella lusitanica

The diversity of mosses is much higher than many people realise. Whereas some moss species have wide ranges that may cross between continents and hemispheres, others are unique to very specific regions and habitats. Among examples of the latter is the European species Bucklandiella lusitanica.

Illustrations of Bucklandiella lusitanica, from Ochyra & Sérgio (1992). Top left: habit; top right: section of stem of hair-leafed form when dry; lower left: section of stem of hairless form and sporophyte when wet.

Bucklandiella lusitanica was only described as a new species (under the name Racomitrium lusitanicum) in 1992 (Ochyra & Sérgio 1992), having gone unnoticed previously despite being a relatively distinctive species. Recent collections of the species have been identified from a single region, the Serra do Gerês mountain rainge and Parque Natural da Peneda-Gerês national park in the northwest of Portugal, at altitudes between 650 and 1000 metres. A single collection from the Serra do Estrela further south in the country was made in the mid-1800s though it went unidentified at the time. Its rarity is such that is has officially been listed as Endangered by the IUCN. Bucklandiella lusitanica is a rheophyte, which is to say that it grows in association with running water. It grows on acidic granite rocks that are periodically or permanently submerged, such as alongside streams and waterfalls. It is particularly abundant on steep rock faces, growing in association with closely related moss species.

Appearance-wise, Bucklandiella lusitanica is a medium-sized moss with irregularly branched stems growing 1.5 to 3.5 centimetres in length. Leaves are rigid, held tight to stem, and two or three millimetres long.One of the species' most distinctive features is a broad, fleshy margin to each leaf that is generally two or three cells thick whereas the lamina of the leaf is mostly only a single cell thick. The alar cells at the base of the sides of the leaf often form inflated, strongly coloured lobes. The leaves commonly end in a fine, colourless hair-point. The structure of the leaves is similar to that of Bucklandiella lamprocarpa, another aquatic moss species, but that species lacks the hair-points. The two species also differ in the form of their spores, those of B. lamprocarpa being larger and more ornate than those of B. lusitanica, and B. lamprocarpa has fatter and often shinier capsules than B. lusitanica.

I mentioned previously that Bucklandiella lusitanica was originally described as a member of the genus Racomitrium. The moss genus Racomitrium was long recognised by a distinctive array of features including leaf lamina cells with distinctly sinuous longitudinal cell walls, a calyptra (the cap of the developing capsule) that is basally frayed into several lobes, and teeth of the peristome (the teeth around the aperture of a mature capsule) that are split into two or more segments (Sawicki et al. 2015). Racomitrium in this sense was a diverse genus with over two hundred species having been named at one time or another, and somewhere between sixty and eighty species recognised as valid in recent years, As a result, Ochyra et al. (2003) proposed the division of Racomitrium in the broad sense between four separate genera. Bucklandiella, the largest of these segregate genera (with about fifty currently known species), was recognised for species with a smooth leaf surface (lacking papillae on the lamina) and relatively short, shallowly divided teeth in the peristome. The division of Racomitrium has not been universally accepted. Larrain et al. (2013) questioned the monophyly and diagnosability of Ochyra et al.'s segregates but Sawicki et al. (2015) reiterated their support for the new system (and added a fifh new segregate genus to boot). It is generally accepted that Racomitrium in the broad sense represents a monophyletic unit, so the question of whether lusitanicum should be assigned to Racomitrium or Bucklandiella may largely be considered a question of just how closely circumscribed you feel a genus should be.

REFERENCES

Larraín, J., D. Quandt, M. Stech & J. Muñoz. 2013. Lumping or splitting? The case of Racomitrium (Bryophytina: Grimmiaceae). Taxon 62 (6): 1117–1132.

Ochyra, R., & C. Sérgio. 1992. Racomitrium lusitanicum (Musci, Grimmiaceae), a new species from Europe. Fragmenta Floristica et Geobotanica 37 (1): 261–271.

Ochyra, R., J. Żarnowiec & H. Bednarek-Ochyra. 2003. Census Catalogue of Polish Mosses. Institute of Botany, Polish Academy of Sciences: Cracow.

Sawicki, J., M. Szczecińska, H. Bednarek-Ochyra & R. Ochyra. 2015. Mitochondrial phylogenomics supports splitting the traditionally conceived genus Racomitrium (Bryophyta: Grimmiaceae). Nova Hedwigia 100 (3–4): 293–317.

Define 'Trichostomum'

The moss in the above photo Icopyright Hermann Schachner) generally goes by the name of Trichostomum crispulum. Trichostomum is a cosmopolitan genus in the Pottiaceae, the largest recognised family of mosses with about 1500 species overall. But with great diversity comes great difficulty of identification. Pottiaceae tend to be small mosses that are common in harsh habitats. Features of pottiaceous mosses are often hard to distinguish and may be quite variable, making it difficult to confidently define taxa. As a result, Pottiaceae is a prime example of what I like to call 'taxonomic blancmange': something that tends to just get prodded nervously then backed away from when it wobbles ominously.

Characteristic features of Trichostomum as it is commonly recognised tend to include symmetric leaves with more or less plane margins, and with the basal cells of the leaf differentiated straight across the blade or in a U-shape. The peristome of the capsule also tends to be short and straight, and the sexual system is usually dioicous (with separate male and female plants) (Flora of North America). However, none of these features are entirely reliable, and some species have been the subject of extensive disagreement about whether they should be placed in Trichostomum, or in a related genus such as Weissia or Tortella.

To date, only a selection of Pottiaceae species have been subject to molecular analysis, but these analyses have confirmed the unsatisfactory nature of the current system. A molecular phylogenetic analysis of the pottiaceous subfamily Trichostomoideae by Werner et al. (2005) did not identify Trichostomum species as a monophyletic clade; instead, various representatives of the 'genus' were scattered throughout the subfamily. The type species of Trichostomum, T. brachydontium, was associated with a few close relatives such as T. crispulum in a broader clade containing numerous species of the genus Weissia. As a result, it has been suggested that the two genera should perhaps be synonymised, in which case the name Trichostomum would be absorbed by the older Weissia. But first, someone would need to work out just how such a genus could be recognised...

REFERENCE

Werner, O., R. M. Ros & M. Grundmann. 2005. Molecular phylogeny of Trichostomoideae (Pottiaceae, Bryophyta) based on nrITS sequence data. Taxon 54 (2): 361–368.

Hypno-Moss

Recent decades have seen a great deal of shifting around in the classification of mosses. As molecular data have become de rigeur in phylogenetic studies, a number of features previously used to distinguish higher groupings have proven to be more labile than previously appreciated. This has lead to a hunt to discern whether other features may be more reliable.

Hypnum cupressiforme, from Andrew's Moss Site.

The Hypnales are one of the major moss groups: as currently recognised, about a third of mosses are Hypnales. They are a major subgroup of the clade of pleurocarpous mosses, i. e. those in which the reproductive sporophytes arise from the sides of gametophyte stems, as explained earlier in this post. In the past, the pleurocarpous mosses have been divided between three orders, the Hypnales, Hookeriales and Leucodontales, on the basis of features of branching habit and the peristome, the array of teeth surrounding the opening of the spore capsule. In the Hookeriales, the teeth of the endostome (the inner ring of the peristome) are connected by a high basal membrane, and molecular phylogenetic analyses have generally supported this order as monophyletic. The Leucodontales were defined by having reduced peristome teeth, and usually sympodial growth (as the primary shoot produces a side-branch, it ceases growing itself and the new branch becomes the new primary shoot). The Hypnales had well-developed peristome teeth, and their growth was generally monopodial (the primary shoot continues growing even after it produces side-branches). The distinction between these latter two orders also correlated with their choice of niches: Leucodontales were mostly epiphytes, whereas Hypnales mostly grew on the ground. However, molecular phylogenetic analyses have not supported the distinction between the Hypnales and Leucodontales, with features such as reduced peristome teeth apparently evolving multiple times with the united clade combining the two orders (Buck et al. 2000). As a result, recent authors have treated the Hypnales as including most members of both the prior orders Hypnales and Leucodontales. A smaller number of pleurocarpous mosses have been placed outside the clade including Hookeriales and Hypnales in the broad sense; there are now known as the Ptychomniales and Hypnodendrales. The broader Hypnales is less well defined morphologically, but its members tend to have differentiated alar cells (distinctly formed cells at the basal corners of the leaves) and smooth spore capsules (Huttunen et al. 2012).

A mat of Leucodon, from here.
This shuffling is not restricted to the higher levels, either. Relationships within the Hypnales remain poorly resolved; indications are that at some point this group went through a quite rapid diversification, resulting in a fairly high level of convergence between lineages and low support for molecular branches. Huttunen et al. (2012) found support for a large clade within the Hypnales including the majority of its Northern Hemisphere members, with a paraphyletic grade outside this containing mostly Southern Hemisphere taxa. Huttunen et al. suggested a Gondwanan origin for the Hypnales, with their diversification in the Northern Hemisphere (where the other pleurocarpous orders never made many inroads) related to the break-up of the Laurasian landmasses. Within the Northern Hemisphere clade, many previously recognised families appear to be polyphyletic; even the type genus of the order, Hypnum, contains species that seem to occupy widely separate places in the hypnalean family tree.

The Azores-endemic moss Echinodium renaudii, copyright Paulo A. V. Borges.

A good example of all this mess is the genus Echinodium, a small genus of six living species whose distinctive appearance lead to it being placed in a family all of its own. Echinodium species grow as fairly stiff plants with long leaves that taper to a narrow point and have thickened margins (the margins are two cell layers thick whereas the body of the leaf is only one cell thick). Echinodium mosses also have a very unusual distribution: two species are found in southeastern Australia and New Zealand, but the other four are restricted to the Macaronesian islands in the Atlantic (that is, the Canaries, the Azores and Madeira). When fossil Echinodium species were discovered in eastern Europe, it was suggested that the genus' current distribution could be a relict of a previously much wider one. However, a molecular analysis of the genus by Stech et al. (2008) identified another explanation: not only were the Australasian and Macaronesian Echinodium species widely separated geographically, they were widely separated phylogenetically. The Australasian species were placed in the family Neckeraceae, whereas the Macaronesian species were related to mosses of the family Lembophyllaceae. What is more, the Macaronesian species did not form a single clade within the Lembophyllaceae: at least one of the species was placed separately from the rest. The supposedly distinctive 'Echinodium' features, it seems, have evolved independently, possibly as an adaptation for wet habitats.

REFERENCES

Buck, W. R., B. Goffinet & A. J. Shaw. 2000. Testing morphological concepts of orders of pleurocarpous mosses (Bryophyta) using phylogenetic reconstructions based on trnL-trnF and rps4 sequences. Molecular Phylogenetics and Evolution 16(2): 180–198.

Huttunen, S., N. Bell, V. K. Bobrova, V. Buchbender, W. R. Buck, C. J. Cox, B. Goffinet, L. Hedenäs, B.-C. Ho, M. S. Ignatov, M. Krug, O. Kuznetsova, I. A. Milyutina, A. Newton, S. Olsson, L. Pokorny, J. A. Shaw, M. Stech, A. Troitsky, A. Vanderpoorten & D. Quandt. 2012. Disentangling knots of rapid evolution: origin and diversification of the moss order Hypnales. Journal of Bryology 34 (3): 187–211.

Stech, M., M. Sim-Sim, M. G. Esquível, S. Fontinha, R. Tangney, C. Lobo, R. Gabriel & D. Quandt. 2008. Explaining the ‘anomalous’ distribution of Echinodium (Bryopsida: Echinodiaceae): independent evolution in Macaronesia and Australasia. Organisms Diversity & Evolution 8 (4): 282–292.