Overview

Brief Summary

Introduction

Diversity of mosses has been classified in approximately 10,000 species, 700 genera, and about 110-120 families. This places the mosses as the third most diverse group of land plants, only after the angiosperms and ferns. Mosses are small plants requiring stereoscopes and compound microscopes for routine examination. The conspicuous green leafy shoots are the gametophytes, haploid organisms, on which the diploid embryo develops into a mature sporophyte (Figure 1). The sporophyte is chlorophyllose and photosynthetic only in early stages of development, and it is mostly dependent on the gametophyte. Moss colonies are a very important element in many ecosystems, from the tundra to the tropical rain forest, reducing soil erosion, capturing water and nutrients, providing shelter for microfauna, and nurseries for seedlings in succession or regeneration processes.

 

 As a lineage, mosses are a historically crucial group in the understanding of the transition to life on land. The green leafy shoots (gametophytes) retain some features of the green algal ancestors (chlorophylls a and b, starch, sperm with two forward undulipodia), but the needle-like shoots that produce the spores (sporophytes) display key innovations for the life outside water, such as stomates, a simple strand of conductive cells [in an unbranched sporophyte], and airborne spores produced in a single apical capsule (sporangium). This is the simplest structural level among all land plants. The next organizational level is found in two fossil groups: Horneophythopsida and Aglaophyton (Rhynia) major, where the sporophyte is branched and produces several sporangia. The sporophyte shows the most complex structural organization in the tracheophytes.

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Comprehensive Description

Characteristics

There is no controversy that the mosses are monophyletic. Synapomorphies for the mosses are: i) leaves in the gametophyte; these green laminar organs are attached densely along the shoot. ii) multicellular rhizoids; these branched filaments composed of a series of multiple cells develop from the surface of the gametophyte axis at the point of contact with the substrate. iii) columnella; this is a cylinder of sterile cells located in the center of the capsule. (Mishler & Churchill, 1984). Other possible synapomorphies are features of male gamete [spermatozoid] ultrastructure (Mishler, Lewis & et al 1994).


Figure 1. Life cycle of a dioicous moss.
Modified from original drawings by Ivy Livingstone. © 1997 BIODIDAC Modified from original drawings by Ivy Livingstone.

The life cycle of a moss alternates a conspicuous gametophyte generation and a dependent sporophyte generation. Some of the salient features of the different life cycle stages are outlined below.

Protonema. Spores germinate and produce a protonema. This is usually filamentous and branched, but in some groups it is tallose or massive. At several places in the protonema, apical cells differentiate and produce the foliose shoots.

Gametophore. An apical cell produces a stem and leaves spirally arranged. The stems produce branches in several combinations of monopodial and sympodial architectures. Leaves are sessile, unlobed, and often with a thickened midrib. Other appendages to the stem are multicellular rhizoids, axillary hairs, paraphyllia, pseudoparaphyllia, and various types of asexual propagules. The patterns of the leaf cell network and leaf cell papillae provide numerous characters for the systematic arrangement of genera and species.

Gametangia. Archegonia and antheridia are produced in groups, with paraphyses among them, and surrounded by perichaetial or perigonial leaves. Autoicous, synoicous, paroicous, heteroicous, and dioicous sexual conditions are found in many taxa.

Seta. The sporophyte consists of the foot, a seta and an apical sporangium. The foot is embedded within the apex of a stem or branch. Ultrastructural details of the transfer zone provide important similarities with other land plants. The seta is short or elongate. It has an internal conductive system that connects the foot and the capsule at both extremes.

Sporangium. A single sporangium or capsule develops distally from an unbranched sporophyte. The sporangium opens by an apical pore, longitudinal splits or most commonly by an operculum. The external cell layer (exothecium) often has stomata, especially in the neck. Underlaying the exothecium there are parenchymatose cells. Both concentric layers constitute the amphithecium (Figure 2A). Internally, the endothecium consists of a cylinder of sporogenous tissue, surrounding a columnella of sterile cells. Moss spores are unicellular, sometimes retaining tetrad marks.


Figure 2. The two basic peristome types of mosses.
A. Cross section of an embryonic sporophyte capsule.
B. Nematodontous peristome (e.g., Polytrichum).
C. Arthrodontous peristome (e.g., Funaria or Bryum).
Drawings copyright © 2000 Efrain De Luna

Peristome. In the majority of mosses, the apex of the capsule (operculum) falls off at maturity and reveals a structure called the peristome (Figure 1). This is a ring of narrow triangular segments surrounding the mouth of the capsule. Changes in moisture conditions cause movements of the peristome and facilitate the dispersion of spores in favorable dry conditions. Two basic types of peristomes are found in mosses: arthrodontous and nematodontous. In arthrodontous peristomes, at the level of the capsule mouth and above, three innermost rings of cells of the amphithecium are involved in the formation of teeth in most taxa. These three concentric rows are known as the "outer", "primary" and "inner" peristomial layers (OPL, PPL, IPL). Most peristomate mosses have the arthrodontous type, in which each tooth is composed of periclinal (tangential) cell wall remnants between two of the three concentric peristomial cell layers (Figure 2C). If the teeth are formed by the tangential walls between the OPL and PPL, the row of teeth is collectively known as the exostome. In the second case, the cell wall remnants are located between the cell rings of the PPL and IPL, therefore the row of segments is known as endostome. A second fundamental type of peristome is nematodontous, structured by narrow columns of entire cell wall remnants. Each tooth consists of agglomerated cylinders formed by the periclinal and anticlinal thickened cell walls (Figure 2B). As in arthrodontous peristomes, peristomial cells derive from the innermost amphithecium, but multiple concentric peristomial layers (four to seven) contribute to the formation of a nematodontous tooth.

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Ecology

Associations

In Great Britain and/or Ireland:
Plant / associate
fruitbody of Dictyonema interruptum is associated with gametophyte of Bryophyta

Plant / associate
fruitbody of Loreleia postii is associated with Bryophyta

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Known predators

Bryophyta (moss cells) is prey of:
Anatidae
scarlet mite
Rangifer
Diptera
Chironomidae
Achorutes
Plectrophenax nivalis
Rangifer tarandus
Anser brachyrhynchus
Henlea
Collembola
Acari
Rhithrogena
Amphinemura
Leuctra
Oligochaeta
Simulium
Chloroperla
Hydropsyche
Heptagenia
Ephemerella
Simuliidae
Cricotopus
Orthocladius
Micrasema
Ephemerella ignita
Perla cephalotes
Leuctra hippopus
Leuctra klapaleki
Hydropsyche instabilis
Crunoecia irrorata
Psychomyia pusilla
Tinodes waeneri
Simulium reptans
Hexatoma
Gallinula chloropus
Ursus arctos
Cervus elaphus
Bos grunniens
Pseudomys higginsi
Rupicapra rupicapra
Rupicapra pyrenaica
Myopus schisticolor
Aphrophila noevaezelandiae
Austroperla cyrene
Pycnocentria
Pycnocentrodes evecta
Amphipoda
Deleatidium
Helicopsyche albescens
Hydora nitida
Acroperla
Aoteapsyche
Maoridiamesea
Polypedellum
Cricotopus I
Hudsonema amabilis
Paranephrops zealandicus
Zealolessica cheira
Austraclima jollyae
Austrosimulium australense
Hydrobiosella
Scirtidae

Based on studies in:
Norway: Spitsbergen (Coastal)
Wales, River Rheidol (River)
UK: Yorkshire, Aire, Nidd & Wharfe Rivers (River)
Finland (River)
New Zealand: Otago, Blackrock, Lee catchment (River)
New Zealand: Otago, Broad, Lee catchment (River)
New Zealand: South Island, Canton Creek, Taieri River, Lee catchment (River)
New Zealand: Otago, Kye Burn (River)
New Zealand: Otago, Stony, Sutton catchment (River)

This list may not be complete but is based on published studies.
  • V. S. Summerhayes and C. S. Elton, Further contributions to the ecology of Spitzbergen, J. Ecol. 16:193-268, from p. 211 (1928).
  • V. S. Summerhayes and C. S. Elton, Further contributions to the ecology of Spitzbergen, J. Ecol. 16:193-268, from p. 217 (1928).
  • J. R. E. Jones, A further ecological study of the river Rheidol: the food of the common insects of the main-stream, J. Anim. Ecol. 19:159-174, from p. 172 (1950).
  • K. Kuusela, Early summer ecology and community structure of the macrozoobenthos on stones in the Javajankoski rapids on the river Lestijoki, Finland, Acta Universitatis Ouluensis (Ser. A, no. 87, Oulu, Finland, 1979).
  • E. Percival and H. Whitehead, 1929. A quantitative study of the fauna of some types of stream-bed. J. Ecol. 17:282-314, from p. 311 & overleaf.
  • Townsend, CR, Thompson, RM, McIntosh, AR, Kilroy, C, Edwards, ED, Scarsbrook, MR. 1998. Disturbance, resource supply and food-web architecture in streams. Ecology Letters 1:200-209.
  • V. S. Summerhayes and C. S. Elton, Contributions to the ecology of Spitsbergen and Bear Island, J. Ecol. 11:214-286, from p. 232 (1923).
  • Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2006. The Animal Diversity Web (online). Accessed February 16, 2011 at http://animaldiversity.org. http://www.animaldiversity.org
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Evolution and Systematics

Evolution

Discussion of Phylogenetic Relationships

View Bryophyta Tree

Tree from Hedderson et al (1996), Mishler & Churchill (1984), and Newton et al (2000)

Early proposals of phylogenetic relationships within the mosses (e.g., Vitt 1984) interpreted three fundamental levels of structural organization, largely based on the morphology of the capsules. In the most basal lineage, the Sphagnopsida, capsules open through an apical pore. In the Andreaeopsida, a very small intermediate group, capsules open through longitudinal valves. In the most derived and diverse lineage, the "true mosses" (commonly classified as the Bryopsida), the capsules are operculate and differentiate a peristome. Characters from the exostome and endostome provided a basis for classification of major groups at the ordinal level and above.

Modern explicit hypotheses for moss relationships are based on several cladistic analyses based on morphological, ultrastructural, and molecular data. A recent issue of the journal "The Bryologist" (Vol 103, issue 2, 2000) includes a number of new phylogenetic analyses that incorporate exemplars from nearly all families of mosses. Each paper was a major collaborative effort by researchers from many countries who brought together sequence data from rbcL, rps4, trnL-F, and 18S rRNA. Newton et al. (2000, Fig. 3E) presented a cladistic analysis of all orders of mosses based on morphological characters and four DNA sequence data sets for 33 exemplar taxa and ten outgroups. Results include the monophyly of the mosses, the inclusion of Takakia as a moss sister to Sphagnum, and the monophyly of the peristomate mosses. In combination, these recent higher level analyses provide the first cladistic framework for the major lineages of mosses.

Five main lineages are currently recognized (Mishler & Churchill 1984, Newton et al., 2000), although their taxonomic ranking as classes or subclasses is still controversial. In most cases, higher-level classification of the mosses is not fully settled because there are different names used for the same major clades. Here we use names for these five lineages considered at the rank of class. A basal split divides the Sphagnopsida (1) sister to a large clade in which the Andreaeopsida (2) is the most basal lineage (Figure 3A, B). Peristomate mosses include three main lineages: Polytrichopsida (3) with nematodontous peristomes are basal to a clade of arthrodontous mosses that includes the Tetraphidopsida (4) and Bryopsida (5) (Figure 3B).

Figure 3. Alternative placements of Polytrichopsida, Tetraphidopsida, Takakiales, and Oedipodiaceae. A. Tree from Mishler & Churchill (1984, p. 412). B. Tree from Beckert et al. (1999, p. 185). C. Tree from Hedderson et al. (1996, p. 217). D. Tree from Mishler et al. (1994, p. 466). E. Tree from Newton et al. (2000, p. 195). F. Tree from Hyvonen et al. (1998, p. 498).

Early schemes of relationships within mosses have not changed substantially, but they have been expanded to accommodate a few groups now recognized at higher taxonomic rankings. Ambuchania, a separate genus closely related to Sphagnum, is now classified in its own order and placed as sister to the Sphagnales (Shaw 2000). Andreaeobryum, a genus related to Andreaea, is also classified in its own order and placed as sister to the Andreaeales (Newton et al. 2000).

Ideas about the relationships within the peristomate clade have oscillated between two views. In some systems, the Polytrichopsida were considered as the most advanced because of the complexity of the gametophyte (Brotherus 1925). In contrast, other arrangements interpreted this group as the most ancestral due to the simplicity of the nematodontous peristome (Vitt 1984). Modern cladistic analyses corroborated the basal position of the nematodontous mosses (Polytrichum) relative to the more recent origin of the diplolepidous mosses (Bryum). However, the phylogenetic positions of certain groups are still controversial. The Tetraphidopsida, for example, has been placed as sister to the Bryopsida or sister to the Polytrichopsida and Bryopsida together (Figure 3A, B, C). Another example of controversial phylogenetic position is the Oedipodiaceae. This family is alternatively placed as sister to the large clade of peristomate mosses (Polytrichopsida, Tetraphidopsida and Bryopsida) or as sister to just the Polytrichales (Figure 3E, F).

The fascinating bryophyte Takakia deserves a special note. Initially it was interpreted as a liverwort, and until recently it was classified in the Calobryales (Schofield 1985). After the discovery of plants with sporophytes, now it is undoubtedly classified as a moss. This placement within the mosses was confirmed with recent cladistic analyses based on morphology and several genes, although it remains uncertain if it should be sister to Andreaeaopsida or Sphagnopsida (Figs. 3C, D, E).

Several factors have made possible such new insight on higher level relationships within mosses. New empirical data have accumulated on morphology, anatomy, ontogeny, ultrastructure, and DNA sequences. Although most of the recent studies at higher levels are based on molecular data, the most important element of the recent progress has been the use of cladistic methodology for the interpretation of such types of evidence. A formal framework of higher-level moss phylogeny is now available, which replaces former intuitive estimates of relationships founded largely on concepts of overall similarity and ad hoc evolutionary scenarios.

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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:2,446Public Records:2,054
Specimens with Sequences:2,325Public Species:987
Specimens with Barcodes:2,157Public BINs:0
Species:1,204         
Species With Barcodes:1,128         
          
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Source: Barcode of Life Data Systems (BOLD)

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Barcode data

Creative Commons Attribution 3.0 (CC BY 3.0)

© Barcode of Life Data Systems

Source: Barcode of Life Data Systems (BOLD)

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Bryophyta

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Source: Barcode of Life Data Systems (BOLD)

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