Bryophyta

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.

Barcode of Life Data Systems (BOLD) Stats
  

Access Published & Released Data: Download FASTA File

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.