Bryophyte - Bryophyte Classification and Phylogeny

Bryophyte Classification and Phylogeny

Traditionally, all living land plants without vascular tissues were classified in a single taxonomic group, often a division (or phylum). More recently, phylogenetic research has questioned whether the bryophytes form a monophyletic group and thus whether they should form a single taxon. A broad consensus among systematists has recently emerged that bryophytes as a whole are not a natural group (i.e. are paraphyletic), although each of the three extant (living) groups is monophyletic. The three lineages are Marchantiophyta (liverworts), Bryophyta (mosses) and Anthocerotophyta (hornworts).

The vascular plants or tracheophytes are the fourth lineage of living land plants. Currently there is some uncertainty over the exact evolutionary relationships among these four lineages, although this may be nearing resolution as data on a variety of coding and non-coding DNA sequences from all three genomes (chloroplast, mitochondrion and nucleus) in addition to protein sequences are brought to bear on the problem. Although a 2005 study supported the traditional view that the bryophytes form a monophyletic group, the preponderance of currently available evidence suggests that the hornworts are sister to vascular plants and liverworts are sister to all other land plants, as shown in the cladogram below.

When extinct plants are taken into account, the picture is slightly altered. There are extinct land plants, such as the horneophytes, which are not bryophytes, but also are not vascular plants since like bryophytes they do not have true vascular tissue. A different distinction is needed. In bryophytes, the sporophyte is a simple unbranched structure with a single spore-forming organ (sporangium). In all other land plants, the polysporangiophytes, the sporophyte is branched and carries many sporangia. It has been argued that this contrast between bryophytes and other land plants is less misleading than the traditional one of non-vascular versus vascular plant, since many mosses have well-developed water-conducting vessels. The contrast is shown in a slightly different cladogram:

The term "bryophyte" thus refers to a grade of lineages defined primarily by what they lack: compared to other living land plants, they lack vascular tissue containing lignin; compared to all other land plants, they lack branched sporophytes bearing multiple sporangia. The prominence of the gametophyte in the life cycle is also a shared feature of the three bryophyte lineages (extant vascular plants are all sporophyte dominant).