Archaeplastida - Taxonomy

Taxonomy

The consensus in 2005, when the group consisting of the glaucophytes, red and green algae and land plants was named 'Archaeoplastida', was that it was a clade, i.e. was monophyletic. Many studies published since this date have provided evidence which is in agreement. On the other hand, other studies have suggested that the group is paraphyletic. To date, the situation appears unresolved, but a strong signal for Plantae (Archaeplastida) monophyly has been demonstrated in a recent study (with an enrichment of red algal genes). The assumption made here is that Archaeplastida is a valid clade.

Various names have been given to the group. Some authors have simply referred to the group as plants or Plantae. However, the name Plantae is ambiguous, since it has also been applied to less inclusive clades, such as Viridiplantae and embryophytes. To distinguish, the larger group is sometimes known as Plantae sensu lato ("plants in the broad sense").

To avoid ambiguity, other names have been proposed. Primoplantae, which appeared in 2004, seems to be the first new name suggested for this group.

Another name that has been applied to this node is Plastida, defined as the clade sharing "plastids of primary (direct prokaryote) origin in Magnolia virginiana Linnaeus 1753".

The name Archaeplastida was proposed in 2005 by a large international group of authors (Adl et al.) who aimed to produce a classification for the eukaryotes which took into account morphology, biochemistry and phylogenetics, and which had "some stability in the near term." They rejected the use of formal taxonomic ranks in favour of a hierarchical arrangement where the clade names do not signify rank. Thus the phylum name 'Glaucophyta' and the class name 'Rhodophyceae' appear at the same level in their classification. The divisions proposed for the Archaeplastida are shown below in both tabular and diagrammatic form.

Archaeplastida:

• Glaucophyta (Glaucocystophyta ) – glaucophytes

Glaucophytes are a small group of freshwater single-celled algae. Their chloroplasts, called cyanelles, have a peptidoglycan layer, making them more similar to cyanobacteria than those of the remaining Archaeplastida.

• Rhodophyceae (Rhodophyta ) – red algae

Red algae form one of the largest groups of algae. Most are seaweeds, being multicellular and marine. Their red colour comes from phycobiliproteins, used as accessory pigments in light capture for photosynthesis.

• Chloroplastida (Viridiplantae ; Chlorobionta ; Chlorobiota )

Chloroplastida is the term chosen by Adl et al. for the group made up of the green algae and land plants (embryophytes). Except where lost secondarily, all have chloroplasts without a peptidoglycan layer and lack phycobiliproteins.

• Chlorophyta – green algae (part)

Adl et al. employ a narrow definition of the Chlorophyta; other sources include the Chlorodendrales and Prasinophytae, which may themselves be combined.

• Ulvophyceae
• Trebouxiophyceae (Pleurastrophyceae ; Microthamniales )
• Chlorophyceae

• Chlorodendrales – green algae (part)
• Prasinophytae – green algae (part)
• Mesostigma (Mesostigmata )
• Charophyta (Charophyceae ) – green algae (part) and land plants

Charophyta sensu lato, as used by Adl et al., is a monophyletic group which is made up of some green algae, including the stoneworts (Charophyta sensu stricto), as well as the land plants (embryophytes).

• Sub-divisions other than Streptophytina (below) were not given by Adl et al.

Other sources would include the green algal groups Chlorokybales, Klebsormidiales, Zygnematales and Coleochaetales.

• Streptophytina – stoneworts and land plants

• Charales (Charophytae ) – stoneworts
• Plantae (Cormophyta ; Embryophyta ) – land plants (embryophytes)