Category:Archaeplastida

The Archaeplastida (or kingdom Plantae sensu lato) are a major group of autotrophic eukaryotes, comprising the red algae (Rhodophyta), the green algae, and the land plants, together with a small group of freshwater unicellular algae called glaucophytes.[4] Unlike red and green algae, glaucophytes have never been involved in secondary endosymbiosis events.[5] All of these organisms have chloroplasts that are surrounded by two membranes, suggesting that they were acquired directly from endosymbiotic cyanobacteria. In all other groups besides the amoeboid Paulinella chromatophora, the chloroplasts are surrounded by three or four membranes, suggesting they were acquired secondarily from red or green algae.

Taxonomy
The consensus in 2005, when the group consisting of the glaucophytes and red and green algae and land plants was named 'Archaeplastida',[1] was that it was a clade, i.e. was monophyletic. Many studies published since then have provided evidence in agreement.[12][13][14][15] Other studies, though, have suggested that the group is paraphyletic.[16][17][18][7] 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).[19] 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.[20][21] 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").

Although many studies have suggested the Archaeplastida form a monophyletic group,[23] a 2009 paper argues that they are in fact paraphyletic.[18] The enrichment of novel red algal genes in a recent study demonstrates a strong signal for Plantae (Archaeplastida) monophyly and an equally strong signal of gene sharing history between the red/green algae and other lineages.[19] This study provides insight on how rich mesophilic red algal gene data are crucial for testing controversial issues in eukaryote evolution and for understanding the complex patterns of gene inheritance in protists.

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.[1]

Morphology
All archaeplastidans have plastids (chloroplasts) that carry out photosynthesis and are believed to be derived from endosymbiotic cyanobacteria. In glaucophytes, perhaps the most primitive members of the group, the chloroplast is called a cyanelle and shares several features with cyanobacteria, including a peptidoglycan cell wall, that are not retained in other members of the group. The resemblance of cyanelles to cyanobacteria supports the endosymbiotic theory.

The cells of most archaeplastidans have walls, commonly but not always made of cellulose.

The Archaeplastida vary widely in the degree of their cell organization, from isolated cells to filaments to colonies to multi-celled organisms. The earliest were unicellular, and many groups remain so today. Multicellularity evolved separately in several groups, including red algae, ulvophyte green algae, and in the green algae that gave rise to stoneworts and land plants.

evolved from the Plants+HC+SAR megagroup