Primoplantae
From Wikipedia, the free encyclopedia
 |
||||
|---|---|---|---|---|
 Algal bloom and land plants
|
||||
| Scientific classification | ||||
|
||||
|
|
||||
|
Primoplantae is a group of organisms that includes green plants (green algae and land plants), red algae, and an obscure group of single-celled algae called the glaucophytes. They derive their energy through photosynthesis using chloroplasts. They differ from other photosynthetic eukaryotes, in that they initially acquired their chloroplasts by engulfing a cyanobacterium. All other eukaryotes with chloroplasts acquired them by way of a primoplant.
Contents |
[edit] Taxonomic history
Some authors have simply referred to this group as plants or Plantae [1] [2] Since the same name has also been applied to less inclusive clades, such as Viridiplantae and embryophytes, this larger group is sometimes known as Plantae sensu lato ("plants in the broad sense").
Because the name Plantae is ambiguous, other names have been proposed. Primoplantae, which appeared in 2004, seems to be the first new name suggested for this group. [3]
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". [4]
Most recently, the name Archaeplastida was proposed. [5]
[edit] Morphology
All primoplants have plastids called chloroplasts that carry out photosynthesis, derived from captured 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 primoplants. The resemblance of cyanelles to cyanobacteria supports the endosymbiotic theory.
Primoplants vary widely in the degree of their cell organization, from isolated cells to filaments to colonies to multi-celled organisms. The earliest primoplants were unicellular, and many groups remain so today. Multicelluarity evolved separately in several groups, including red algae, ulvophyte green algae, and in the green algae that gave rise to stoneworts and land plants. The cells of most primoplants have walls, commonly but not always made of cellulose.
[edit] Endosymbiosis
Because the ancestral primoplant acquired its chloroplasts directly by engulfing cyanobacteria, the event is known as a primary endosymbiosis. Evidence for this includes the presence of a double membrane around the chloroplasts; one membrane belonged to the bacterium, and the other to the eukaryote that captured it. Over time, many genes from the chloroplast have been transferred to the nucleus of the host cell. The presence of such genes in the nuclei of eukaryotes without chloroplasts suggests this transfer happened early in the primoplants' evolution. [6]
All other eukaryotes with chloroplasts gained them by engulfing a single-celled primoplant with its own bacterially-derived chloroplasts. The chloroplasts of euglenids and chlorarachniophytes appear to be captured green algae. Other photosynthetic eukaryotes have chloroplasts that are captured (primoplant) red algae, and include heterokont algae, cryptophytes, haptophytes, and dinoflagellates. Because these involve endosymbiosis of cells that have their own endosymbionts, the process is called secondary endosymbiosis. The chloroplasts of these eukaryotes are typically surrounded by more than two membranes, reflecting their history of multiple engulfment.
[edit] Fossil record
Perhaps the most ancient remains of Primoplantae are microfossils from the Roper group in northern Australia. The structure of these single-celled fossils resemble that of modern green algae. These date to the Mesoproterozoic Era, about 1500 to 1300 Ma (million years ago) [7] These fossils are consistent with a molecular clock study that calculated that this clade diverged about 1500 Ma. [8] The oldest fossil that can be assigned to a specific modern group is the red alga Bangiomorpha, from 1200 Ma. [9]
In the late Neoproterozoic Era, algal fossils became more numerous and diverse. Eventually, in the Paleozoic Era, plants emerged onto land, and have continued to flourish up to the present.
[edit] References
- ^ T. Cavalier-Smith (1981). "Eukaryote Kingdoms: Seven or Nine?". BioSystems 14: 461-481.
- ^ Bhattacharya, Debashish; Yoon, Hwan Su; Hackett, Jeremiah (2003). "Photosynthetic eukaryotes unite: endosymbiosis connects the dots.". BioEssays 26: 50-60.
- ^ Palmer, Jeffrey D.; Soltis, Douglas E.; & Chase, Mark W. (2004). "The plant tree of life: an overview and some points of view". American Journal of Botany 91: 1437-1445.
- ^ Simpson, A.G.B. (2004). "{{{title}}}". First International Phylogenetic Nomenclature Meeting. Paris, July 6-9, 2004..
- ^ Sina M. Adl et al (2005). "The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists". Journal of Eukaryotic Microbiology 52 (5): 399. DOI:10.1111/j.1550-7408.2005.00053.x.
- ^ Andersson, Jan O., & Roger, Andrew J. (2002). "A cyanobacterial gene in non-photosynthetic protists--An early chloroplast acquisition in eukaryotes?". Current Biology 12: 115-119..
- ^ Javaux, Emmanuelle J, Knoll, Andrew H, & Walter, Malcolm R. (2004). "TEM evidence for eukaryotic diversity in mid-Proterozoic oceans". Geobiology 2: 121-132.
- ^ Yoon, Hwan Su; Hackett, Jeremiah D., Ciniglia, Claudia; Pinto, Gabriele; & Bhattacharya, Debashish} (2004). "A molecular timeline for the origin of photosynthetic eukaryotes.". Molecular Biology & Evolution 21 (5): 809-818.
- ^ Butterfield, Nicholas J. (2000). "Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes.". Paleobiology 26 (3): 386–404.
