Chromalveolate

Chromalveolate
Clockwise from top-left: a haptophyte, some diatoms, a water mold, a cryptomonad, and Macrocystis, a phaeophyte
Scientific classification
Domain:	Eukarya
(unranked)	Bikonta
(unranked):	Corticata
Kingdom:	Chromalveolata*
Phyla
Heterokontophyta Haptophyta Cryptophyta Alveolata (Superphylum) Ciliophora Apicomplexa Dinoflagellata

Chromalveolata is a eukaryote supergroup first proposed by Thomas Cavalier-Smith as a refinement of his kingdom Chromista, which was first put forward in 1981. Chromalveolata was proposed to represent the result of a single secondary endosymbiosis between a line descending from a bikont and a red alga^[1] that became the progenitor of chlorophyll c containing plastids. In a major classification produced in 2005, Chromalveolata was regarded as one of the six major groups within the eukaryotes.^[2] However this has been increasingly challenged. Thus two papers published in 2008 have phylogenetic trees in which the chromalveolates are split up.^[3][4]

Groups and classification

Historically, many chromalveolates were considered plants, because of their cell walls, photosynthetic ability, and in some cases their morphological resemblance to the land plants (Embryophyta). However, when the five-kingdom system took prevalence over the animal-plant dichotomy, most chromalveolates were put into the kingdom Protista, with the water molds and slime nets put into the kingdom Fungi, and the brown algae staying in the plant kingdom.

In 2005, in a classification reflecting the consensus at the time, the Chromalveolata were regarded as one of the six major clades of eukaryotes.^[2] Although not given a formal taxonomic status in this classification, elsewhere the group has been treated as a Kingdom. The Chromalveolata were divided into four major subgroups:

• Cryptophyta
• Haptophyta
• Stramenopiles (or Heterokontophyta)
• Alveolata

Other groups which may be included within, or related to, chromalveolates, are:

• Telonemia^[5]

Though several groups, such as the ciliates and the water molds, have lost the ability to photosynthesize, most are autotrophic. All photosynthetic chromalveolates use chlorophylls a and c, and many use accessory pigments. Chromalveolates share similar glyceraldehyde 3-phosphate dehydrogenase proteins.^[6]

However, as early as 2005, doubts were being expressed as to whether the Chromalveolata was monophyletic,^[7] and a review in 2006 noted the lack of evidence for several of the supposed six major eukaryote groups, including the Chromalveolata.^[8] As of 2010^[update] there seems to be an emerging consensus that the group is not monophyletic. The four original subgroups fall into two categories, one comprising the Cryptophyta and the Haptophyta, the other the Stramenopiles and the Alveolata.^[3][4]

Halvaria

Main article: Halvaria

Analyses in 2007 and 2008 agree that the Stramenopiles and the Alveolata are related, forming a reduced chromalveolate clade.

Hacrobia

Main article: Hacrobia

The Haptophyta and Cryptophyta are related in these analyses,^[3][4] forming a clade which has been called 'Hacrobia'.

Position of SAR and Hacrobia

The Hacrobia appear to be more closely related to the Archaeplastida (plants in the very broad sense), being a sister group in one analysis,^[3] and actually nested inside this group in another.^[4] (Earlier, Cavalier-Smith had suggested a clade called Corticata for the grouping of all the chromalveolates and the Archaeplastida.)^[9] Halvaria and Rhizaria together form the SAR supergroup. The SAR supergroup and the Archaeplastida/Hacrobia combination are grouped in the bikonts, which all appear to descend from a heterotrophic eukaryote with two flagella.

See also: Eukaryote#Phylogeny

Morphology

Chromalveolates, unlike other groups with multicellular representatives, do not have very many common morphological characteristics. Each major subgroup has certain unique features, including the alveoli of the Alveolata, the haptonema of the Haptophyta, the ejectisome of the Crytophyta, and the two different flagella of the Heterokontophyta. However, none of these features are present in all of the groups.

The only common chromalveolate features are these:

• The shared origin of chloroplasts, as mentioned above
• Presence of cellulose in most cell walls

Since this is such a diverse group, it is difficult to summarize shared chromalveolate characteristics.

Ecological role

A potato plant infected with Phytophthora infestans.

Many chromalveolates affect our ecosystem in enormous ways. Some of these organisms can be very harmful. Dinoflagellates produce red tides which can devastate fish populations and intoxicate oyster harvests. Apicomplexans are some of the most successful specific parasites to animals. Water molds cause several plant diseases. In fact, it was a water mold, Phytophthora infestans, that caused the Irish potato famine.

A Californian kelp forest.

However, many chromalveolates are vital members of our ecosystem. Diatoms are one of the major photosynthetic producers, and as such produce much of the oxygen we breathe, and also take in much of the carbon dioxide from the atmosphere. Brown algae, most specifically kelps, create underwater "forest" habitats for many marine creatures, and provide a large portion of the diet of coastal communities.

Chromalveolates also provide many products that we use. The algin in brown algae is used as a food thickener, most famously in ice cream. The siliceous shells of diatoms have many uses, such as in reflective paint, in toothpaste, or as a filter, in what is known as diatomaceous earth.

References

1. ^ Keeling PJ (2009). "Chromalveolates and the evolution of plastids by secondary endosymbiosis". J. Eukaryot. Microbiol. 56 (1): 1–8. doi:10.1111/j.1550-7408.2008.00371.x. PMID 19335769. 
2. ^ ^{a b} Adl, Sina M.; 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, http://www.blackwell-synergy.com/doi/abs/10.1111/j.1550-7408.2005.00053.x 
3. ^ ^{a b c d} Burki, Fabien; Shalchian-Tabrizi, Kamran & Pawlowski, Jan (2008). "Phylogenomics reveals a new 'megagroup' including most photosynthetic eukaryotes". Biology Letters 4 (4): 366–369. doi:10.1098/rsbl.2008.0224. PMC 2610160. PMID 18522922. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2610160 
4. ^ ^{a b c d} Kim, E; Graham, LE (Jul 2008). Redfield, Rosemary Jeanne. ed. "EEF2 analysis challenges the monophyly of Archaeplastida and Chromalveolata." (Free full text). PLoS ONE 3 (7): e2621. doi:10.1371/journal.pone.0002621. PMC 2440802. PMID 18612431. http://www.plosone.org/article/info:doi/10.1371/journal.pone.0002621. 
5. ^ Shalchian-Tabrizi K, Eikrem W, Klaveness D, Vaulot D, Minge M, Le Gall F, Romari K, Throndsen J, Botnen A, Massana R, Thomsen H, Jakobsen K (2006). "Telonemia, a new protist phylum with affinity to chromist lineages". Proc Biol Sci 273 (1595): 1833–42. doi:10.1098/rspb.2006.3515. PMC 1634789. PMID 16790418. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1634789. 
6. ^ Takishita K, Yamaguchi H, Maruyama T, Inagaki Y (2009). Zhang, Baohong. ed. "A hypothesis for the evolution of nuclear-encoded, plastid-targeted glyceraldehyde-3-phosphate dehydrogenase genes in "chromalveolate" members". PLoS ONE 4 (3): e4737. doi:10.1371/journal.pone.0004737. PMC 2649427. PMID 19270733. http://dx.plos.org/10.1371/journal.pone.0004737. 
7. ^ Harper, J. T., Waanders, E. & Keeling, P. J. 2005. On the monophyly of chromalveolates using a six-protein phylogeny of eukaryotes. Int. J. System. Evol. Microbiol., 55, 487-496. [1]
8. ^ Laura Wegener Parfrey, Erika Barbero, Elyse Lasser, Micah Dunthorn, Debashish Bhattacharya, David J Patterson, and Laura A Katz (2006 December). "Evaluating Support for the Current Classification of Eukaryotic Diversity". PLoS Genet. 2 (12): e220. doi:10.1371/journal.pgen.0020220. PMC 1713255. PMID 17194223. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1713255. 
9. ^ Cavalier-Smith, T. (2002). "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". International journal of systematic and evolutionary microbiology 52 (Pt 2): 297–354. PMID 11931142.  edit

External links

• Tree of Life Eukaryotes