Chromista

Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles ( plastids). It includes all protists whose plastids contain chlorophyll ''c'', such as some algae, diatoms, oomycetes, and protozoans. It is probably a polyphyletic group whose members independently arose as a separate evolutionary group from the common ancestor of all eukaryotes. As it is assumed the last common ancestor already possessed chloroplasts of red algal origin, the non-photosynthetic forms evolved from ancestors able to perform photosynthesis. Their plastids are surrounded by four membranes, and are believed to have been acquired from some red algae.

Chromista as a biological kingdom was created by British biologist Thomas Cavalier-Smith in 1981 to differentiate some protists from typical protozoans and plants. According to Cavalier-Smith, the kingdom originally included only algae, but his later analysis indicated that many protozoa also belong to the new group. As of 2018, the kingdom is as diverse as kingdoms Plantae and Animalia, consisting of eight phyla. Notable members include marine algae, potato blight, dinoflagellates, ''Paramecium'', the brain parasite '' Toxoplasma,'' and the malarial parasite ''Plasmodium''.

Biology

Members of Chromista are single-celled and multicellular eukaryotes having basically either or both features:
#plastid(s) that contain chlorophyll ''c'' and lie within an extra (periplastid) membrane in the lumen of the rough endoplasmic reticulum (typically within the perinuclear cisterna);
#cilia with tripartite or bipartite rigid tubular hairs.

Even though the kingdom includes diverse organisms from algae to malarial parasites (''Plasmodium''), they are genetically related and are believed to have evolved from a common ancestor with all other eukaryotes but in an independent line of evolution. As a result of evolution, many have retained their plastids and cilia, while some have lost them. Molecular evidence indicate that the plastids in chromists were derived from red algae through secondary symbiogenesis in a single event. (In contrast, plants acquired their plastids from cyanobacteria through primary symbiogenesis.) These plastids are now enclosed in two extra cell membranes, making a four-membrane envelope, as a result of which they acquired many other membrane proteins for transporting molecules in and out of the organelles. The diversity of chromists arose from degeneration, loss or replacement of the plastids in some lineages. There was also additional symbiogenesis of green algae, the genes of which are retained in some members (such as heterokonts), as well as bacterial chlorophyll (indicated by the presence of ribosomal protein L36 gene, ''rpl36'') in haptophytes and cryptophytes.

History and groups

The name Chromista was first introduced by Cavalier-Smith in 1981; the earlier names Chromophyta, Chromobiota and Chromobionta correspond to roughly the same group.

It has been described as consisting of three different groups:

*Heterokonts or stramenopiles: brown algae, diatoms, water moulds, etc.
*Haptophytes
*Cryptomonads

Cavalier-Smith later (in 2009) stated his reason for making a new kingdom, saying:

Since then Chromista has been defined in different ways at different times. In 2010, Cavalier-Smith himself indicated his desire to move Alveolata, Rhizaria and Heliozoa into Chromista.

Some examples of classification of the Chromista and related groups are shown below.

Chromophycées (Chadefaud, 1950)

The Chromophycées (Chadefaud, 1950), renamed Chromophycota (Chadefaud, 1960), included the current Ochrophyta (autotrophic Stramenopiles), Haptophyta (included in Chrysophyceae until Christensen, 1962), Cryptophyta, Dinophyta, Euglenophyceae and Choanoflagellida (included in Chrysophyceae until Hibberd, 1975).

Chromophyta (Christensen 1962, 1989)

The Chromophyta (Christensen 1962, 2008), defined as algae with chlorophyll c, included the current Ochrophyta (autotrophic Stramenopiles), Haptophyta, Cryptophyta, Dinophyta and Choanoflagellida. The Euglenophyceae were transferred to the Chlorophyta.

Chromophyta (Bourrelly, 1968)

The Chromophyta (Bourrelly, 1968) included the current Ochrophyta (autotrophic Stramenopiles), Haptophyta and Choanoflagellida. The Cryptophyceae and the Dinophyceae were part of Pyrrhophyta (= Dinophyta).

Chromista (Cavalier-Smith, 1981)

The Chromista (Cavalier-Smith, 1981) included the current Stramenopiles, Haptophyta (grouped together as Chromophyta) and Cryptophyta.

Chromalveolata (Adl ''et al''., 2005)

The Chromalveolata (Cavalier-Smith, 1981) included Stramenopiles, Haptophyta, Cryptophyta and Alveolata.

Classification

In his original classification in 1981, Cavalier-Smith included three phyla under Chromista:
#Heterokonta
#Haptophyta
#Cryptomonada

In 2010, Cavalier-Smith reorganised Chromista to include SAR supergroup (such as Stramenopiles, Alveolata and Rhizaria) and Hacrobia (Haptista, Cryptista).

In 2015, he and his colleagues made a new higher-level grouping of all organisms as a revision of seven kingdoms model. In it, they classified Chromista into 2 subkingdoms and 11 phyla, namely:
#Subkingdom Harosa (=Supergroup SAR)
##Phylum Ciliophora
##Phylum Miozoa
##Phylum Bigyra
##Phylum Ochrophyta
##Phylum Pseudofungi (=Oomycota)
##Phylum Cercozoa
##Phylum Retaria
#Subkingdom Hacrobia
##Phylum N.N. (=''nomen nominandum'', name unknown)—i.e. the classes Endohelea, Picomonadea and Telonemea
##Phylum Cryptista
##Phylum Haptophyta
##Phylum Heliozoa

Cavalier-Smith made a new analysis of Chromista in 2018 in which he classified all chromists into 8 phyla (Gyrista corresponds to the above phyla Ochrophyta and Pseudofungi, Cryptista corresponds to the above phyla Cryptista and "N.N.", Haptista corresponds to the above phyla Haptophyta and Heliozoa):
#Subkingdom Harosa
##Phylum Miozoa
##Phylum Ciliophora
##Phylum Gyrista
##Phylum Bigyra
##Phylum Cercozoa
##Phylum Retaria
#Subkingdom Hacrobia
##Phylum Cryptista
##Phylum Haptista

Phylogeny

Controversy

Molecular trees have had some difficulty resolving relationships between the different groups. All three may share a common ancestor with the alveolates (see chromalveolates), but there is evidence that suggests that the haptophytes and cryptomonads do not belong together with the heterokonts or the SAR clade, but may be associated with the Archaeplastida. Cryptista specifically may be sister or part of Archaeplastida.

See also

* Cabozoa
* Cavalier-Smith's system of classification
* List of Chromista by conservation status

References

External links

UCMP: Introduction to the Chromista

{{Taxonbar, from=Q862296

Obsolete eukaryote taxa
Kingdoms (biology)