Chromista

Structure of typical chromists compared with plant cell (left)

Members of Chromista are single-celled and multicellular eukaryotes having basically either or both features:[3]

1. plastid(s) that contain chlorophyll c and lies within an extra (periplastid) membrane in the lumen of the rough endoplasmic reticulum (typically within the perinuclear cisterna);
2. 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.[5] Molecular evidences indicate that the plastids in chromists were derived from red algae through secondary symbiogenesis in a single event.[6] (In contrasts, plants acquired their plastids from cyanobacteria through primary symbiogenesis.[7]) 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.[8] There was also additional symbiogenesis of green algae, the genes of which are retained in some members (such as heterokonts),[9] as well as bacterial chlorophyll (indicated by the presence of ribosomal protein L36 gene, rpl36) in haptophytes and cryptophytes.[10]

The name Chromista was first introduced by Cavalier-Smith in 1981;[3] the earlier names Chromophyta, Chromobiota and Chromobionta correspond to roughly the same group.

It has been described as consisting of three different groups:[11]

• 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:

I established Chromista as a kingdom distinct from Plantae and Protozoa because of the evidence that chromist chloroplasts were acquired secondarily by enslavement of a red alga, itself a member of kingdom Plantae, and their unique membrane topology.[5]

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.[12][13]

Chromista classification

In his original classification in 1981, Cavalier-Smith included three phyla under Chromista, such as:[3]

1. Heterokonta
2. Haptophyta
3. Cryptomonada

In 2010, Cavalier-Smith reorganised Chromista to include SAR supergroup such as (Stramenopiles, Alveolata and Rhizaria) and Hacrobia (Haptista, Cryptista).[5]

In 2017, 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 10 phyla, namely:[16]

1. Subkingdom Hacrobia
   1. Phulym N.N. (=nomen nominandum, name unknown)
   2. Phylum Cryptista
   3. Phylum Haptophyta
   4. Phylum Heliozoa
2. Subkingdom Harosa (=Supergroup SAR)
   1. Phylum Ciliophora
   2. Phylum Miozoa
   3. Phylum Bigyra
   4. Phylum Pseudofungi (=Oomycota)
   5. Phylum Cercozoa
   6. Phylum Retaria

Cavalier-Smith made a new analysis of Chromista in 2018 in which he classified all chromists into 8 phyla, as follows:[4]

1. Phylum Miozoa
2. Phylum Ciliophora
3. Phylum Gyrista
4. Phylum Bigyra
5. Phylum Cercozoa
6. Phylum Retaria
7. Phylum Cryptista
8. Phylum Haptista

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.[1][17] Cryptista specifically may be sister or part of Archaeplastida.[18]

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

Wikispecies has information related to Chromista

• UCMP: Introduction to the Chromista