Nucleomorph
Encyclopedia
Nucleomorphs are small, reduced eukaryotic nuclei found in certain plastid
s. So far, only two groups of organisms are known to contain a nucleomorph: the cryptomonads of the supergroup Chromista
and the chlorarachniophyte
s of the supergroup Rhizaria
. The nucleomorphs support the endosymbiotic theory
, and are an evidence that the plastids of these organisms are so-called complex plastids. Studies of the genomic organization and of the molecular phylogeny
have shown that the nucleomorph of the cryptomonads formerly was the nucleus
of a red alga, whereas the nucleomorph of the chlorarchniophytes formerly was the nucleus of a green alga. In both groups of organisms the plastids originate from engulfed photoautotrophic eukaryote
s. After the red or green alga was engulfed by its host cell, it was reduced. Nucleomorphs retained only three chromosomes and many genes were transferred to the nucleus of the host cell.
The unique combination of host cell and complex plastid results in cells with four genomes: two prokaryotic genomes (mitochondrion
and plastid
of the red or green alga) and two eukaryotic
genomes (nucleus of host cell and nucleomorph).
----
According to GenBank release 164 (Feb 2008), there are 13 Cercozoa and 181 Cryptophyta entries (an entry is the submission of a sequence to the DDBJ/EMBL/GenBank public database of sequences).
Most sequenced organisms were:
Guillardia theta : 54;
Rhodomonas salina : 18;
Cryptomonas sp. : 15;
Chlorarachniophyceae sp. :10;
Cryptomonas paramecium : 9;
Cryptomonas erosa :7.
Note that the taxonomy used in the first section is probably outdated. See links below to NCBI TaxBrowser for present taxonomy
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
s. So far, only two groups of organisms are known to contain a nucleomorph: the cryptomonads of the supergroup Chromista
Chromista
The Chromista are a eukaryotic supergroup, probably polyphyletic, which may be treated as a separate kingdom or included among the Protista. They include all algae whose chloroplasts contain chlorophylls a and c, as well as various colorless forms that are closely related to them...
and the chlorarachniophyte
Chlorarachniophyte
Chlorarachniophytes are a small group of algae occasionally found in tropical oceans. They are typically mixotrophic, ingesting bacteria and smaller protists as well as conducting photosynthesis. Normally they have the form of small amoebae, with branching cytoplasmic extensions that capture prey...
s of the supergroup Rhizaria
Rhizaria
The Rhizaria are a species-rich supergroup of unicellular eukaryotes. This supergroup was proposed by Cavalier-Smith in 2002. They vary considerably in form, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods...
. The nucleomorphs support the endosymbiotic theory
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...
, and are an evidence that the plastids of these organisms are so-called complex plastids. Studies of the genomic organization and of the molecular phylogeny
Molecular phylogeny
Molecular phylogenetics is the analysis of hereditary molecular differences, mainly in DNA sequences, to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree...
have shown that the nucleomorph of the cryptomonads formerly was the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
of a red alga, whereas the nucleomorph of the chlorarchniophytes formerly was the nucleus of a green alga. In both groups of organisms the plastids originate from engulfed photoautotrophic eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s. After the red or green alga was engulfed by its host cell, it was reduced. Nucleomorphs retained only three chromosomes and many genes were transferred to the nucleus of the host cell.
The unique combination of host cell and complex plastid results in cells with four genomes: two prokaryotic genomes (mitochondrion
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
and plastid
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
of the red or green alga) and two eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
genomes (nucleus of host cell and nucleomorph).
----
According to GenBank release 164 (Feb 2008), there are 13 Cercozoa and 181 Cryptophyta entries (an entry is the submission of a sequence to the DDBJ/EMBL/GenBank public database of sequences).
Most sequenced organisms were:
Guillardia theta : 54;
Rhodomonas salina : 18;
Cryptomonas sp. : 15;
Chlorarachniophyceae sp. :10;
Cryptomonas paramecium : 9;
Cryptomonas erosa :7.
Note that the taxonomy used in the first section is probably outdated. See links below to NCBI TaxBrowser for present taxonomy