Nucleomorph
This article is missing information about inclusions including nucleolus (plus rDNA!), division behavior, mentioned in refs of doi:10.1007/s00709-017-1153-5 (RG link). (October 2021) |
Nucleomorphs are small, vestigial eukaryotic
Organisms with known nucleomorphs
So far, only two
Of the two known plastids that contain nucleomorphs, both have four membranes, the nucleomorph residing in the periplastidial compartment, evidence of being engulfed by a eukaryote through phagocytosis.[1]
In addition, some species within the dinoflagellates that have gone through tertiary endosymbiosis also have endosymbionts with both a nucleus and mitochondria present.[5]
Nucleomorph genome
Nucleomorphs represent some of the smallest genomes ever sequenced. After the red or green alga was engulfed by a
genomes (nucleus of host cell and nucleomorph).The model cryptomonad Guillardia theta became an important focus for scientists studying nucleomorphs. Its complete nucleomorph sequence was published in 2001, coming in at 551 Kbp. The G. theta sequence gave insight as to what genes were retained in nucleomorphs. Most of the genes that moved to the host cell involved protein synthesis, leaving behind a compact genome with mostly single-copy “housekeeping” genes (affecting transcription, translation, protein folding and degradation and splicing) and no mobile elements. The genome contains 513 genes, 465 of which code for protein. Thirty genes are considered “plastid” genes, coding for plastid proteins.[1][7]
The genome sequence of another organism, the chlorarachniophyte Bigelowiella natans indicates that its nucleomorph is probably the vestigial nucleus of a green alga, whereas the nucleomorph in G. theta probably came from a red alga. The B. natans genome is smaller than that of G. theta, with about 373 Kbp and contains 293 protein-coding genes as compared to the 465 genes in G. theta. B. natans also only has 17 genes that code for plastid proteins, again fewer than G. theta. Comparisons between the two organisms have shown that B. natans contains significantly more introns (852) than G. theta (17). B. natans also had smaller introns, ranging from 18-21 bp, whereas G. theta’s introns ranged from 42-52 bp.[1]
Both the genomes of B. natans and G. theta display evidence of genome reduction besides elimination of genes and tiny size, including elevated composition of adenine (A) and thymine (T), and high substitution rates.[4][7][8]
Persistence of nucleomorphs
There are no recorded instances of vestigial nuclei in any other secondary plastid-containing organisms, yet they have been retained independently in the cryptomonads and chlorarachniophytes. Plastid gene transfer happens frequently in many organisms, and it is unusual that these nucleomorphs have not disappeared entirely. One theory as to why these nucleomorphs have not disappeared as they have in other groups is that
Nucleomorphs also often code for many of their own critical functions, like transcription and translation.[9] Some say that as long as there exists a gene in the nucleomorph that codes for proteins necessary for the plastid’s functioning that are not produced by the host cell, the nucleomorph will persist.[1] In cryptophytes and chlorarachniophytes all DNA transfer between the nucleomorph and host genome seems to have ceased, but the process is still going on in a few dinoflagellates.[10]
See also
References
- ^ PMID 19617523.
- S2CID 8966320.
- ^ PMID 18077423.
- ^ PMID 16760254.
- ^ Tertiary Endosymbiosis in Two Dinotoms Has Generated Little Change in the Mitochondrial Genomes of Their Dinoflagellate Hosts and Diatom Endosymbionts - PLOS
- PMID 24709562.
- ^ PMID 17373660.
- PMID 11323671.
- ^ Curtis, Bruce et al. "Algal genomes reveal evolutionary mosaicism and the fate of nucleomorphs." Nature 492 :59-65
- ^ Organellogenesis still a work in progress in novel dinoflagellates
External links
- Insight into the Diversity and Evolution of the Cryptomonad Nucleomorph Genome
- Cryptophyta at NCBI taxbrowser
- Cercozoa at NCBI taxbrowser
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.