Editing Nucleomorph (section)

==Nucleomorph genome==
Nucleomorphs represent some of the smallest genomes ever sequenced. After the red or green alga was engulfed by a [[cryptomonad]] or [[chlorarachniophyte]], respectively, its genome was reduced. The nucleomorph genomes of both cryptomonads and chlorarachniophytes converged upon a similar size from larger genomes.  They retained only three chromosomes and many genes were transferred to the nucleus of the host cell, while others were lost entirely.<ref name=archibald1/> Chlorarachniophytes contain a nucleomorph genome that is diploid and cryptomonads contain a nucleomorph genome that is tetraploid.<ref>{{Cite journal|title = Polyploidy of Endosymbiotically Derived Genomes in Complex Algae|url= |journal = Genome Biology and Evolution|date = 2014-04-01|issn = 1759-6653|pmc = 4007541|pmid = 24709562|pages = 974–980|volume = 6|issue = 4|doi = 10.1093/gbe/evu071|first1 = Yoshihisa|last1 = Hirakawa|first2 = Ken-Ichiro|last2 = Ishida}}</ref> 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 algae) and two [[Eukaryote|eukaryotic]] genomes (nucleus of host cell and nucleomorph).

The model [[cryptomonad]] ''[[Guillardia|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.<ref name=archibald1/><ref name=archibald2>{{cite journal | last1 = Archibald | first1 = John M | year = 2007 | title = Nucleomorph Genomes: Structure, Function, Origin and Evolution | doi = 10.1002/bies.20551 | pmid = 17373660 | journal = BioEssays | volume = 29 | issue = 4| pages = 392–402 }}</ref> It has three chromosomes with eukaryotic telomeres subtended by rRNA.<ref name=McFadden/>

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.<ref name=archibald1/>

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.<ref name=Gilson2006/><ref name=archibald2/><ref name=douglas2>{{cite journal | last1 = Douglas | first1 = SE | last2 = Zauner | first2 = S | last3 = Fraunholz | first3 = M | last4 = Beaton | first4 = M | last5 = Penny | first5 = S | display-authors = etal   | year = 2001 | title = The highly reduced genome of an enslaved algal nucleus | bibcode = 2001Natur.410.1091D | doi = 10.1038/35074092| pmid = 11323671 | journal = [[Nature (journal)|Nature]] | volume = 410 | issue = 6832| pages = 1091–1096 | doi-access = free }}</ref>