Nucleomorphs are small, vestigial eukaryotic nuclei found between the inner and outer pairs of membranes in certain plastids.
They are thought to be vestiges of red and green algal nuclei that were engulfed by a larger eukaryote.
[1] In 2020, genetic work identified the plastid in Lepidodinium and two previously undescribed dinoflagellates ("MGD" and "TGD") as being most closely related to the green alga Pedinomonas.
Plastid-targeted proteins encoded in the host genome must cross all four membranes to reach the plastid.
[7] The chlorarachniophytes, on the other hand, has no such thing as a cER, hence the initial import into the epiplastid space must occur by some other mechanism.
Based on research done on apicomplexa, which also has 4 membranes but no cER, it's possible that the protein is first sent into the ER, then sent to the epiplastid space by the endomembrane sorting system.
The model cryptomonad Guillardia theta became an important focus for scientists studying 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.
[4][13][14] 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.
[7] 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 (MGD and TGD).
[16] The standard nucleomorph is the result of secondary endosymbiosis: a cyanobacterium first became the chloroplast of ancestral plants, which diverged into green and red algae among other groups; the algal cell is then captured by another eukaryote.
However, in the case of dinotoms (i.e. those having diatom endosymbionts), the symbiont's nucleus appears to be of normal size with a large amount of DNA, surrounded by plenty of cytoplasm.