nucleomorphs

A nucleomorph is a remnant nucleus of a secondary endosymbiont that persists within certain unicellular algae and protists. It resides in the periplastid compartment, a narrow space between the endosymbiont’s plastid envelope and the host cytoplasm, and is surrounded by a small nuclear envelope derived from the endosymbiont. Nucleomorphs reflect the long-term integration of a secondary endosymbiotic partner.

Nucleomorphs occur in two major groups: cryptophytes (cryptomonads) and chlorarachniophytes. In cryptophytes the plastid derives from a red alga; in chlorarachniophytes from a green alga. In both cases the endosymbiont’s nucleus was retained and subsequently reduced to a nucleomorph, maintaining a minimal set of genes needed for its own expression and maintenance.

The nucleomorph is bounded by its own envelope and sits inside the host cytoplasm within the periplastid compartment, adjacent to the plastid. It remains connected to the host’s transcriptional and translational machinery to express a subset of essential genes, while many endosymbiont functions have been lost or transferred to the host nucleus.

Genomes of nucleomorphs are extremely compact for eukaryotes, typically on the order of a few hundred kilobases and encoding a limited set of genes for replication, transcription, and translation. Many ancestral genes have been transferred to the host nucleus or are no longer present. The plastid-targeted proteins are mostly encoded in the host genome and imported into the plastid; the nucleomorph retains a reduced gene set essential for its maintenance.

Nucleomorphs provide insight into the evolution of secondary endosymbiosis, organelle genome reduction, and endosymbiotic gene transfer, and are studied to understand how complex cells sustain integrated endosymbiotic partners.