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. 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. 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. 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 introns present in nucleomorphs are not recognized by host spliceosomes because they are too small and therefore cannot be cut and later incorporated into host DNA. Nucleomorphs also often code for many of their own critical functions, like transcription and translation. 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. See also Endosymbiont References 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