Although previous endosymbiotic events resulted in the increase in the number of membranes, tertiary plastids can have 3-4 membranes. The most largely studied tertiary plastids are found in dinoflagellates. Tertiary plastids are believed to have been derived from a red algae replacing secondary plastids. Consistent with our previous rules for reduction in genome size, and incorporation of genes into the host genome, tertiary plastid genome consists of about 14 genes. These genes are broken down further into small minicircles that contain 1-3 genes. These genomes are circular like prokaryotic genomes. Further, they only encode atpA, atpB, petB, perD, psaA, psaB, psbA-E, psbI, 16S and 23S rRNA. These genes play vital proteins used in photosystem I and II, indicating further their cyanobacterial origin.
