text: The first plastid is highly accepted within the scientific community to be derived from the engulfment of cyanobacteria ancestor into a eukaryotic organism.  Evidence supporting this belief is found in many morphological similarities such as the presence of a two plasma membranes. It is thought that the first membrane belonged to the cyanobacteria ancestor. During phagocytosis, a vesicle engulfs a molecule with its plasma membrane to allow safe import. When the cyanobacteria became engulfed, the bacterium avoided digestion and led to the double membrane found in primary plastids.  However, in order to live in symbiosis, the eukaryotic cell that engulfed the cyanobacterium must now provide proteins and metabolites to maintain the functions of the bacteria in exchange for energy. Thus, an engulfed cyanobacterium must give up some of its genetic material to allow for endosymbiotic gene transfer to the eukaryote, a phenomenon that is thought to be extremely rare due to the "learned nature" of the interactions that must occur between the cells to allow for processes such as; gene transfer, protein localization, excretion of highly reactive metabolites, and DNA repair.  This would mean, a reduction in genome size, for the cyanobacteria, but also an increase in cytobacterial genes within the eukaryotic genome. The genus of Synechocystis sp., strain PCC6803 is a unicellular fresh water cyanobacteria that encodes 3725 genes, and a 3.9 Mb sized genome. However, most plastids rarely exceed 200 protein coding genes. A recent study sequenced the genome of a cyanobacterium that was living extracellularly in endosymbiosis with the water-fern Azolla filiculoides. Endosymbiosis was supported by the fact that the cyanobacterium was unable to grow autonomously, and the observance of the cyanobacterium being vertically transferred between succeeding generations. After cyanobacterium genome analysis, the researchers found that over 30% of the genome was made up of pseudogenes. In addition, roughly 600 transposable elements were found within the genome. The pseudogenes were found in genes such as dnaA, DNA repair genes, glycolysis and nutrient uptake genes. dnaA is essential to initiation of DNA replication in prokaryotic organisms, thus Azolla filiculoides is thought to provide nutrients, and transcriptional factors for DNA replication in exchange for fixed nitrogen that is not readily available in water. Although the cyanobacterium had not been completely engulfed in the eukaryotic organism, the relationship is thought to demonstrate the precursor to endosymbiotic primary plastids.