Paulinella


Paulinella is a genus of about nine species of freshwater amoeboids.
Its most famous members are the three photosynthetic species P. chromatophora, P. micropora and P. longichromatophora, the first two being freshwater forms and the third a marine form, which has recently taken on a cyanobacterium as an endosymbiont. The event to permanent endosymbiosis probably occurred with a cyanobiont. The plastid is often referred to as the 'cyanelle' or chromatophore. The endosymbiotic event happened about 90–140 million years ago in a bacterial species which diverged about 500 million years ago from some known cyanobacteria. This is striking because the chloroplasts of all other known photosynthetic eukaryotes derive ultimately from a single cyanobacterium endosymbiont, which was taken in probably over a billion years ago by an ancestral archaeplastidan. The P. chromatophora symbiont was related to the Prochlorococcus and Synechococcus cyanobacteria. The chromatophore genome has gone through a reduction, and is now just one third the size of the genome of its closest free living relatives, but still 10-fold larger than most plastid genomes. Some of the genes have been lost, others have migrated to the amoeba's nucleus through endosymbiotic gene transfer. Other genes have degenerated due to Muller's ratchet - accumulations of harmful mutations due to genetic isolation, and have probably been replaced with genes from other microbes through horizontal gene transfer. The nuclear genes of P. chromatophora are most closely related to the heterotrophic P. ovalis. P. ovalis also have at least two cyanobacterial-like genes, which were probably integrated into their genome through horizontal gene transfer from its cyanobacterial prey. Similar genes could have made the photosynthetic species pre-equipped to accept the chromatophore.