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 (in evolutionary terms) taken on a cyanobacterium as an endosymbiont. The plastid is often referred to as the 'cyanelle' or chromatophore. The endosymbiotic event happened about 100 million years 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 (and subsequently adopted into other eukaryote groups, by further endosymbiosis events). The P. chromatophora symbiont was related to the Prochlorococcus and Synechococcus cyanobacteria (sister to the group consisting of the living members of those two genera). 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 (those regions not modified by the symbiont) 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 speices pre-equipped to accept the chromatophore.