''Paulinella'' is a genus of at least eleven species including both freshwater and marine

amoeboid An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopo ...

s. 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 have recently (in evolutionary terms) taken on a

cyanobacterium Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

as an

endosymbiont An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...

. As a result they are no longer able to perform

phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

like their non-photosynthetic relatives. The event to permanent endosymbiosis probably occurred with a cyanobiont. The resulting

organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' th ...

is a

photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosymbiotic cyan ...

that is often referred to as a 'cyanelle' or chromatophore, and is the only other known primary endosymbiosis event of photosynthetic cyanobacteria, although primary endosymbiosis with a non-photosynthetic cyanobacterial symbiont have occurred in the

diatom A diatom ( Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising se ...

family ''Rhopalodiaceae''. The endosymbiotic event happened about 90–140 million years ago in a cyanobacterial species which diverged about 500 million years ago from the ancestors of its sister clade that consist of the living members of the '' Prochlorococcus'' and '' Synechococcus'' cyanobacteria genera, which is a different clade to the plastids belonging to the

Archaeplastida The Archaeplastida (or kingdom Plantae '' sensu lato'' "in a broad sense"; pronounced /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor gr ...

. This is striking because the

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...

s of all other known photosynthetic

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...

s derive ultimately from a single cyanobacterium endosymbiont, which was taken in about 1.6 billion years ago by an ancestral

archaeplastida The Archaeplastida (or kingdom Plantae '' sensu lato'' "in a broad sense"; pronounced /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor gr ...

n (and subsequently adopted into other eukaryote groups through secondary endosymbiosis events, and later tertiary and quaternary endosymbiosis, etc). The only exception is the

ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a differen ...

''Pseudoblepharisma tenue'', which in addition to a photosynthetic symbiont that is a captured green algae, also has a photosynthetic prokaryote as a symbiont; a

purple bacteria Purple bacteria or purple photosynthetic bacteria are Gram-negative proteobacteria that are phototrophic, capable of producing their own food via photosynthesis. They are pigmented with bacteriochlorophyll ''a'' or ''b'', together with variou ...

with a reduced genome, instead of a 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 Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...

. Some of the genes the nucleus received from the chromatophore were multiplied many times over through a “copy-paste” mechanism called

retrotransposition A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Tra ...

, enabling them to function more efficiently and making them more tolerant against toxic compounds associated with photosynthesis. This changed the metabolism of the amoeba so much that it could no longer feed on microbes like its ancestors, and it became completely dependent on its endosymbiont, which in turn has lost so many genes it can no longer survive outside its host cell. The nuclear genes of ''P. chromatophora'' (those regions not modified by the symbiont) are most closely related to the

heterotrophic A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

''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.

References

{{Taxonbar, from=Q3815918 Cercozoa genera Imbricatea