Paulinella Multipora
   HOME

TheInfoList



OR:

''Paulinella'' is a genus of at least eleven species including both freshwater and marine
amoeboid An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and r ...
s. Like many members of euglyphids it is covered by rows of siliceous scales, and use filose
pseudopods A pseudopod or pseudopodium (: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filaments and ...
to crawl over the substrate of the
benthic zone The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...
. 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 ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteria' ...
as an
endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...
. As a result they are no longer able to perform
phagocytosis Phagocytosis () is the process by which a cell (biology), 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 ph ...
like their non-photosynthetic relatives. ''P. chromatophora'' was discovered in sediments of the river Rhine on Christmas Eve 1894 by German biologist
Robert Lauterborn Robert Lauterborn (23 October 1869 - 11 September 1952) was a German botanist, limnologist and protozoologist. Lauterborn was born in Ludwigshafen where his father was a publisher. His mother died when he was two and he was taken care of by his au ...
, who named it Paulinella after his stepmother Pauline. The event to permanent endosymbiosis probably occurred with a
cyanobiont Cyanobionts are cyanobacteria that live in symbiosis with a wide range of organisms such as terrestrial or aquatic plants; as well as, algal and fungal species. They can reside within extracellular or intracellular structures of the host. In ord ...
. The resulting
organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...
is a
photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...
plastid A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria. Examples of plastids include chloroplasts ...
that is often referred to as a 'cyanelle' or chromatophore, and it represents the only known primary endosymbiosis event of photosynthetic cyanobacteria (other than the origin of
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
), although primary endosymbiosis with a non-photosynthetic cyanobacterial symbiont have occurred in the
diatom A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...
family ''Rhopalodiaceae'' and the algae ''
Braarudosphaera bigelowii ''Braarudosphaera bigelowii'' is a coastal coccolithophore in the fossil record going back 100 million years to the Late Cretaceous. Coccolithophore The Family (biology), family Braarudosphaeraceae consist of single-celled coastal phytopla ...
''. The endosymbiotic event happened about 90–140 million years ago when an α-cyanobacterium (rather than a β-cyanobacterium which the plastids in
Archaeplastida The Archaeplastida (or kingdom Plantae '' sensu lato'' "in a broad sense"; pronounced ) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group glaucophytes. It als ...
originates from), who diverged about 500 million years ago from the ancestors of its sister clade that consist of the living members of the cyanobacteria genera ''
Prochlorococcus ''Prochlorococcus'' is a genus of very small (0.6  μm) marine cyanobacteria with an unusual pigmentation ( chlorophyll ''a2'' and ''b2''). These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosyn ...
'' and ''
Synechococcus ''Synechococcus'' (from the Greek ''synechos'', in succession, and the Greek ''kokkos'', granule) is a unicellular cyanobacterium that is very widespread in the marine environment. Its size varies from 0.8 to 1.5  μm. The photosynthetic ...
'', was permanently established within the amoeba. It is estimated the last common ancestor of extant photosynthetic species lived about 60 million years ago. This is striking because the
chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s of all other known photosynthetic
eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
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 ) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group glaucophytes. It als ...
n (and subsequently adopted into other eukaryote groups through
secondary endosymbiosis Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibly ...
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 flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...
'' 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 bacterium with a reduced genome, instead of a cyanobacterium. 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. It is estimated that 0.3-0.8% of Paulinella's genes were derived from its endosymbiont, in addition to a small amount of genes from other organisms. Other genes have degenerated due to
Muller's ratchet In evolutionary genetics, Muller's ratchet (named after Hermann Joseph Muller, by analogy with a ratchet effect) is a process which, in the absence of recombination (especially in an asexual population), results in an accumulation of irreversibl ...
– 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 organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
. 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), also transposon, or jumping gene, is a type of mobile genetic element, a nucleic acid sequence in DNA that can change its position within a genome. The discovery of mobile genetic elements earned Barbara McClinto ...
, 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 amoebae have evolved KR01 genes that enhance
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
transport to the chromatophores. ''Paulinella'' show both a very slow growth rate and sensitivity to light, divide every 6–7 days, and prefer low light conditions, probably as a protection against oxidative stress and other light related stress as it doesn't have the same degree of
photoprotection Photoprotection is the biochemical process that helps organisms cope with molecular damage caused by sunlight. Plants and other oxygenic phototrophs have developed a suite of photoprotective mechanisms to prevent photoinhibition and oxidative str ...
mechanisms found in organisms with a photosynthetic apparatus of Archaeplastid origin, which has a much longer evolutionary history. The nuclear genes of ''P. chromatophora'' (those regions not affected by endosymbiotic gene transfer) 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'' is a marine heterotrophic species of Paulinella that has been shown to eat cyanobacteria and bacteria. ''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. The presence of extant heterotrophic lineages makes ''Paulinella'' a valuable model for unravelling early stages of primary endosymbiosis event and studying the post symbiotic genome evolution of both the plastid and the host.


References

{{Taxonbar, from=Q3815918 Rhizaria genera Imbricatea Endosymbiotic events