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An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis (Greek: endo = inner and biosis = living). For instance, some nitrogen fixing bacteria (e.g. in Rhizobium, Sinorhizobium and Bradyrhizobium ) live in plants, reef-building corals contain single-celled algae, and several insect species contain bacterial endosymbionts. Many other examples of endosymbiosis exist.

Many instances of endosymbiosis are obligate, where neither the endosymbiont nor the host can survive without the other. One example is represented by gutless marine worms which can not survive without their endosymbiotic bacteria. However, not all endosymbioses are obligate. Also, some endosymbioses can be harmful to either of the organisms involved. See symbiosis for further discussion of this issue.

It is generally agreed that certain organelles of the eukaryotic cell, especially mitochondria and chloroplasts, originated as bacterial endosymbionts. This theory is known as the endosymbiotic theory, confirmed and popularized by Lynn Margulis.


Bacterial obligate endosymbionts in marine oligochaetes

Endosymbiosis is a term which is used for both intra- and extracellular symbionts. In marine oligochaetes (e.g Olavius or Inanidrillus) the endosymbionts are extracellular and they fill the entire body of their host. These marine worms are nutritionally dependent on their symbiotic chemoautotrophic bacteria lacking any digestive or excretory system (no gut, mouth or nephridia).

Bacterial endosymbionts in other marine invertebrates

Extracellular endosymbionts are also represented in all 5 extant classes of Echinodermata (Crinoidea, Ophiuroidea, Asteroidea, Echinoidea, and Holothuroidea). Little is known of the nature of the association (mode of infection, transmission, metabolic requirements, etc.) but phylogenetic analysis indicates that these symbionts belong to the alpha group of the class Proteobacteria, relating them to Rhizobium and Thiobacillus. Other studies indicate that these subcuticular bacteria may be both abundant within their hosts and widely distributed among the Echinoderms in general.

Symbiodinium dinoflagellate endosymbionts in marine metazoa and protists

Dinoflagellate endosymbionts of the genus Symbiodinium, commonly known as zooxanthellae, are found in corals, molluscs (esp. giant clams , the Tridacna), and foraminifera. These endosymbionts drive the amazing formation of coral reefs by capturing sunlight and providing their hosts with energy for carbonate deposition.

Previously thought to be a single species, molecular phylogenetic evidence over the past couple decades has shown there to be great diversity in Symbiodinium. In some cases there is specificity between host and Symbiodinium clade. More often, however, there is a ecological distribution of Symbiodinium, the symbionts switching between hosts with apparent ease. When reefs become environmentally stressed, this distribution of symbionts is related to the observed pattern of coral bleaching and recovery. Thus the distribution of Symbiodinium on coral reefs and its role in coral bleaching presents one of the most complex and interesting current problems in reef ecology.

Bacterial obligate endosymbionts in insects

Among bacterial endosymbionts of insects, the best studied are the pea aphid Acyrthosiphon pisum and its endosymbiont Buchnera sp. APS, and the tsetse fly Glossina morsitans morsitans and its endosymbiont Wigglesworthia glossinidia brevipalpis . As with endosymbiosis in other insects, the symbiosis is obligate in that neither the bacteria nor the insect is viable without the other. Scientists have been unable to cultivate the bacteria in lab conditions outside of the insect. With special nutritionally-enhanced diets, the insects can survive, but are unhealthy, and at best survive only a few generations.

The endosymbionts live in specialized insect cells called bacteriocytes (also called mycetocytes), and are maternally-transmitted, i.e. the mother transmits her endosymbionts to her offspring. In some cases, the bacteria are transmitted in the egg, as in Buchnera; in others like Wigglesworthia, they are transmitted via milk to the developing insect embryo.

The bacteria are thought to help the host by either synthesizing nutrients that the host cannot make itself, or by metabolizing insect waste products into safer forms. For example, the primary role of Buchnera is thought to be to synthesize essential amino acids that the aphid cannot acquire from its natural diet of plant sap. The evidence is (1) when aphids' endosymbionts are killed using antibiotics, they appear healthier when their plant sap diet is supplemented with the appropriate amino acids, and (2) after the Buchnera genome was sequenced, analysis uncovered a large number of genes that likely code for amino acid biosynthesis genes; most bacteria that live inside other organisms do not have such genes, so their existence in Buchnera is noteworthy. Similarly, the primary role of Wigglesworthia is probably to synthesize vitamins that the tsetse fly does not get from the blood that it eats.

The benefit for the bacteria is that it is protected from the environment outside the insect cell, and presumably receives nutrients from the insect. Genome sequencing reveals that obligate bacterial endosymbionts of insects have among the smallest of known bacterial genomes and have lost many genes that are commonly found in other bacteria. Presumably these genes are not needed in the environment of the host insect cell. (A complementary theory as to why the bacteria may have lost genes, Muller's ratchet, is that since the endosymbionts are maternally transmitted and have no opportunity to exchange genes with other bacteria, it is more difficult to keep good genes in all individuals in a population of these endosymbionts.) Research in which a parallel phylogeny of bacteria and insects was inferred supports the belief that the obligate endosymbionts are transferred only vertically (i.e. from the mother), and not horizontally (i.e. by escaping the host and entering a new host).

Attacking obligate bacterial endosymbionts may present a way to control their insect hosts, many of which are pests or carriers of human disease. For example aphids are crop pests and the tsetse fly carries the organism (trypanosome protozoa) that causes African sleeping sickness. Other motivations for their study is to understand symbiosis, and to understand how bacteria with severely depleted genomes are able to survive, thus improving our knowledge of genetics and molecular biology.


Obligate bacterial endosymbiosis in marine oligochaetes:

  • Endosymbiotic sulphate-reducing and sulphide-oxidizing bacteria in an oligochaete worm. Dubilier N., Mülders C.,Ferdelman T., De Beer D.,Pernthaler A.,Klein M., Wagner M., Erseus C., Thiermann F., Krieger J., Giere O & Amann R.

Bacterial endosymbionts in echinoderms:

  • Subcuticular bacteria from the brittle star Ophiactis balli (Echinodermata: Ophiuroidea) represent a new lineage of extracellular marine symbionts in the alpha subdivision of the class Proteobacteria. Burnett, W J and J D McKenzie

Symbiodinium dinoflagellate endosymbionts in marine metazoa and protists

Obligate bacterial endosymbionts in insects:

  • PLOS Biology Primer- Endosymbiosis: lessons in conflict resolution
  • A general review of bacterial endosymbionts in insects. P. Baumann, N. A. Moran and L. Baumann, Bacteriocyte-associated endosymbionts of insects in M. Dworkin, ed., The prokaryotes, Springer, New York, 2000.
  • An excellent review of insect endosymbionts that focuses on genetic issues. Jennifer J. Wernegreen (2002), Genome evolution in bacterial endosymbionts of insects, Nature Reviews Genetics, 3, pp. 850-861.
  • A review article on aphids and their bacterial endosymbionts. A. E. Douglas (1998), Nutritional interactions in insect-microbial symbioses: Aphids and Their Symbiotic Bacteria Buchnera, Annual Reviews of Entomology, 43, pp. 17-37.
  • Describes possible methods to control the human pathogen causing African sleeping sickness, which is transmitted by tsetse flies. Focuses on methods using the primary and secondary endosymbionts of the tsetse fly. Serap Aksoy, Ian Maudlin, Colin Dale, Alan S. Robinsonand and Scott L. O’Neill (2001), Prospects for control of African trypanosomiasis by tsetse vector, TRENDS in Parasitology, 17 (1), pp. 29-35.
  • Announces and analyzes the full genome sequence of Buchnera sp. APS, the endosymbiont of the pea aphid, and the first endosymbiont to have its genome sequenced. S. Shigenobu, H. Watanabe, M. Hattori, Y. Sakaki and H. Ishikawa (2000), Genome sequence of the endocellular bacterial symbiont of aphids Buchnera sp. APS, Nature, 407, pp. 81-86.
  • An article that discusses one theory on how obligate endosymbionts may have their genomes degraded, in a freely-available journal. Nancy A. Moran (1996), Accelerated evolution and Muller’s ratchet in endosymbiotic bacteria, Proceedings of the National Academy of Sciences of the USA, 93, pp. 2873-2878.
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