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An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualistic relationship. Examples are nitrogen-fixing bacteria, which live in the root nodules of legumes, single-cell algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to insects.
Symbiosis is any type of a close and long-term biological interaction, between two organisms of different species. The two organisms, termed symbionts, can be either in a mutualistic, a commensalistic, or a parasitic relationship. In 1879, Heinrich Anton de Bary defined symbiosis as "the living together of unlike organisms".
Symbiogenesis 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 other organelles of eukaryotic cells are descended from formerly free-living prokaryotes taken one inside the other in endosymbiosis. Mitochondria appear to be phylogenetically related to Rickettsiales bacteria, while chloroplasts are thought to be related to cyanobacteria.
Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A typical life cycle involves flightless females giving live birth to female nymphs—who may also be already pregnant, an adaptation scientists call telescoping generations—without the involvement of males. Maturing rapidly, females breed profusely so that the number of these insects multiplies quickly. Winged females may develop later in the season, allowing the insects to colonize new plants. In temperate regions, a phase of sexual reproduction occurs in the autumn, with the insects often overwintering as eggs.
The froghoppers, or the superfamily Cercopoidea, are a group of hemipteran insects in the suborder Auchenorrhyncha. Adults are capable of jumping many times their height and length, giving the group their common name, but many species are best known for their plant-sucking nymphs which produce foam shelters, and are referred to as "spittlebugs".
Spiroplasma is a genus of Mollicutes, a group of small bacteria without cell walls. Spiroplasma shares the simple metabolism, parasitic lifestyle, fried-egg colony morphology and small genome of other Mollicutes, but has a distinctive helical morphology, unlike Mycoplasma. It has a spiral shape and moves in a corkscrew motion. Many Spiroplasma are found either in the gut or haemolymph of insects where they can act to manipulate host reproduction, or defend the host as endosymbionts. Spiroplasma are also disease-causing agents in the phloem of plants. Spiroplasmas are fastidious organisms, which require a rich culture medium. Typically they grow well at 30 °C, but not at 37 °C. A few species, notably Spiroplasma mirum, grow well at 37 °C, and cause cataracts and neurological damage in suckling mice. The best studied species of spiroplasmas are Spiroplasma poulsonii, a reproductive manipulator and defensive insect symbiont, Spiroplasma citri, the causative agent of citrus stubborn disease, and Spiroplasma kunkelii, the causative agent of corn stunt disease.
A bacteriome is a specialized organ, found mainly in some insects, that hosts endosymbiotic bacteria. Bacteriomes contain specialized cells, called bacteriocytes, that provide nutrients and shelter to the bacteria while protecting the host animal. In exchange, the bacteria provide essentials like vitamins and amino acids to the host insect. Bacteriomes also protect the bacteria from the host's immune system, with insects secreting antimicrobial peptides such as the coleoptericin secreted by weevils to keep bacteria within the bacteriome.
Genome size is the total amount of DNA contained within one copy of a single complete genome. It is typically measured in terms of mass in picograms or less frequently in daltons, or as the total number of nucleotide base pairs, usually in megabases. One picogram is equal to 978 megabases. In diploid organisms, genome size is often used interchangeably with the term C-value.
Buchnera aphidicola, a member of the Pseudomonadota and the only species in the genus Buchnera, is the primary endosymbiont of aphids, and has been studied in the pea aphid, Acyrthosiphon pisum. Buchnera is believed to have had a free-living, Gram-negative ancestor similar to a modern Enterobacterales, such as Escherichia coli. Buchnera is 3 μm in diameter and has some of the key characteristics of its Enterobacterales relatives, such as a Gram-negative cell wall. However, unlike most other Gram-negative bacteria, Buchnera lacks the genes to produce lipopolysaccharides for its outer membrane. The long association with aphids and the limitation of crossover events due to strictly vertical transmission has seen the deletion of genes required for anaerobic respiration, the synthesis of amino sugars, fatty acids, phospholipids, and complex carbohydrates. This has resulted not only in one of the smallest known genomes of any living organism, but also one of the most genetically stable.
A bacteriocyte, also known as a mycetocyte, is a specialized adipocyte found primarily in certain insects such as aphids, tsetse flies, German cockroaches, weevils, and ants. These cells contain endosymbiotic organisms such as bacteria and fungi, which provide essential amino acids and other chemicals to their host. Bacteriocytes may aggregate into a specialized organ called the bacteriome.
Wigglesworthia glossinidia is a species of gram-negative bacteria that is a bacterial endosymbiont of the tsetse fly. Because of this relationship, Wigglesworthia has lost a large part of its genome, leaving it with one of the smallest genomes of any living organism, consisting of a single chromosome of 700,000 bp and a plasmid of 5,200. Together with Buchnera aphidicola, Wigglesworthia has been the subject of genetic research into the minimal genome necessary for any living organism.
Symbiotic bacteria are bacteria living in symbiosis with another organism or each other. For example, rhizobia living in root nodules of legumes provide nitrogen fixing activity for these plants.
"Candidatus Carsonella ruddii" is an obligate endosymbiotic Gammaproteobacterium with one of the smallest genomes of any characterised bacteria.
Acyrthosiphon pisum, commonly known as the pea aphid, is a sap-sucking insect in the family Aphididae. It feeds on several species of legumes worldwide, including forage crops, such as pea, clover, alfalfa, and broad bean, and ranks among the aphid species of major agronomical importance. The pea aphid is a model organism for biological study whose genome has been sequenced and annotated.
The minimal genome is a concept which can be defined as the set of genes sufficient for life to exist and propagate under nutrient-rich and stress-free conditions. Alternatively, it can also be defined as the gene set supporting life on an axenic cell culture in rich media, and it is thought what makes up the minimal genome will depend on the environmental conditions that the organism inhabits.
Blochmannia is a genus of symbiotic bacteria found in carpenter ants and their allies in the tribe Camponotini. As of 2014, Blochmannia has been discovered in the guts of over 60 species across 6 genera within the Camponotini, and is predicted to be pervasive throughout the tribe. Blochmannia was first discovered by zoologist Friedrich Blochmann in 1887, who described "bacteria-like structures" in the ovaries and midgut of Camponotus ligniperdus in 1887. In 2000, Candidatus Blochmannia was proposed as its own genus.
"Candidatus Karelsulcia muelleri" is an aerobic, gram-negative, bacillus bacterium that is a part of the phylum Bacteroidota. "Ca. K. muelleri" is an obligate and mutualistic symbiotic microbe commonly found occupying specialized cell compartments of sap-feeding insects called bacteriocytes. A majority of the research done on "Ca. K. muelleri" has detailed its relationship with the host Homalodisca vitripennis. Other studies have documented the nature of its residency in other insects like the maize leafhopper (Cicadulina) or the spittlebug (Cercopoidea). "Ca. K. muelleri" is noted for its exceptionally minimal genome and it is currently identified as having the smallest known sequenced Bacteroidota genome at only 245 kilobases.
Microbial symbiosis in marine animals was not discovered until 1981. In the time following, symbiotic relationships between marine invertebrates and chemoautotrophic bacteria have been found in a variety of ecosystems, ranging from shallow coastal waters to deep-sea hydrothermal vents. Symbiosis is a way for marine organisms to find creative ways to survive in a very dynamic environment. They are different in relation to how dependent the organisms are on each other or how they are associated. It is also considered a selective force behind evolution in some scientific aspects. The symbiotic relationships of organisms has the ability to change behavior, morphology and metabolic pathways. With increased recognition and research, new terminology also arises, such as holobiont, which the relationship between a host and its symbionts as one grouping. Many scientists will look at the hologenome, which is the combined genetic information of the host and its symbionts. These terms are more commonly used to describe microbial symbionts.
Vertical transmission of symbionts is the transfer of a microbial symbiont from the parent directly to the offspring. Many metazoan species carry symbiotic bacteria which play a mutualistic, commensal, or parasitic role. A symbiont is acquired by a host via horizontal, vertical, or mixed transmission.
Reductive evolution is the process by which microorganisms remove genes from their genome. It can occur when bacteria found in a free-living state enter a restrictive state or are completely absorbed by another organism becoming intracellular (symbiogenesis). The bacteria will adapt to survive and thrive in the restrictive state by altering and reducing its genome to get rid of the newly redundant pathways that are provided by the host. In an endosymbiont or symbiogenesis relationship where both the guest and host benefit, the host can also undergo reductive evolution to eliminate pathways that are more efficiently provided for by the guest.
References
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- Kirchberger, Paul C.; Schmidt, Marian L.; Ochman, Howard (2020-09-08). "The Ingenuity of Bacterial Genomes". Annual Review of Microbiology . 74 (1). Annual Reviews: 815–834. doi:10.1146/annurev-micro-020518-115822. ISSN 0066-4227.
- Bennett, Gordon M.; Moran, Nancy A. (31 July 2013). "Small, smaller, smallest: the origins and evolution of ancient dual symbioses in a phloem-feeding insect". Genome Biology and Evolution. 5 (9). Oxford Journals: 1675–1688. doi:10.1093/gbe/evt118. PMC 3787670 . PMID 23918810. Archived from the original on 2 June 2014.
- ↑ Brown, Terrence A. (2002). "Genomes". National Center for Biotechnology Information. Oxford: Wiley-Liss.
- 1 2 Zimmer, Carl (23 August 2013). "And the Genomes Keep Shrinking..." National Geographic. National Geographic Magazine. Archived from the original on August 23, 2013.
- 1 2 3 Ishii, Yoshiko; Matsuura, Yu; Kakizawa, Shigeyuki; Nikoh, Naruo; Fukatsu, Takema (August 2013). "Diversity of Bacterial Endosybionts Associated with Macrosteles Leafhoppers Vectoring Phytopathogenic Phytoplasmas". Applied and Environmental Microbiology. 79 (16): 5013–5022. doi:10.1128/aem.01527-13. PMC 3754707 . PMID 23770905.
- 1 2 Noda, Hiroaki; Watanabe, Kenji; Kawai, Sawako; Yukohiro, Fumiko; Miyoshi, Takaharu (2012-06-04). "Bacteriome-associated endosymbionts of the green rice leafhopper Nephotettix cincticeps (Hemiptera: Cicadellidae)". Applied Entomology and Zoology. 47 (3): 217–225. doi:10.1007/s13355-012-0110-1. S2CID 18485847 . Retrieved 2021-03-15.
- ↑ Bennett, Gordon M.; Moran, Nancy A. (2013). "Small, Smaller, Smallest: The Origins and Evolution of Ancient Dual Symbioses in a Phloem-Feeding Insect". Genome Biology and Evolution. 5 (9): 1675–1688. doi: 10.1093/gbe/evt118 . ISSN 1759-6653. PMC 3787670 . PMID 23918810.