Bacteriocyte

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Bacteriocyte
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Post-larval Bathymodiolus azoricus gill filament with prominent bacteriocyte
Anatomical terms of microanatomy

A bacteriocyte (Greek for bacteria cell), also known as a mycetocyte, is a specialized adipocyte found primarily in certain insect groups such as aphids, tsetse flies, German cockroaches, weevils. 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.

Contents

Endosymbiosis with microorganisms is common in insects. More than 10% of insect species rely upon intracellular bacteria for their development and survival. [1] Endosymbionts and their relationships with their hosts are diverse both functionally and genetically. However, the host cell in which bacterial and fungal endosymbionts reside is mostly unknown.

Location

Bacteriocyte location varies depending on the insect and endosymbiont type. These cells often inhabit fat bodies inside the midgut epithelium. The proximity to the insects' digestive system facilitates the absorption of bacteriocyte-produced nutrients. However, fungal-infected bacteriocytes and some bacteria-infected bacteriocytes can sometimes populate the hemocoel, a blood-containing cavity between the organs of most arthropods. [2]

Development

Transmission of endosymbionts

Vertical transmission of endosymbionts from the maternal bacteriocytes is thought to occur in the blastula stage of development. Blastula (PSF).jpg
Vertical transmission of endosymbionts from the maternal bacteriocytes is thought to occur in the blastula stage of development.

Bacteriocyte microorganism transfer occurs through vertical transmission from mother to offspring. Horizontal transmission or infection does not typically appear because insects with bacteriocytes depend so significantly on their symbiotic relationships to survive. Hosts without bacteriocytes do not usually survive and reproduce into adulthood. In some cases, the bacteria and fungi are transmitted in the egg, as in Buchnera ; [3] in others, like Wigglesworthia , they are transmitted via a milky substance that is fed to the developing insect embryo. Suppression of the Ultrabithorax gene in embryos led to the disappearance of bacteriocytes in Nysius plebius, while manipulation of the Antennapedia gene impacted the formation of bacteriomes but did not halt the formation of bacteriocytes altogether. [4]

Although vertical transmission of the symbionts is pivotal, the underlying and cellular mechanisms of this process are relatively unknown. However, there are several existing hypotheses. One theory is that the microorganisms circulating in the hemolymph of the mother migrate to a posterior region of the offspring blastula containing enlarged follicle cells. Other studies suggest that symbionts are directly transferred from the maternal bacteriocyte to the follicular region of the blastula through exocytic and endocytic transport. A newer hypothesis suggests that a membranous conduit forms between the maternal bacteriocyte and blastula which acts as a bridge for symbionts. Additionally, some studies show that the recognition of stem cell niches and association with dynein, kinesin, and microtubules are crucial for transmission from the parent to the offspring germline as well as segregation to host daughter cells. [5]

Growth

Bacteriocyte tissue grows considerably during nymphal and larval development as it organizes into two regular clusters near the gut and developing embryonic chains. As some insects grow older, such as aphids, they begin to exhibit disorganized architecture in the bacteriocyte tissue. Eventually, this trend leads to progressive dis-aggregation of the tissue caused by an increasing lack of intercellular adhesion of the cells that only increases as the insect ages. Dis-aggregation appears prominently in reproductively active as well as senescent adults. Some bacteriocyte nuclei, like those in aphids, also follow this pattern of development. They are initially round and centrally positioned but progressively become more deformed and move to the periphery of the cell. [6]

Death

Aphids are closely evolutionarily linked to their endosymbionts and depend on them for survival throughout their entire lifetime, resulting in a distinct form of cell death. Aphid Vectors.jpg
Aphids are closely evolutionarily linked to their endosymbionts and depend on them for survival throughout their entire lifetime, resulting in a distinct form of cell death.

Bacteriocytes can undergo a controlled form of cell death distinct from apoptosis. Elimination of bacteriocytes usually begins when the insect reaches reproductive maturity. Bacteriocyte degeneration begins with cytoplasmic hypervacuolation, meaning an excess of organelles called vacuoles form in the cytoplasm and then progressively expand throughout the entire cell. These vacuoles, which originate from the endoplasmic reticulum, also contain large acidic compartments are thought to aid in cellular degeneration. Hypervacuolation is a common feature in cells that undergo an autophagic, or "self-eating" death. Bacteriocytes, however, do not experience an autophagic death based on the lack of digested cellular components in the vacuoles. Bacteriocytes do develop some organelles to break down cellular components, called autophagosomes, but research suggests that their development is a stress response to adverse cellular conditions caused by the acidic hypervacuolation and not a contributor to cell death. This form of bacteriocyte death is also nonapoptotic, based on the irregular shape of the adult nucleus as well as a lack of chromatin condensation during degeneration and other characteristic features. Genetic testing also reveals a significant inhibition of the apoptotic pathway. Some other cell death characteristics found in bacteriocytes include acid-induced mitochondrial dysfunction, high levels of reactive oxygen species, and In the late phase of cell death, the digestion of endosymbionts by lysosomes. [6]

Other bacteriocytes, like those found in weevils, undergo a different form of cell death. Unlike aphids, weevils lose their bacteriocytes in adulthood. In these species, both apoptotic and autophagic mechanisms quickly eliminate bacteriomes associated with the gut. This form of cell death is more common in insects with a smaller dependence on their endosymbionts. Aphids, on the other hand, are closely evolutionarily tied to bacterial endosymbiosis resulting in a more complicated form of cell death. [6]

Function

Nutrition

The main function of bacteriocytes is to indirectly provide nutrients to the insect through the use of symbionts. The microorganisms housed in these specialized cells produce essential nutrients for their hosts in exchange for an enclosed environment to live. The health of these endosymbionts is crucial for the host's biology as their presence changes the balance of amino acid metabolism and mitochondrial phosphorylation. Both of these processes are essential for insect flight ability and performance. Insects housing symbionts develop best when fed on a diet with a lower protein-to-carbohydrate ratio than other insects because the symbionts already add considerable amounts of amino acid and nitrogen nutrition to the host. [2] Because of this nutritional imbalance, bacteriocytes are more prevalent in insects that utilize diets consisting of an excess of one compound while lacking some nutrients, like amino acids and proteins. [1]

Other functions

While some endosymbionts directly provide their hosts with food, others secrete enzymes to help aid in digestion of materials the insect cannot break down itself such as wood. [7] Additionally, some bacteriocyte endosymbionts serve an immune function are known to prime the immune system especially against trypanosomes. [8]

Examples

Symbionts (Buchnera aphidicola) within a bacteriocyte of a pea aphid (Acyrthosiphon pisum). The central object is the host nucleus; Buchnera cells are round and packed into the cytoplasm. Journal.pbio.0050126.g001.png
Symbionts (Buchnera aphidicola) within a bacteriocyte of a pea aphid (Acyrthosiphon pisum). The central object is the host nucleus; Buchnera cells are round and packed into the cytoplasm.

Aphids

Development of aphid ( Acyrthosiphon pisum ) bacteriocytes has been studied with the host cells that contain the endosymbiotic bacteria, Buchnera aphidicola. Bacteriocytes of aphids have a subpopulation of the bacteriocytes that is chosen prior to the maternal transmission of the bacteria to the embryo. Even later in the aphid's life, a second population of adipose cells are selected to become bacteriocytes. Bacteriocyte development has been maintained in aphids for 80–150 million years. [10]

Tsetse flies

The Tsetse fly's most prominent and vital endosymbiont is the bacterium, Wigglesworthia glossinidia, These bacteria are housed in the fly's bacteriocytes and produce B vitamins (B1, B6, and B9). The Tsetse fly itself lacks the ability to obtain these nutrients because of its hematophagous diet. W. glossinidia population in the maternal milk glands also help prime the immune system during the larval stage. Tsetse flies with bacteriocytes containing W. glossinidia are less susceptible to trypanosome infection later in life. [8]

Related Research Articles

<span class="mw-page-title-main">Endosymbiont</span> Organism that lives within the body or cells of another organism

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", σύν syn "together" and βίωσις biosis "living".) 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.

<span class="mw-page-title-main">Aphid</span> Superfamily of insects

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.

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.

<i>Buchnera aphidicola</i> Species of bacterium

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.

Wigglesworthia glossinidia is a species of gram-negative bacteria which was isolated from the gut of the tsetse fly. W. glossinidia is a bacterial endosymbiont of the tsetse fly. Because of this relationship, Wigglesworthia has lost a large part of its genome and has one of the smallest known 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. Wigglesworthia also synthesizes key B-complex vitamins which the tsetse fly does not get from its diet of blood. Without the vitamins Wigglesworthia produces, the tsetse fly has greatly reduced growth and reproduction. Since the tsetse fly is the primary vector of Trypanosoma brucei, the pathogen that causes African trypanosomiasis, it has been suggested that W. glossinidia may one day be used to help control the spread of this disease.

<i>Acyrthosiphon pisum</i> Species of true bug

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.

<span class="mw-page-title-main">Trophosome</span> Organ containing endosymbionts

A trophosome is a highly vascularised organ found in some animals that houses symbiotic bacteria that provide food for their host. Trophosomes are contained by the coelom of the vestimentiferan tube worms and in the body of symbiotic flatworms of the genus Paracatenula.

The hologenome theory of evolution recasts the individual animal or plant as a community or a "holobiont" – the host plus all of its symbiotic microbes. Consequently, the collective genomes of the holobiont form a "hologenome". Holobionts and hologenomes are structural entities that replace misnomers in the context of host-microbiota symbioses such as superorganism, organ, and metagenome. Variation in the hologenome may encode phenotypic plasticity of the holobiont and can be subject to evolutionary changes caused by selection and drift, if portions of the hologenome are transmitted between generations with reasonable fidelity. One of the important outcomes of recasting the individual as a holobiont subject to evolutionary forces is that genetic variation in the hologenome can be brought about by changes in the host genome and also by changes in the microbiome, including new acquisitions of microbes, horizontal gene transfers, and changes in microbial abundance within hosts. Although there is a rich literature on binary host–microbe symbioses, the hologenome concept distinguishes itself by including the vast symbiotic complexity inherent in many multicellular hosts. For recent literature on holobionts and hologenomes published in an open access platform, see the following reference.

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. By one early investigation, the minimal genome of a bacterium should include a virtually complete set of proteins for replication and translation, a transcription apparatus including four subunits of RNA polymerase including the sigma factor rudimentary proteins sufficient for recombination and repair, several chaperone proteins, the capacity for anaerobic metabolism through glycolysis and substrate-level phosphorylation, transamination of glutamyl-tRNA to glutaminyl-tRNA, lipid biosynthesis, eight cofactor enzymes, protein export machinery, and a limited metabolite transport network including membrane ATPases. Proteins involved in the minimum bacterial genome tend to be substantially more related to proteins found in archaea and eukaryotes compared to the average gene in the bacterial genome more generally indicating a substantial number of universally conserved proteins. The minimal genomes reconstructed on the basis of existing genes does not preclude simpler systems in more primitive cells, such as an RNA world genome which does not have the need for DNA replication machinery, which is otherwise part of the minimal genome of current cells.

Hamiltonella defensa is a species of bacteria. It is maternally or sexually transmitted and lives as an endosymbiont of whiteflies and aphids, meaning that it lives within a host, protecting its host from attack. It does this through bypassing the host's immune responses by protecting its host against parasitoid wasps. However, H. defensa is only defensive if infected by a virus. H. defensa shows a relationship with Photorhabdus species, together with Regiella insecticola. Together with other endosymbionts, it provides aphids protection against parasitoids. It is known to habitate Bemisia tabaci.

<i>Angomonas deanei</i> Species of parasitic flagellate protist in the Kinetoplastea class

Angomonas deanei is a flagellated trypanosomatid protozoan. As an obligate parasite, it infects the gastrointestinal tract of insects, and is in turn a host to symbiotic bacteria. The bacterial endosymbiont Ca. "Kinetoplastibacterium crithidii" maintains a permanent mutualistic relationship with the protozoan such that it is no longer able to reproduce and survive on its own. The symbiosis, subsequently also discovered in varying degrees in other protists such as Strigomonas culicis, Novymonas esmeraldas, Diplonema japonicumand Diplonema aggregatum are considered as good models for the understanding of the evolution of eukaryotes from prokaryotes, and on the origin of cell organelles.

Arsenophonus nasoniae is a species of bacterium which was previously isolated from Nasonia vitripennis, a species of parasitoid wasp. These wasps are generalists which afflict the larvae of parasitic carrion flies such as blowflies, houseflies and flesh flies. A. nasoniae belongs to the phylum Pseudomonadota and family Morganellaceae. The genus Arsenophonus, has a close relationship to the Proteus (bacterium) rather than to that of Salmonella and Escherichia. The genus is composed of gammaproteobacterial, secondary-endosymbionts which are gram-negative. Cells are non-flagellated, non-motile, non-spore forming and form long to highly filamentous rods. Cellular division is exhibited through septation. The name 'Arsenophonus nasoniae gen. nov., sp. nov.' was therefore proposed for the discovered bacterium due to its characteristics and its microbial interaction with N. vitripennis. The type strain of A. nasoniae is Strain SKI4.

Sodalis is a genus of bacteria within the family Pectobacteriaceae. This genus contains several insect endosymbionts and also a free-living group. It is studied due to its potential use in the biological control of the tsetse fly. Sodalis is an important model for evolutionary biologists because of its nascent endosymbiosis with insects.

Nasuia deltocephalinicola was reported in 2013 to have the smallest genome of all bacteria, with 112,091 nucleotides. For comparison, the human genome has 3.2 billion nucleotides. The second smallest genome, from bacteria Tremblaya princeps, has 139,000 nucleotides. While N. deltocephalinicola has the smallest number of nucleotides, it has more protein-coding genes (137) than some bacteria.

<i>Paracatenula</i> Genus of flatworms

Paracatenula is a genus of millimeter sized free-living marine gutless catenulid flatworms.

The initial acquisition of microbiota is the formation of an organism's microbiota immediately before and after birth. The microbiota are all the microorganisms including bacteria, archaea and fungi that colonize the organism. The microbiome is another term for microbiota or can refer to the collected genomes.

"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.

<span class="mw-page-title-main">Symbiosome</span>

A symbiosome is a specialised compartment in a host cell that houses an endosymbiont in a symbiotic relationship.

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.

<i>Strigomonas culicis</i> Species of parasitic flagellate protist in the Kinetoplastea class

Strigomonas culicis is a protist and member of flagellated trypanosomatids. It is an obligate parasite in the gastrointestinal tract of mosquito, and is in turn a host to symbiotic bacteria. It maintains strict mutualistic relationship with the bacteria as a sort of cell organelle (endosymbiont) so that it cannot lead an independent life without the bacteria. This and other symbiont-harbouring trypanosomatids such as Angomonas deanei are considered as "excellent models for the study of cell evolution because the host protozoan co-evolves with an intracellular bacterium in a mutualistic relationship", and "the origin of new organelles".

References

  1. 1 2 Baumann P, Moran NA, Baumann L, editors. (2000) Bacteriocyte-associated endosymbionts of insects. In: Dworkin M, editor. The prokaryotes [online]. New York: Springer. Available: http://link.springer.de/link/service/books/10125/ .
  2. 1 2 Thompson, S.N.; Simpson, S.J. (2009). "Nutrition". Encyclopedia of Insects (2 ed.). Amsterdam: Academic Press. pp. 715–720.
  3. ↑ Douglas, A E (1998). "Nutritional interactions in insect-microbial symbioses: Aphids and their symbiotic bacteria Buchnera". Annual Review of Entomology. 43: 17–38. doi:10.1146/annurev.ento.43.1.17. ISSN   0066-4170. PMID   15012383.
  4. ↑ Matsuura, Yu; Kikuchi, Yoshitomo; Miura, Toru; Fukatsu, Takema (July 28, 2015). "Ultrabithorax is essential for bacteriocyte development". Proceedings of the National Academy of Sciences of the United States of America. 112 (30): 9376–9381. Bibcode:2015PNAS..112.9376M. doi: 10.1073/pnas.1503371112 . JSTOR   26464197. PMC   4522796 . PMID   26170303.
  5. ↑ Koga; Meng; Tsuchida; Fukatsu (2012). "Cellular mechanism for selective vertical transmission of an obligate insect symbiont at the bacteriocyte–embryo interface". Proc Natl Acad Sci USA. 109 (20): 1230–1237. doi: 10.1073/pnas.1119212109 . PMC   3356617 . PMID   22517738.
  6. 1 2 3 Calevro, Federica; Callaerts, Patrick; Charles, Hubert; Heddi, Abdelaziz; Febvay, Gérard; Vulsteke, Veerle; Duport, Gabrielle; Buhler, Kurt; Parisot, Nicolas (2018-02-20). "Bacteriocyte cell death in the pea aphid/Buchnera symbiotic system". Proceedings of the National Academy of Sciences. 115 (8): E1819–E1828. doi: 10.1073/pnas.1720237115 . ISSN   0027-8424. PMC   5828623 . PMID   29432146.
  7. ↑ Brune, Andreas (2009). "Symbionts aiding digestion". Encyclopedia of Insects (2 ed.). Amsterdam: Academic Press. pp. 978–983.
  8. 1 2 Sloan, Megan; Ligoxygakis, Petros (2017). "Immunology of insect vectors: midgut interactions of sandflies and tsetse with kinetoplastid parasites as a paradigm for establishing infection". Advances in Insect Physiology. 52: 231–248. doi:10.1016/bs.aiip.2017.04.003.
  9. ↑ Hoff, Mary (2007-04-10). "When Bacteria Lose a Single DNA Base, Aphids Suffer". PLOS Biol. 5 (5): e126. doi:10.1371/journal.pbio.0050126. PMC   1847844 . PMID   20076671.
  10. ↑ Braendle, Christian; Miura, Toru; Bickel, Ryan; Shingleton, Alexander W; Kambhampati, Srinivas; Stern, David L (2003-10-13). "Developmental Origin and Evolution of Bacteriocytes in the Aphid–Buchnera Symbiosis". PLOS Biol. 1 (1): e21. doi:10.1371/journal.pbio.0000021. PMC   212699 . PMID   14551917.
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