Wolbachia
Encyclopedia
Wolbachia is a genus
of bacteria which infects arthropod
species, including a high proportion of insects (~60% of species), as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere
. Its interactions with its hosts are often complex, and in some cases have evolved to symbiotic rather than parasitic. One study concludes that more than 16% of neotropical insect species carry this bacterium and as many as 25-70% of all insect species are estimated to be potential hosts.
, a species of mosquito
. Hertig formally described the genus in 1936 as Wolbachia pipientis. There was little interest after the discovery until 1971 when Janice Yen and A. Ralph Barr of the University of California, Los Angeles
discovered that Culex mosquito eggs were killed by a cytoplasmic incompatibility
when the sperm of Wolbachia-infected males fertilized infection-free eggs.
In 1990, Richard Stouthamer of the University of California, Riverside
discovered that Wolbachia can make males dispensable in some species.
It is today of considerable interest due to its ubiquitous distribution and many different evolutionary interactions.
Wolbachia are known to cause four different phenotypes:
Several species are so dependent on Wolbachia that they are unable to reproduce effectively without the bacteria in their bodies.
Wolbachia are present in mature eggs, but not mature sperm. Only infected females pass the infection on to their offspring. One study on infected woodlice
showed that the broods of infected organisms had a higher proportion of females than their uninfected counterparts. It is thought that Wolbachia, especially Wolbachia-caused cytoplasmic incompatibility, may be important in promoting speciation. Wolbachia strains that distort the sex ratio may alter their host's pattern of sexual selection in nature, and also engender strong selection to prevent their action, leading to some of the fastest examples of natural selection in natural populations
and mosquito species. Flies infected with the bacteria are more resistant to RNA viruses such as Drosophila C Virus, Nora Virus, Flock House virus, cricket paralysis virus, Chikungunya virus, and West Nile Virus
In the common house mosquito, higher levels of Wolbachia density were correlated with more insecticide resistance. Another interesting interaction is seen in leaf-miners Phyllonorycter blancardella where Wolbachia help their host produce green islands on yellowing tree leaves which allows the host to continue feeding while growing to their adult form. Larvae treated with tetracycline, which kills Wolbachia, lose this ability and subsequently only 13% emerge successfully as adult moths. Lastly, in the nematode
species Brugia malayi, Wolbachia has become an obligate endosymbiont and provides the host with chemicals necessary to its survival.
flies. This genome was sequenced at The Institute for Genomic Research
in a collaboration between Jonathan Eisen
and Scott O'Neill. The second Wolbachia genome to be determined was one that infects Brugia malayi
nematodes. Genome sequencing projects for several other Wolbachia strains are in progress. A complete copy of the Wolbachia genome sequence was found within the genome sequence of the fruit fly Drosophila ananassae and large segments were found in 7 other Drosophila species.
In an application of DNA barcoding
to the identification of species of Protocalliphora
flies, it was found that several distinct morphospecies had identical cytochrome c oxidase I gene sequences, most likely through horizontal gene transfer
by Wolbachia species as they jump across host species.
As a result, Wolbachia can cause misleading results in molecular
cladistical
analyses.
Wolbachia has been found to confer Drosophila hosts with resistance against certain RNA virus infections.
Wolbachia also harbor a temperate bacteriophage
called WO. Comparative sequence analyses of bacteriophage WO offer some of the most compelling examples of large-scale horizontal gene transfer
between Wolbachia coinfections in the same host. It is the first bacteriophage implicated in frequent lateral transfer between the genomes of bacterial endosymbionts. Gene transfer by bacteriophages could drive significant evolutionary change in the genomes of intracellular bacteria that are typically considered highly stable and prone to genomic degradation.
s, mite
s, and many species of filarial nematode
s (a type of parasitic worm
), including those causing onchocerciasis
("River Blindness") and elephantiasis
in humans as well as heartworm
s in dogs. Not only are these disease-causing filarial worms infected with Wolbachia, but Wolbachia seem to play an inordinate role in these diseases. A large part of the pathogenicity of filarial nematodes is due to host immune response toward their Wolbachia. Elimination of Wolbachia from filarial nematodes generally results in either death or sterility of the nematode. Consequently, current strategies for control of filarial nematode diseases include elimination of Wolbachia via the simple doxycycline
antibiotic rather than far more toxic anti-nematode medications.
The use of the naturally existing strains of Wolbachia to control mosquito populations has also been a topic of research. Wolbachia can be used to control dengue and malaria by eliminating older insects that contain more parasites. Allowing younger insects to survive lessens selection pressure for evolution of resistance.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of bacteria which infects arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
species, including a high proportion of insects (~60% of species), as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...
. Its interactions with its hosts are often complex, and in some cases have evolved to symbiotic rather than parasitic. One study concludes that more than 16% of neotropical insect species carry this bacterium and as many as 25-70% of all insect species are estimated to be potential hosts.
History
The bacterium was first identified in 1924 by Marshall Hertig and S. Burt Wolbach in Culex pipiensCulex pipiens
Culex pipiens is a species of blood-feeding mosquito of the family Culicidae. It is a vector of some diseases, such as Japanese encephalitis, meningitis, Urticaria...
, a species of mosquito
Mosquito
Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...
. Hertig formally described the genus in 1936 as Wolbachia pipientis. There was little interest after the discovery until 1971 when Janice Yen and A. Ralph Barr of the University of California, Los Angeles
University of California, Los Angeles
The University of California, Los Angeles is a public research university located in the Westwood neighborhood of Los Angeles, California, USA. It was founded in 1919 as the "Southern Branch" of the University of California and is the second oldest of the ten campuses...
discovered that Culex mosquito eggs were killed by a cytoplasmic incompatibility
Cytoplasmic incompatibility
Cytoplasmic incompatibility is a phenomenon that results in sperm and eggs being unable to form viable offspring. The effect arises from changes in the gamete cells caused by intracellular parasites like Wolbachia, which infect a wide range of insect species...
when the sperm of Wolbachia-infected males fertilized infection-free eggs.
In 1990, Richard Stouthamer of the University of California, Riverside
University of California, Riverside
The University of California, Riverside, commonly known as UCR or UC Riverside, is a public research university and one of the ten general campuses of the University of California system. UCR is consistently ranked as one of the most ethnically and economically diverse universities in the United...
discovered that Wolbachia can make males dispensable in some species.
It is today of considerable interest due to its ubiquitous distribution and many different evolutionary interactions.
Role in sexual differentiation of hosts
Within arthropods, Wolbachia is notable for significantly altering the reproductive capabilities of its hosts. These bacteria can infect many different types of organs, but are most notable for the infections of the testes and ovaries of their hosts.Wolbachia are known to cause four different phenotypes:
- Male killing: males are killed during larval development, which increase the rate of born females.
- Feminization: infected males develop as females or infertile pseudo-females.
- ParthenogenesisParthenogenesisParthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
: reproduction of infected females without males. Some scientists have suggested that parthenogenesis may always be attributable to the effects of Wolbachia. An example of a parthenogenic species is the TrichogrammaTrichogrammaThe wasps of genus Trichogramma, commonly known as "stingless wasps", are some of the most widely-studied agents of biological control in the field of entomology. Trichogramma wasps are tiny Hymenopteran insects, measuring 1 millimeter in length or less, that parasitize the eggs of many types of...
wasp, which has evolved to procreate without males with the help of Wolbachia. Males are rare in this tiny species of insect, possibly because many have been killed by that very same strain of Wolbachia. - Cytoplasmic incompatibilityCytoplasmic incompatibilityCytoplasmic incompatibility is a phenomenon that results in sperm and eggs being unable to form viable offspring. The effect arises from changes in the gamete cells caused by intracellular parasites like Wolbachia, which infect a wide range of insect species...
: the inability of Wolbachia-infected males to successfully reproduce with uninfected females or females infected with another Wolbachia strainStrain (biology)In biology, a strain is a low-level taxonomic rank used in three related ways.-Microbiology and virology:A strain is a genetic variant or subtype of a micro-organism . For example, a "flu strain" is a certain biological form of the influenza or "flu" virus...
.
Several species are so dependent on Wolbachia that they are unable to reproduce effectively without the bacteria in their bodies.
Wolbachia are present in mature eggs, but not mature sperm. Only infected females pass the infection on to their offspring. One study on infected woodlice
Oniscus asellus
Oniscus asellus, the common woodlouse, is one of the largest and most common species of woodlouse in the British Isles and Western and Northern Europe, growing to lengths of 16 mm and widths of 6 mm.-Distribution:...
showed that the broods of infected organisms had a higher proportion of females than their uninfected counterparts. It is thought that Wolbachia, especially Wolbachia-caused cytoplasmic incompatibility, may be important in promoting speciation. Wolbachia strains that distort the sex ratio may alter their host's pattern of sexual selection in nature, and also engender strong selection to prevent their action, leading to some of the fastest examples of natural selection in natural populations
Wolbachia infections confer fitness advantages
Wolbachia has been linked to viral resistance in Drosophila melanogasterDrosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
and mosquito species. Flies infected with the bacteria are more resistant to RNA viruses such as Drosophila C Virus, Nora Virus, Flock House virus, cricket paralysis virus, Chikungunya virus, and West Nile Virus
West Nile virus
West Nile virus is a virus of the family Flaviviridae. Part of the Japanese encephalitis antigenic complex of viruses, it is found in both tropical and temperate regions. It mainly infects birds, but is known to infect humans, horses, dogs, cats, bats, chipmunks, skunks, squirrels, domestic...
In the common house mosquito, higher levels of Wolbachia density were correlated with more insecticide resistance. Another interesting interaction is seen in leaf-miners Phyllonorycter blancardella where Wolbachia help their host produce green islands on yellowing tree leaves which allows the host to continue feeding while growing to their adult form. Larvae treated with tetracycline, which kills Wolbachia, lose this ability and subsequently only 13% emerge successfully as adult moths. Lastly, in the nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
species Brugia malayi, Wolbachia has become an obligate endosymbiont and provides the host with chemicals necessary to its survival.
Horizontal gene transfer and genomics
The first Wolbachia genome to be determined was that of one that infects Drosophila melanogasterDrosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
flies. This genome was sequenced at The Institute for Genomic Research
The Institute for Genomic Research
The Institute for Genomic Research was a non-profit genomics research institute founded in 1992 by Craig Venter in Rockville, Maryland, United States. It is now a part of the J. Craig Venter Institute.-History:...
in a collaboration between Jonathan Eisen
Jonathan Eisen
Jonathan A. Eisen is an American evolutionary biologist, currently working at University of California, Davis. His academic research is in the fields of evolutionary biology, genomics and microbiology and he is the academic editor-in-chief of the open-access journal PLoS Biology.In 2011 Eisen was...
and Scott O'Neill. The second Wolbachia genome to be determined was one that infects Brugia malayi
Brugia malayi
Brugia malayi is a nematode , one of the three causative agents of lymphatic filariasis in humans. Lymphatic filariasis, also known as elephantiasis, is a condition characterized by swelling of the lower limbs. The two other filarial causes of lymphatic filariasis are Wuchereria bancrofti and...
nematodes. Genome sequencing projects for several other Wolbachia strains are in progress. A complete copy of the Wolbachia genome sequence was found within the genome sequence of the fruit fly Drosophila ananassae and large segments were found in 7 other Drosophila species.
In an application of DNA barcoding
DNA barcoding
DNA barcoding is a taxonomic method that uses a short genetic marker in an organism's DNA to identify it as belonging to a particular species. It differs from molecular phylogeny in that the main goal is not to determine classification but to identify an unknown sample in terms of a known...
to the identification of species of Protocalliphora
Protocalliphora
Protocalliphora or bird blowflies are a blow fly genus containing many species which are obligate parasites of birds. The larvae suck the blood of nestlings and are found in the nests of birds...
flies, it was found that several distinct morphospecies had identical cytochrome c oxidase I gene sequences, most likely through horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
by Wolbachia species as they jump across host species.
As a result, Wolbachia can cause misleading results in molecular
Molecular phylogeny
Molecular phylogenetics is the analysis of hereditary molecular differences, mainly in DNA sequences, to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree...
cladistical
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
analyses.
Wolbachia has been found to confer Drosophila hosts with resistance against certain RNA virus infections.
Wolbachia also harbor a temperate bacteriophage
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...
called WO. Comparative sequence analyses of bacteriophage WO offer some of the most compelling examples of large-scale horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
between Wolbachia coinfections in the same host. It is the first bacteriophage implicated in frequent lateral transfer between the genomes of bacterial endosymbionts. Gene transfer by bacteriophages could drive significant evolutionary change in the genomes of intracellular bacteria that are typically considered highly stable and prone to genomic degradation.
Applications to human health
Outside of insects, Wolbachia infects a variety of isopod species, spiderSpider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s, mite
Mite
Mites, along with ticks, are small arthropods belonging to the subclass Acari and the class Arachnida. The scientific discipline devoted to the study of ticks and mites is called acarology.-Diversity and systematics:...
s, and many species of filarial nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
s (a type of parasitic worm
Parasitic worm
Parasitic worms or helminths are a division of eukaryoticparasites that, unlike external parasites such as lice and fleas, live inside their host. They are worm-like organisms that live and feed off living hosts, receiving nourishment and protection while disrupting their hosts' nutrient...
), including those causing onchocerciasis
Onchocerciasis
Onchocerciasis , also known as river blindness and Robles' disease, is a parasitic disease caused by infection by Onchocerca volvulus, a nematode . Onchocerciasis is the world's second-leading infectious cause of blindness. It is not the nematode, but its endosymbiont, Wolbachia pipientis, that...
("River Blindness") and elephantiasis
Elephantiasis
Elephantiasis is a disease that is characterized by the thickening of the skin and underlying tissues, especially in the legs and male genitals. In some cases the disease can cause certain body parts, such as the scrotum, to swell to the size of a softball or basketball. It is caused by...
in humans as well as heartworm
Heartworm
Heartworm is a parasitic roundworm that is spread from host to host through the bites of mosquitoes. The heartworm is a type of filaria, a small thread-like worm. The definitive host is the dog but it can also infect cats, wolves, coyotes, foxes and other animals, such as ferrets, sea lions and...
s in dogs. Not only are these disease-causing filarial worms infected with Wolbachia, but Wolbachia seem to play an inordinate role in these diseases. A large part of the pathogenicity of filarial nematodes is due to host immune response toward their Wolbachia. Elimination of Wolbachia from filarial nematodes generally results in either death or sterility of the nematode. Consequently, current strategies for control of filarial nematode diseases include elimination of Wolbachia via the simple doxycycline
Doxycycline
Doxycycline INN is a member of the tetracycline antibiotics group, and is commonly used to treat a variety of infections. Doxycycline is a semisynthetic tetracycline invented and clinically developed in the early 1960s by Pfizer Inc. and marketed under the brand name Vibramycin. Vibramycin...
antibiotic rather than far more toxic anti-nematode medications.
The use of the naturally existing strains of Wolbachia to control mosquito populations has also been a topic of research. Wolbachia can be used to control dengue and malaria by eliminating older insects that contain more parasites. Allowing younger insects to survive lessens selection pressure for evolution of resistance.
External links
- Virtual Museum of Bacteria
- Wolbachia research portal National Science FoundationNational Science FoundationThe National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...
- Howard Hughes Medical Institute High School Lab Series