Wolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects (~60% of species).
It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere. 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.
The bacterium was first identified in 1924 by Marshall Hertig and S. Burt Wolbach in Culex pipiens, 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 at Los Angeles discovered that Culex mosquito eggs were killed by a cytoplasmic incompatibility (see below) when the sperm of Wolbachia-infected males fertilized infection-free eggs. In 1990, Richard Stouthamer of the University of California at 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 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. Male embryos (in the case of cytoplasmic inherited bacteria) or male larvae (in the case of Microsporidia) are killed. In the case of embryo death, this diverts investment from males to females who can transmit these cytoplasmic elements (for instance, in ladybird beetles, infected female hosts eat their dead male brothers, which is positive from the viewpoint of the bacterium). In the case of microsporidia-induced larval death, the agent is transmitted out of the male lineage (through which it cannot be transmitted) into the environment, where it may be taken up again infectiously by other individuals. Male-killing occurs in many insects. In the case of male embryo death, a variety of bacteria have been implicated, including Wolbachia.
- Feminization: infected males develop as females or infertile pseudo-females. Male organisms are converted into females by cytoplasmic inherited protists (Microsporidia) or bacteria (Wolbachia), regardless of nuclear sex-determining factors. This occurs in amphipod and isopod Crustacea and Lepidoptera.
- Parthenogenesis: 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 Trichogramma 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. In certain haplodiploid Hymenoptera and mites, in which males are produced asexually, Wolbachia and Cardinium can induce duplication of the chromosomes and thus convert the organisms into females. The cytoplasmic bacterium forces haploid cells to go through mitosis to produce diploid cells which therefore will be female. This produces an entirely female population. Interestingly, if antibiotics are administered to populations which have become asexual in this way, they revert back to sexuality instantly, as the cytoplasmic bacteria forcing this behaviour upon them is removed.
- Cytoplasmic incompatibility: the inability of Wolbachia-infected males to successfully reproduce with uninfected females or females infected with another Wolbachia strain. In many arthropods, zygotes produced by sperm of infected males and ova of non-infected females can be killed by Wolbachia or Cardinium. Cytoplasmic Incompatibility (CI) 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. As the reproductive incompatibility is caused by bacteria that reside in the cytoplasm of the host cells, it is referred to as cytoplasmic incompatibility. In 1971, Janice Yen and A. Ralph Barr of UCLA demonstrated the etiologic relationship of Wolbacchia infection and cytoplasmic incompatibility in Culex mosquitos when they found that eggs were killed when the sperm of Wolbachia-infected males fertilized infection-free eggs.
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.
Applications to Human Health
Outside of insects, Wolbachia infects a variety of isopod species, spiders, mites, and many species of filarial nematodes (a type of parasitic worm), including those causing onchocerciasis ("River Blindness") and elephantiasis in humans as well as heartworms 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. 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.
Topics of Interest
Intragenomic conflict: The selfish gene theory postulates that natural selection will increase the frequency of those genes whose phenotypic effects ensure their successful replication. Generally, a gene achieves this goal by building, in cooperation with other genes, an organism capable of transmitting the gene to descendants. Intragenomic conflict arises when genes inside a genome are not transmitted by the same rules, or when a gene causes its own transmission to the detriment of the rest of the genome (this last kind of gene is usually called selfish genetic element, or ultraselfish gene or parasitic DNA).
This article deals with conflict between nuclear and cytoplasmic genes. Mitochondria represent one such example of a set of cytoplasmic genes, as do plasmids and bacteria which have integrated themselves into another species' cytoplasm.
Infection in heartworms: Wolbachia pipientis is an intracellular bacterium that is an endosymbiont of Dirofilaria immitis. It is thought that all heartworms are infected with Wolbachia to some degree. Research indicates that the inflammation that occurs at the die-off of adult heartworms or larvae is in part due to the release of Wolbachia bacteria or protein into the tissues. This may be particularly significant in cats, in which disease seems to be more related to larval death than living adult heartworms (see below). Pre-treating heartworm positive animals with an antibiotic such as doxycycline to remove Wolbachia may prove to be beneficial, but further studies are necessary.
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