Wolbachia is a genus of inherited bacteria which infects arthropod species, including a high proportion of insects. It is one of the world's most common parasitic microbes and is potentially 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 M. Hertig and S. B. Wolbach in Culex pipiens
, a species of mosquito
. Hertig formally described the genus 1936 as Wolbachia pipientis
. There was little interest after the discovery until 1971 when it was discovered that Culex
mosquito eggs were killed when the sperm of Wolbachia
infected males fertilized infection-free eggs (Cytoplasmic incompatibility
). It is today of considerable interest due to the nature of interactions and evolutionary consequences.
It was discovered that Wolbachia makes males dispensable in 1990 by Richard Stouthamer of the University of Califonia, Riverside.
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
of their hosts.
Wolbachia are known to cause four different phenotypes:
- Male killing: death of infected males.
- Feminization: infected males develop as females or infertile pseudo-females.
- 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 Pathogenic species would be the Trichogramma wasp.
- Cytoplasmic incompatibility: the inability of Wolbachia-infected males to successfully reproduce with uninfected females or females infected with another Wolbachia strain.
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. It is thought that the phenotypes caused by Wolbachia, especially cytoplasmic incompatibility, may be important in promoting speciation. Wolbachia can also cause misleading results in molecular cladistical analyses.
Horizontal gene transfer and genomics
The first Wolbachia
genome to be determined was that of one that infects Drosophila melanogaster
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
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.
Applications to human health
Outside of insects, Wolbachia
infects a variety of isopod
, 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 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 modified strains of Wolbachia to control mosquito populations has also been a topic of research.