Wolbachia pipientis are gram-negative bacteria that form intracellular inherited infections in many invertebrate hosts. They are extremely common with at least 20% of all insects being infected. Since insect species comprise ~85% of all animal species on the planet, Wolbachia pipientis are one of the most common bacterial endosymbionts in the biosphere and can be of major importance in ecological and evolutionary processes. Moreover they infect numerous non-insect invertebrates including filarial nematodes, terrestrial crustaceans, mites, and spiders. They are predominantly transmitted through females to developing eggs, but can also undergo some horizontal transmission between host species. The limits of the host range are not fully appreciated at this time. Much of the success of Wolbachia can be attributed to the diverse phenotypes that result from infection. These include classical mutualism in nematodes in which the bacteria are required for fertility and larval development; and reproductive parasitism in arthropods as characterized by the ability of Wolbachia to override chromosomal sex determination, induce parthenogenesis, selectively kill males, influence sperm competition and generate cytoplasmic incompatibility. Reproductive parasitism enhances the spread of Wolbachia through host arthropod populations by increasing the number of infected females, the transmitting sex of this bacterium. Wolbachia are present in mature eggs, but not mature sperm. It is thought that the phenotypes caused by Wolbachia, especially cytoplasmic incompatibility, may be important in promoting rapid speciation events in insects. The unique biology of Wolbachia has attracted a growing number of researchers and science educators interested in questions ranging from the evolutionary implications of infection to the use of this endosymbiont for human disease control and discovery-based projects in high school classrooms.
Unlike other symbionts that spread through host populations by enhancing the fitness of their host, Wolbachia can spread by reducing the fitness of their host. In arthropods, Wolbachia parasitize host reproductive strategies in four basic ways: male killing, feminization, parthenogenesis, and cytoplasmic incompatibility (CI). Because these bacteria are inherited through egg cytoplasm, they are selected to increase the number of infected females (i.e., the transmitting sex) in a population, even at the expense of males. Such examples illustrate the ongoing cytonuclear conflict over sex determination and sex ratios, which can in turn play an important role in rapid evolutionary changes.
Briefly, male killing occurs when infected male embryos die such as in the ladybird Adalia bipunctata, the butterfly Acraea encedon, and Drosophila bifasciata and D. innubila. This effect imparts a fitness advantage to infected female siblings, perhaps through reducing the fitness cost of competition with siblings. Feminization occurs when infected genetic males are converted to phenotypic females who are able to transmit the bacteria. Parthenogenesis induction (PI) typically occurs in haplodiploid wasps in which infected virgin females asexually produce all female offspring. Unlike male-killing, PI can give rise to all female populations that can persist. Finally, CI is the most common alteration and occurs in all the major insect orders as well as in mites and other arthropods. This effect is typically characterized by a sperm modification that leads to abnormalities in post-fertilization paternal chromosome behavior and, ultimately, embryonic mortality. It is typically expressed in crosses between an infected male and uninfected female, thereby reducing the fitness of uninfected females. The key point here is that Wolbachia are in various ways in the business hijacking host reproduction to enhance their own maternal spread.
Physiology and Cell Biology
Wolbachia pipientis have small circular chromosomes that contain approximately one to one and a half million base pairs (A,T,G,C) of DNA. The chromosomes of Wolbachia from Drosophila melanogaster flies (wMel) and Brugia malayi nematodes (wBm) have been completely sequenced, and several other genome sequencing projects are in progress. Like other intracellular bacteria, the DNA of Wolbachia is disproportionally composed of A and T base pairs. Approximately 65% of the base pairs are A and T. The total number of protein-coding genes is 1308 in wMel and 1218 in wBm. In contrast, the number of protein-coding genes in free-living (non-intracellular) bacteria typically ranges from 3500-4500. Major genetic differences between the two Wolbachia genomes are the presence of mobile genetic elements (bacteriophages, transposons) and ankyrin-repeat protein encoding genes in wMel that may assist Wolbachia's ability to hijack sexual reproduction in arthropods.
How to Grow
Wolbachia are obligate intracellular bacteria that naturally grow in the cytoplasm of host cells. As a result, Wolbachia can not be grown artificially on agar plates because they require a complex mix of nutrients from host cells. Scientists, educators, and students can culture Wolbachia in the lab using insects or insect cell lines infected with the bacteria. Wolbachia can be easily killed with an antibiotic treatment using tetracycline, doxcycline, and other antibiotics. High temperatures above 30C also kill Wolbachia.
Wolbachia pipientis are obligate intracellular bacteria that replicate exclusively inside host cells. Under no conditions have Wolbachia been cultured outside of the host animal environment. As a result, Wolbachia are naturally cultured by rearing infected insects in a laboratory or in vitro cultivated in cells of insect and mammalian cell lines (see video). Wolbachia can survive, but not replicate, outside the host cytosol in insect cell medium and within insect hemolymph.
Relevance to Humans and Ecosystems
Toxicity, Symptoms and Treatment
Outside of Insecta, Wolbachia pipientis infects a variety of other arthropods: terrestrial crustaceans (isopods or woodlice), and terrestrial arachnids (spiders and mites). Wolbachia infected arthropods display a wide range of effects, from apparent tolerance to extreme phenotypes that result from effective reproductive manipulation by the bacterial parasite. The reproductive manipulations include killing of genetically male embryos, induction of parthenogenesis, conversion of genetic females into functional and morphological females, and induction of reproductive incompatibility between infected and uninfected animals. These effects serve to promote the spread of the infection through host populations by assisting infected animals in having more offspring than uninfected ones. Carrying a Wolbachia infection is not without its costs, and infected animals are sometimes less successful when isolated than are uninfected (or antibiotic-cured) relatives.
Wolbachia have also been identified in many species of filarial nematodes (a type of parasitic roundworm), including those causing the human diseases onchocerciasis ("River Blindness") and elephantiasis, and heartworm disease of dogs. Not only are these disease-causing nematodes infected with Wolbachia, but Wolbachia seem to play an important role in these diseases. The filarial worms do not display the sorts of symptoms (outlined above) observed in infected arthropods. However, elimination of Wolbachia from filarial nematodes, using tetracycline-family antibiotics such as doxycycline, generally results in either death or sterility of the worm. This suggests that the Wolbachia supply some essential service to the worms, though the exact nature of the service is not yet known. Because of this, doxycycline treatment has been added to the set of drugs that are used to treat the approximately 150 million human infections with filarial nematodes.
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