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Wolbachia is a type of parasitic bacteria that most commonly infects arthropods , the animal phylum that includes insects and arachnids, and nematodes, the animal phylum that includes parasitic worms known as roundworms. In arthropods, Wolbachia greatly impacts these animals’ reproduction and survives by its transmission from parent to offspring, usually through a female's eggs. In male embryonic hosts, Wolbachia fundamentally changes the biological makeup of the organisms, turning them into females that will mature to lay eggs and ultimately sustain Wolbachia's life indefinitely. Because of these abilities, Wolbachia is known to increase the reproductive capabilities of its female hosts.

Biology of Wolbachia

Wolbachia is a microscopic parasitic bacteria. It is unique among other bacteria in that it does not live on the exteriors of other organisms but rather penetrates and overtakes an organism's individual cells to begin living off of them. Wolbachia can then control the host cells that it inhabits; once inside a cell, the bacteria strain ensures its spread by attaching itself to the spindles that form between the existing cells and the cell copies that separate from them. Spindles are structures that distribute chromosomes , or DNA, in cells. In this way, Wolbachia infects all new cells created from the original host cell.

Because the primary purpose of such parasites is survival, Wolbachia typically travels to an organism's gonads, the reproductive centers of most living things. In female hosts, ensuring its own reproduction is a relatively simple task for Wolbachia, as it only has to secrete itself in the organism's egg cells to be passed on to offspring. The strain of bacteria has also developed a number of techniques for overcoming most obstacles it encounters in the process of perpetuating itself.

Cytoplasmic Incompatibility

Few options exist for Wolbachia when it infects a male host. Because of the bacteria's cytoplasmic incompatibility with a male's sperm, it cannot be passed on to offspring through the reproductive processes of males alone. This is manifested in the way in which Wolbachia-infected sperm merges with the healthy eggs of a female but ultimately prevents the eventual embryo from dividing and developing.

Thus, attaching itself to male sperm serves no biological purpose for Wolbachia unless the infected sperm joins to a female organism's Wolbachia-infected eggs to form infected offspring. For this reason, it is more beneficial for Wolbachia to invade the cells of female hosts, where it has more possibilities for survival.

Feminization

Infecting a female host's eggs is the most direct way for Wolbachia to set the conditions for its reproduction. Wolbachia cannot guarantee its survival in this way because the embryos that form from the female's eggs and the male's sperm could become male offspring. Due to cytoplasmic incompatibility, Wolbachia would have nowhere else to go and would ultimately die.

To circumvent this potential outcome, Wolbachia can induce the feminization of the embryos it has infected. During feminization, Wolbachia can biologically convert developing male embryos into females. In some embryonic hosts, it does this by attacking the organ that creates male sex hormones, thus ensuring that the embryo—by default—will develop into a female. In other kinds of hosts, particularly insects, Wolbachia can directly control the sex that the embryo will become, using methods not fully known to biologists.

Male Killing

Sometimes, alongside converting male embryos to females, Wolbachia kills developing males that could pose threats to their female siblings simply by receiving more care and resources from their parents. As Wolbachia has no use for male embryos and must ensure the survival of the females so the females can carry the bacteria inside of them, killing males is in Wolbachia's interest.

The strain of bacteria can only do this, however, only in organisms whose genetic makeup allows it. Insects, for example, like many other organisms, develop into either males or females depending on the amount of chromosomes they receive from their parents. In some insects, Wolbachia can safely manipulate these chromosome counts to turn males into females without causing the host any harm.

In other kinds of insects, however, the numbers of chromosomes are more closely linked to the sex the embryos will become. In these insect varieties, if embryos suddenly begin developing as sexes that don't match their determined number of chromosomes, they eventually die from receiving either too much or too little genetic information. Where it can do so, Wolbachia takes advantage of this trait by feminizing and killing male host embryos. Ideally, the parents of the offspring can then devote more attention to keeping the female embryos—and Wolbachia—alive.

Genetic Components

Despite Wolbachia's ability to accomplish these genetic manipulations, scientists found that Wolbachia is a largely unremarkable parasite in the overall biological world. After sequencing its genome—an organism's entire layout of DNA and other genetic information—scientists discovered that Wolbachia's genes contain a significant number of redundancies such as replicated viruses and other useless, superfluous elements. These conventional traits appeared to place Wolbachia in a class with other, similar parasitic bacteria, but scientists also identified several characteristics that set Wolbachia into a grouping of its own.

First, the unique proteins in Wolbachia's DNA were thought to function as the hormones that control host organisms. Scientists also believed other proteins on Wolbachia's surface served a similar purpose: allowing the parasite to establish connections to its host's biological systems. Last, some of Wolbachia's proteins are genetically structured so they can overtake the proteins of a host organism. This is likely what permits Wolbachia to hijack a host's cell-division system for reproduction.

Regarding this final genetic trait, scientists also noted a biological benefit of a host organism being infected with Wolbachia: After experimenting on Wolbachia-carrying fruit flies, scientists discovered that female flies hosting a specific strain of Wolbachia produced four times as many eggs as uninfected flies. Scientists concluded from the experiment that due to the bacteria's own need to survive, Wolbachia greatly increased the insects' abilities to reproduce. Consequently, in the cases of female hosts, some in the scientific community refer to Wolbachia as more of a mutualistic organism than a harmful parasite.

Bibliography

Timmer, John. "Meet Wolbachia: The Male-Killing, Gender-Bending, Gonad-Eating Bacteria." Ars Technica. Condé Nast. 24 Oct. 2011. Web. 30 Mar. 2015. http://arstechnica.com/science/2011/10/meet-wolbachia-the-male-killing-gender-bending-gonad-chomping-bacteria/

"What Is a Genome?" Genetics Home Reference. U.S. National Library of Medicine. 30 Mar. 2015. Web. 30 Mar. 2015. http://ghr.nlm.nih.gov/handbook/hgp/genome

"Wolbachia Biology." University of Rochester Department of Biology. University of Rochester. Web. 30 Mar. 2015. http://www.rochester.edu/college/bio/labs/WerrenLab/WerrenLab-WolbachiaBiology.html

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