Wolbachia: The most abundant parasite in nature favors females and harms males

It is the most abundant reproductive parasite in nature. More than half of all insect and arthropod species are infected Wolbachiana parasite that Marshall Hertig and Samuel Wolbach, pathologists at Harvard University, first observed in 1924, in the ovary of the mosquito Culex pipiens.

Wolbachian it has inspired collections of science books and is a protagonist in many international conferences. Today it acts as a brake on a devastating disease, dengue. And all because of the strange way in which it is transmitted from generation to generation, damaging the reproductive success of the males (or directly destroying them) and favoring the reproduction of the females of the species it parasites.

It is transmitted through the ovules

Wolbachian can only live inside the cell. Therefore, it is transmitted to offspring through ova, but not through sperm, which lack a developed cytoplasm. And over millions of years of coevolution with its hosts, it has evolved extraordinary reproductive strategies, favoring female success (which translates into its own success) and male failure.

Wolbachia-infected insect ovarian tissue cell. ¬© 2004 Public Library of Science. Courtesy of Scott O’Neill, CC BY

The strategy of Wolbachian in favor of females

In some insect species, male embryos infected with Wolbachian they die, female embryos do not. The parasitic bacterium makes no concessions to males.

It has recently been described as Wolbachian inhibits a protein necessary for masculinization, which means the death of the male embryo in the process of its development.

But the strategy can be more sophisticated: not to kill male embryos, but to promote their feminization. infection from Wolbachian disrupts the endocrine system of some infected crustaceans, which causes the feminization of offspring that would otherwise be male according to their genetic endowment.

Wolbachian it can also “encourage” the multiplication of females without relying on males. One of the most striking consequences of the infection is that in some cases it induces parthenogenesis, i.e. the reproduction of infected females without the need for males, causing only females in the offspring.

Finally, the bacterium becomes a particular contraceptive system: it makes infected sperm not compatible with uninfected eggs. This cytoplasmic incompatibility is the most common strategy. If infected males try to breed with uninfected females, they will fail. In this way only infected females reproduce, spreading the bacterium in the population.

Currently, the cellular and molecular mechanisms of cytoplasmic incompatibility are beginning to be revealed. Infected sperm cells, when they enter the healthy egg, have difficulty reorganizing their nucleus, which is highly condensed. This prevents the initiation of embryonic development. On the contrary, factors are present in the infected egg which counteract this anomaly and allow the normal union of the male and female nuclei.

The four strategies developed by Wolbachia to increase their reproductive success by increasing the frequency of infected females in populations. author provided

The important role of Wolbachian

The ability to Wolbachian increasing their reproductive success explains their extraordinary expansion in the wild. However, the consideration of this bacterium as a parasite must not hide the fact that in many cases the relationship with the host can be considered a form of mutualism, i.e. advantageous for both parties.

Somehow little known, Wolbachian it can protect the host from other infections and increase its longevity and fecundity. This has potential biomedical importance. It can also synthesize nutrients needed by the host. Some heartworms (parasitic roundworms) can only survive or reproduce if infected. In a specific case (brugia malayiwhich causes lymphatic filariasis) it has been shown that the bacterium, but not its host, synthesizes the heme group, an essential cofactor for the functioning of many enzymes.

The cytoplasmic incompatibility that induces the infection of Wolbachian it does not only occur between infected males and uninfected females. It also occurs between males and females infected with different strains of the bacterium. This can lead to segmentation of insect populations, which favors the emergence of new species. It is conceivable that infection with Wolbachian is related to the diversification of these animals, although the extent to which this was relevant remains to be determined.

Ally against dengue and other diseases

Wolbachian protects its host from other pathogens. In some cases it reduces the replication of the virus, or shortens the life cycle of the host, limiting the viral multiplication. This converts to Wolbachian a potential ally to stop the spread of insect-borne diseases, mainly from mosquitoes. One strategy could be to release infected males, who will try unsuccessfully to breed with uninfected females, reducing the mosquito population. Or they release both infected males and females who spread the bacteria throughout the population, reducing the overall pathogen burden.

Significant progress is already being made in this area. In a recent study, the introduction of infected mosquitoes into Yogyakarta, Indonesia reduced the incidence of dengue by 77%.

Chagas disease, yellow fever, leishmaniasis, the Zika virus and even malaria are other potential targets of this biological warfare which uses Wolbachian as an ally.

Some heartworms depend on infection from Wolbachian to complete its life cycle. Heartworms are implicated in very serious tropical diseases, such as lymphatic filariasis or onchocerciasis. His addiction to Wolbachian raised the possibility of antibiotic treatments (doxycycline or rifampicin), which attack bacteria, complementary to antiparasitic treatments.

There has been progress in this direction, but the treatments are very time consuming, have contraindications and cannot be applied to children or to women who are pregnant or breastfeeding their babies. Therefore, it is essential to develop more effective antibiotics against Wolbachian to alleviate this terrible health problem, since heartworm disease affects more than 150 million people worldwide, mainly in countries with low economic resources.

And so, we are faced with an indisputably successful reproductive parasite, essential mutualist, endocrine manipulator, evolutionary engine and therapeutic target. All this, favoring women without question.

Ramon Munoz-Chapuli Oriol. Professor of Animal Biology (retired), University of Malaga
This article was originally published in The Conversation. Read the original.

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