Evolution has given some parasites incredible abilities that allow them to survive and reproduce by “subduing” their host. These include not only the Cordyceps fungus, which turns ants into zombies. Another striking example is the Wolbachia bacteria. They do not “zombify” insects, but they do create parasitic populations consisting exclusively of females. In a recent study, scientists were able to figure out exactly how they manage to get rid of males.
Scientists have discovered how bacteria change the sex of insects. Photo source: sputnik.by
Why bacteria get rid of male insects
Wolbachia were first discovered in mosquitoes back in 1924. Incidentally, the bacterium was named after Simeon Burt Wolbach, one of the scientists who discovered it. These microbes reproduce exclusively inside the cells of arthropods and nematodes. Moreover, they are quite common – according to some data, about 2% of all insect species are infected with them.
It has long been known that these bacteria destroy males in the population. Why do bacteria dislike them so much? The thing is that Wolbachia can only be transmitted in one way – from parent to offspring through female eggs. This means that an infected male cannot infect another insect with the bacteria. Accordingly, the microbes that end up inside the male are sooner or later forced to die as a whole colony.
Wolbachia bacteria, which infects up to 20% of insects. Photo source: labroots.com
Scientists first discovered that Wolbachia are capable of changing the ratio of males and females in populations about 30 years ago. However, only now have they managed to figure out exactly how this happens.
How bacteria change insect populations
Wolbachia does not use any one tool to get rid of males, but uses many different “tricks.” For example, they destroy male embryos and prevent the development of eggs. In addition, adult males of some species are forced to develop female characteristics, that is, to turn into females.
However, as a recent study published in Genome Biology has shown, one of the main ways the bacteria can create an all-female population is by manipulating the reproduction of their host, allowing them to produce only female offspring. So how does the colony then continue to exist? The bacteria, it turns out, have everything they need to ensure that the females themselves can reproduce. At least, that's the case with the wasps.
Bacteria changes the sex of an insect during its development in an egg. Photo source: firma-zarya.ru
It has long been known that wasps of different sexes have different numbers of chromosomes. Females contain two copies of chromosomes and develop from fertilized eggs. Males differ in that they have only one set of chromosomes, respectively, they have only one copy of each gene. But most importantly, they develop from unfertilized eggs.
As previous studies have shown, chromosomes are initially duplicated, but then split into separate cells. As a result, daughter cells receive only half of the chromosomes of the parent cell.
How bacteria change the sex of male wasps
Scientists have discovered that the bacterium changes the sex of males at a very early stage of development. Using a mechanism that scientists still cannot fully understand, Wolbachia prevents chromosome division. As a result, the embryo in the egg receives two copies of the chromosomes.
Infected wasp populations consist exclusively of females. Photo source: tver.aif.ru
In other words, instead of males, females emerge from the eggs, which then pass the bacteria on to their offspring. As a result, the population can do without males entirely. But how does the bacterium do this? Another study, published in Current Biology in April 2024, partially answers this question.
During this study, scientists discovered a gene within the DNA of Wolbachia that the bacterium most likely borrowed from insects. This gene encodes a protein responsible for the sex of the offspring. Thus, Wolbachia does not even manipulate the sex of its host, but rather takes part in the formation of the embryo.
The current study has confirmed these findings – similar genes responsible for sex determination have been found in other insects. This means that in the distant past, Wolbachia bacteria infected some insect and borrowed from it a “tool” for fighting males in the form of sex determination genes.
Scientists use bacteria to fight dangerous insects. Image source: nea.gov.sg
As we can see, parasites sometimes use truly incredible strategies for their survival. For example, the horsehair worm makes its host commit suicide, and the Entomophthora muscae fungus controls the behavior of fruit flies and makes them perform certain actions before they die.
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It must be said that the current study not only reveals the amazing ability of bacteria to adapt to various conditions, but also has practical significance. The fact is that Wolbachia are already used to reduce the number of insects that carry dangerous diseases, such as mosquitoes. A better understanding of how the bacteria controls insect populations will make these projects even more effective.