They wipe them while still in the egg, switch their sex to female, and kill them ‎in a variety of bizarre ways – the Wolbachia bacteria, a common insect infection, do ‎not require males for their distribution – so they kill them off and protect the females

The most common infectious bacteria in the world hate males and have a weakness for the true stronger sex – females. Named Wolbachia, they infect insects, other arthropods, like mites, spiders, scorpions, and more, as well as parasitic worms. Seven out of ten insects carry the bacteria; a true epidemic. Once it infects an organism, the bacterium transforms it for its needs, taking over the organism’s reproductive system, protecting it from competing invaders, and sometimes even endowing its host with special abilities.

The bacterium was discovered in 1924 by Marshall Hertig and Simeon Burt Wolbach in the reproductive cells of the common house mosquito (Culex pipiens). Several years after its discovery, it was named Wolbachia pipientis, after Wolbach and the mosquito. For decades following its discovery, Wolbachia was almost forgotten, partly because it does not seem to be able to infect humans. It was only when it became clear just how common the bacterium, along with its strange properties, is in nature, that intensive research began.

Wolbachia can only survive and reproduce inside a host’s cells. This type of relationship is called endosymbiosis, the most intimate relationship that can exist between two organisms, where one organism lives inside another. In fact, we humans owe our lives to such relationships. Most cells of our bodies contain organelles called mitochondria, which provide energy by breaking down sugars in the presence of oxygen. They also control cell death and processes related to the aging of cells and of the organisms comprised of these cells.

Mitochondria are found in all multicellular organisms, animals, plants, and fungi, as well as in many unicellular organisms. They are missing in non-nucleated unicellular organisms such as bacteria and archaea, since mitochondria were originally bacteria that penetrated approximately a billion and a half years ago into a large host cell, which ultimately became the ancestor of all nucleated cells. Today mitochondria cannot live outside the cell so this event is now thought of as the ancestor of all nucleated cells. These bacteria – distant relatives of Wolbachia – developed a close partnership with their host cell. Over time, they transformed from independent bacteria living inside cells into organelles. Although they retained some of their bacterial genetic material, they transferred some of it to the DNA in the host cell’s nucleus.



A very intimate relationship, which changes nature’s equilibrium. Wolbachia bacteria | Source: Science Photo Library

Who needs males?

Mitochondria are inherited solely from the mother. If you are a male, your mitochondria are in trouble, since even if you do have offspring, you are a dead-end from their perspective; your mitochondria will die with you without being passed on to the next generation. Wolbachia bacteria suffer from a similar problem. Although they occasionally may be able to infect new organisms “conventionally,” where a healthy animal is infected by bacteria from another animal, their main way of transmission is from mother to offspring.

Wolbachia bacteria pass on from one generation to the next via infected eggs. The sperm cells are just too small for them and there is no place for the bacteria to hide inside them. So as it happens, only female carriers can ensure the continuation of the Wolbachia lineage. Males are the end of the road for it, and hence its “hatred of males.”

During evolution, the Wolbachia bacteria developed several ways to overcome this male problem and ensure their survival. For example, the bacterium behaves as a reproductive parasite – taking over the host’s reproductive system and changing it, so that the reproductive results will match its needs. In some species of Hymenoptera (ants, bees, wasps, etc.), mites and thrips (an order of tiny insects), in which males develop from unfertilized eggs, Wolbachia completely eliminates the need for males. The parasite enables its hosts to reproduce via parthenogenesis: The unfertilized eggs the mother lays develop into females that are her clone, instead of into males. No need for fertilization, and therefore – not for the useless males.

This Wolbachia ability was discovered in 1990, when Richard Stouthamer from the University of California and his colleagues noticed that antibiotics “cured” the parthenogenesis of Trichogramma parasitic wasps. After antibiotic treatment, the males reappeared and even mated with the females. Three years after the discovery, Stouthamer and his colleagues identified the guilty culprits – the Wolbachia bacteria.

Nip them in the bud

In the arthropods that they infect, such as butterflies, moths, beetles, flies, and pseudoscorpions, the bacteria kill males ‘the biblical way’, while they are still larvae. Emily Dyson and Gregory Hurst from the University College London found in 2001, for example, that the nymph butterfly Hypolimnas bolina found on the Samoan Islands, Upolu, and Savai’i consisted almost exclusively of females. Wolbachia bacteria massacred the males, so that for every 99 females there was only one male.

At first glance, a bacterium killing its male hosts seems to be a waste of time, because the Wolbachia dies with it. But the hatched infected females no longer need to compete with their brothers over food – after the latter died as eggs. Moreover, in many cases, the dead brothers become the first meal of the young females, and a dead brother also reduces the risk of incest. Taken together, all of this slightly improves the females’ chances to survive, reproduce – and transmit the bacteria to the next generation.

According to the theory of evolution, it is widely expected that such mass killings will lead to a strong selection towards the appearance of a mutant male that is resistant to such killing. Indeed, already in 2007, Hurst reported that in the islands of Upolu and Savai’I, the ratio of males to females regained balance at 50:50. In less than ten generations, the butterfly population exhibited a resistant gene; an example of very rapid evolution. Hurst says the gene may have developed within the population of butterflies on the islands or was imported somehow from a population of the same species in Southeast Asia, which were already resistant to the male slaughter by Wolbachia.



At some point there were 99 females for every male. Female Hypolimnas bolina | Source: Wikipedia

A preference for female carriers

Despite the return of the males, Wolbachia have not lost the war. The bacteria employ additional ruses and are still thriving in the seemingly resistant butterfly population. When they allow males to live, or when males develop resistance and survive, the bacteria move to a different tactic called “cytoplasmic incompatibility.” While the bacteria may not be able to penetrate sperm, they can and do affect their production. They modify the sperm cells so that they cannot fertilize eggs that are not infected with the bacteria of the same strain. Therefore, when a healthy female – which doesn’t carry Wolbachia – mates with an infected male, her eggs will not be fertilized. In contrast, when a female carrier mates with another male – carrier or not – it will lay fertile, and Wolbachia-infected, eggs.

This situation provides a huge advantage to female carriers over healthy females. Female carriers can mate and produce offspring with all males, whereas healthy females can do so only with healthy males. As a result, the infected females become increasingly prevalent as generations pass, and soon much of the population will be comprised of male and female carriers. This manipulation is the most common one performed by the bacteria and can be found in almost all organisms that the Wolbachia infect.

Gender reassignment surgery

The fourth way in which the bacteria deal with males is by turning them into females. This phenomenon is seen in some species of butterflies, bugs, and isopod crustaceans such as the pill-bug (Armadillidium vulgare).

In pill-bugs, like in humans, gender is determined according to the sex chromosomes. In humans, the Y chromosome dictates the fetus’ sex. Except for extraordinary cases, females have two identical sex chromosomes (XX) and males have two different chromosomes (XY). In pill-bugs, the situation is reversed, females have two different sex chromosomes (ZW) while males have an identical pair of chromosomes (ZZ).

However, in many populations of pill-bugs, females are the majority. In such populations, the Wolbachia bacteria damage special glands that the males carry, inducing hormonal changes that cause eggs carrying the male ZZ chromosomes to develop into fertile females. In all of these populations, all infected fertilized eggs develop into females, regardless of their genetic composition. In subsequent generations, the female sex chromosome that is not being used in the infected population, will disappear: The females are infected genetic males without female chromosomes, so they do not pass it to their offspring. Thus, offspring sex is determined only by the infection status of the Wolbachia – females are infected pill-bugs and males are healthy pill-bugs.

However, some populations of pill-bugs found be no longer infected with Wolbachia are still comprised of a female majority. The females in these populations are genetically males, but they do not carry the bacteria.

According to a study presented in September 2016 at the International Conference of Entomology, the cause of this strange situation is yet again Wolbachia. It turns out that the bacterium does not release its grip easily. The Wolbachia make female pill-bugs by donating DNA directly to the pill-bug genes. The genetic fragment from the bacteria that is responsible for turning male pill-bugs into females is incorporated into the sex chromosomes of the males. In fact, within this pill-bug population, there are two sex chromosomes – the old Z chromosome and another Z chromosome that contains the bacterial DNA that makes it a female. This is an evolution of a new chromosome, one might call it W.



Sex reassignment surgery: The bacteria turn males into females via a hormonal modification. A pill-bug | Source: Wikipedia

The virus within the bacteria

The transfer of Wolbachia DNA fragments into the genome of hosts occurs occasionally and is probably mediated by a bacteriophage (phage, for short) – a virus that attacks the bacteria. Jonathan Swift wrote at the time, “Big fleas have little fleas, upon their backs to bite 'em, and little fleas have lesser fleas, and so, ad infinitum.” This sentence is true for all organisms: Phages called WO, after the bacteria, can be found in almost all Wolbachia that live inside cells of arthropods.

These phages can multiply inside the bacteria in two cycles. One of them is called the lytic cycle, where the phages recruit the bacterial systems to generate more copies of themselves, finally breaking out as they dissolve the bacteria and kill it. In the second cycle, called the lysogenic cycle, phage DNA integrates into the genome of the host bacterium and multiplies as the bacterium replicates and divides. Sometimes a dormant lysogenic phage is awakened and switches to the lytic cycle.

When the phage breaks out of Wolbachia bacteria into their host cell, it might find itself in trouble. If there are no “free” Wolbachia bacteria around, it must leave the host cell, infiltrate another, find Wolbachia bacteria within it and penetrate them. This is no simple challenge. Viruses are almost always limited to one domain of organisms. For example, viruses that infect bacteria are not able to penetrate archaea (a type of unicellular organism) or nucleated cells (such as those of humans and other multicellular organisms) and infect them. However, it seems that WO found a way to overcome the eukaryotic cells on the way to infect its bacterial host.

To find out how it does this, Seth and Sara Bordenstein of Vanderbilt University in Tennessee mapped the phage genome. To their surprise, they found that half of the phage’s genetic material originates in animals. One DNA sequence revealed in it is part of a gene responsible for producing one of the components of widow spiders’ venom that can pierce animal cells.

The Bordensteins identified additional animal-derived genes that are related to causing cell death, pathogen recognition, and immune system control. They speculate that the stolen genes and their new combinations generated in the phage, allow them to leave and enter animal cells and evade the immune system and other defense mechanisms of the organisms that hosts the bacteria.

The phage’s ability to adopt genes from Wolbachia hosts may benefit the bacteria themselves. After all, when the virus is latent, its DNA is integrated into the bacterial genome, and could therefore be considered as a passing of segments of DNA from the hosts to the Wolbachia. In an interview with Ed Yong, the Bornsteins discussed evidence they found indicating that some of the genes involved in the bacteria’s ability to control hosts’ reproductive systems lies in the genome of the phage and not the bacteria. However, these findings have yet to be published in the scientific literature.

Fertile cooperation

When not tinkering with their hosts’ reproductive systems, Wolbachia help them in many different ways, effectively enhancing their chances of survival: Moth larvae from the Phyllonorycter blancardella species are leaf miners that live on the leaves of apple trees. They prevent the section of leaf they live on from turning yellow and dying. The Wolbachia bacteria inhabiting hungry larvae secrete substances onto the leaves that create “green islands” – patches of living tissue that allow the larva to continue eating and reach maturity; and bed bugs get a supply of B vitamins generally lacking in their blood meals, from the Wolbachia.

The genes for B vitamin production do not exist in most Wolbachia. They can only be found in those that inhabit bed bugs or their close relatives, the bat bugs. Transferred to the Wolbachia from the genome of other bacteria, these genes’ presence is beneficial to the host insects and therefore, also to the bacteria that infect them, rendering the relationship between Wolbachia and bed bugs mutually exploitative – a partnership.

Even humans can take advantage of the bacteria. Although Wolbachia are not able to infect vertebrates such as humans, they certainly influence organisms that cause disease in them. For example, parasitic worms that host the bacterium are unable to live without it. Treatment against worms of this sort, such as those that cause river blindness or lymphatic filariasis, is difficult and can harm the patients themselves, because worms and humans are quite similar. However, we can apply antibiotics to destroy the bacteria living in the worms, and thus stop the worms without hurting the person.

For us, Wolbachia’s most valuable feature is the protection they grant their host against other pathogens. In mosquitoes, for example, the bacteria activate genes that enhance the activity of their immune system to make it difficult for viruses and other parasites to infect the mosquito. Furthermore, the bacterium directly competes, and successfully so, with pathogens for nutrients – and thus may inhibit pathogens that eluded the mosquito's immune system. And mosquitoes not infected by pathogens will naturally not transfer them on to the person they feed off.

In order to exploit Wolbachia for our purposes, all that is needed is to bring them to areas affected, for example by zika or dengue, and allow them to infect a group of mosquitoes. Their ability to control the hosts’ reproductive system will ensure its rapid spread, infecting the majority of the population, if not all of it, thus halting the spread of the disease-causing agents.