Parasites: enslavers of the brain

US, WASHINGTON (ORDO NEWS) — Undoubtedly, it is a terrible and at the same time bewitching sight to watch how the miserable ladybug (Coleomegilla maculata) turns into a zombie. Typically, these insects behave as insatiable and sophisticated predators. Throughout life, one beetle is able to destroy a myriad of aphids.

In search of a victim, he “tunes” his antennas, where the chemical receptors are located, and picks up the signals coming from the plant aphids eaten. Having attacked the trail, the bug adjusts to the perception of the smells of its prey. The matter remains small – he sneaks up and tears the victim apart with sharp jagged jaws.

Ladybug is well protected from most predators. Her bright spotted carapace, so cute to the human eye, warns everyone: “Bite – you will regret it.” If the bird or other enemy nevertheless dares to attack, the insect gives off a very bitter tasting – and poisonous – hemolymph of orange color, having barely tried it, the unlucky predator immediately spits out the nasty prey and learns the lesson for life: it will never touch again ” speckled “bugs.

Well, what is not a perfectly protected creature? However, he also found an Achilles heel: parasitic hymenoptera – riders – adapted to lay eggs directly into the body of a ladybug. When the three-millimeter female rider, Dinocampus coccinellae, is ready for breeding, it lands next to the beetle and with a deft movement plunges the sting-ovipositor into his body, simultaneously injecting a portion of the chemicals there, which we will discuss below.

The hatched larva begins to feed on the host’s internal juices. Moreover, no external changes occur with the ladybug: she continues to hunt aphids tirelessly. However, overcooked prey goes to feed the parasite hidden inside.

Parasites enslavers of the brain

Parasites enslavers of the brain

Three weeks later, the larva grows and prepares to leave the host’s body in order to degenerate into an adult. She squeezes through the gap in the shell of a bug, gets out and starts to weave herself a silk cocoon right under his belly. It would seem that now the body has freed itself from the parasite, but the brain remains enslaved: the zombie ladybug, which, when the threat approaches, begins to jerk its paws, now serves to scare away predators. Indeed, for a rider it is a matter of life and death: a sedentary chrysalis in a cocoon is practically defenseless and can become easy prey for its enemies like a larva with a fragile-looking lacewing.

In the role of a forced bodyguard, the beetle will stay for another week until an adult rider is finally formed, which will get out of the cocoon and fly away. At this moment, most of the ladybugs die, having fulfilled their mission of carrier and nanny for the rider’s larva. Particularly persistent individuals manage to survive by going through all the circles of hell.

This terrifying story is not at all a figment of someone’s imagination. And this is by no means an isolated case: the host organism — whether it be an insect, a fish or a mammal — often provides “both a table and a house” (and a brain) to a parasite that has settled in it, sometimes being on the verge of death.

The function of the outer protective shell is one of the few services that the owners unwittingly provide to uninvited guests. For example, the Costa Rican knitting spider Leucauge argyra, hanging its web for hunting, is being transformed beyond recognition under the influence of another parasite rider Hymenoepimecis argyraphaga.

The female of this hymenoptera glues an egg to the victim’s body, and the hatched larva gnaws at the spider’s abdomen and begins to feed on its blood. Feeding lasts several weeks, after which the host spider suddenly completely redraws its skillful snares: a sprawling web turns into a plexus of several thick tows tied at a central point. There, the larva that has sucked all the juices from the victim is fixed in order to weave its own cocoon. Is it possible to come up with the best way to protect yourself from attack?

Some parasites manage to even more skillfully manipulate the behavior of the owners. So, a unicellular creature – a blood sporum, or malarial plasmodium (Plasmodium malariae and related species), before infecting a person, spends part of the life cycle in the body of a mosquito drinking blood, including human.

Satisfying food needs is associated with a risk to the life of a bloodsucker, and hence the plasmodia living in it, because a person irritated by a squeaky squeak will not leave a wet place from a mosquito, depriving the blood spore of a chance of moving to the next stage of development – already inside the human body.

In order not to die with a bloodsucker, the parasite tries to moderate its appetite, making it less likely to go hunting and not to itch at the victim’s ear, if the first attempts to get a portion of blood were unsuccessful. However, as soon as plasmodium forms a generation of sporozoites – cells that are ready for asexual reproduction and concentrated in the salivary glands of the mosquito, the parasite begins to get the exact opposite from the host: it excites blood thirst in the insect and pushes for risky attempts to bite again, even when there is more blood in it does not climb. At the same time, the fate of the bloodsucker does not concern the parasite, which has reached its goal and moved into the body of a new host for further development.

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Plasmodia cause only insignificant changes in the behavior of mosquitoes, while other parasites can even bring their puppet to death. Small fish from the family of cyprinids, well-known to aquarists in small African reservoirs, in the natural environment stay away from the water surface so as not to get into the beak of some bird wandered into a quiet backwater.

But as soon as they pick up the fluke worm, they begin to swim to the surface, even roll over to shine with a silver belly in the sun, thereby turning into a noticeable target. Birds are much more likely to eat infected fish, along with which worms get into their stomach, having the opportunity to grow and multiply.

The most famous manipulator parasite follows a very similar strategy on land. Rats and mice (as well as other mammals) can become infected with Toxoplasma gondii, a relative of malarial plasmodium. These parasites form numerous (up to several thousand) cysts – cells that are hidden under the protective membrane in the victim’s brain. But to complete the life cycle, toxoplasma must enter the intestinal lining of the cat. How can she achieve this, because the sporovi simply cannot move on her own? To ensure the continuation of the genus, the parasite has to sacrifice its intermediate host – a rodent.

Rats or mice infected with Toxoplasma do not fear the smell of a cat, this “aroma” even attracts them.

So, in search of a cat, they become easy prey: a pair of sweeps of clawed paws, and the rodent finds itself in the stomach of the predator along with the parasite that has prepared for him such a fate.

How can unicellular species like sporozoans, as a result of natural selection, develop such impressive abilities to control the behavior of highly organized animals? While this remains one of the most interesting mysteries of evolutionary biology. One hypothesis was suggested by biologist Richard Dawkins, author of the popular science bestselling book The Selfish Gene (1976).

Dawkins believes that genes only evolve to reproduce themselves more and more successfully. Our bodies mean something to ourselves, but from the point of view of genes, they play the role of a controlled membrane necessary for the transfer of DNA from one generation to another. The set of genes of any person or other organism is called the genotype; all external signs of the body and its functions, predetermined by the genotype, but acquiring their final form as a result of the development of the individual, are the phenotype.

According to Dawkins, the phenotype is not limited to the external characteristics of the body – it also includes behavior due to genes. Say, in the beaver’s genes, his bones, muscles and skin are “encoded”. But besides this, genes also determine the structure of neural networks in the animal’s brain, forcing it to sharpen tree trunks and erect dams.

The whole life of the beaver and his family depends on the dam that he will be able to create: the water surrounding the beaver hole protects his home from the attacks of predators. If a gene mutation allows a beaver to build a dam better, this will increase the chances of a particular phenotype – the carrier of such a gene to survive and to have a large (average) fertility of the same gene. This means that a particular mutation will have the opportunity to gain a foothold and spread in the population over several generations.

But if the effect of the gene materializes in the physical world, for example, in the form of a dam and an artificial reservoir, then Dawkins suggests, why not extend to the control of other living creatures? For an example he gives all the same parasites. Indeed, the ability to control the behavior of the owners is programmed in their DNA, the mutation of at least one of the genes can affect the entire behavioral chain associated with the influence on the host. Whether this turns out to be beneficial for the parasite or, on the contrary, will hinder it, depends entirely on the type of mutation.

So, if a person with influenza, instead of sneezing and infecting others, suddenly locks himself in a room until his death, then such a strain of the virus will not be able to spread among the new hosts and very quickly disappear from the population. More successful are those mutations that change the behavior of the hosts in a manner that is favorable for parasites. For example, if the rider’s genes change so that his larva succeeds in instilling habits of the ladybug that contribute to the protection of the parasite, then individuals with this mutation will gain an advantage (they will be less likely to die from predators), and the offspring of such a rider will be more numerous.

According to the Dawkins hypothesis expressed in 1982 in the book Enhanced Phenotype, some of the invader genes turn out to be more successful than the victim’s own genes responsible for their behavioral habits. The book is much ahead of its time: in those years, scientists were just beginning to study parasites that change the behavior of the owners. Decades later, researchers finally managed to lift the veil of secrecy and unravel the tricks of the puppeteer parasites.

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A group of scientists led by Fred Libersat of Ben Gurion University is studying in detail the wasp Ampulex compressa, which stings cockroaches, turns them into obedient zombies and leads into its mink, holding by its antennae, like a dog on a leash. At the same time, the cockroach does not lose its ability to walk, it simply disappears from the desire to move independently.

The wasp-ampuleicide, meanwhile, lays eggs on its abdomen, and the poor victim obediently waits until the larva hatches and penetrates inside. But how does the wasp manage to master the cockroach so masterfully? The Libersat group found that its thin sting penetrates with surgical accuracy in the area of ​​the insect’s brain, which are responsible for its motor functions, and generously processes nerve cells with a cocktail of neurotransmitters that have a similar effect to psychotropic drugs.

The experiments put forward by Liebersat point to the suppression of the activity of the neurons responsible for fleeing when a cockroach is in danger. And although scientists were able to unravel the neurosurgical manipulations of wasp-ampulicides in great detail, they are far from a complete understanding of the enslavement process.

The wasp elixir consists of a mixture of different substances, among which the Libersat group has yet to identify compounds that affect the behavior of the cockroach and describe the mechanism of action itself. However, already at this stage of research, they are in good agreement with the idea of ​​Dawkins’s “expanded phenotype”: genes encoding the molecules of the neurotransmitter introduced by the wasp make a forced cockroach part of the plan to continue its genus: it turns into a cradle for an aspen larva.

In some cases, scientists were able to understand which genes allow parasites to manipulate the behavior of the host. Baculoviruses that infect unpaired silkworm caterpillars and other butterflies transform their cells into factories for the production of new viruses. The infected caterpillar looks no different from the healthy one: it still gnaws at the leaves, but at the same time it climbs up the tree, which healthy individuals never do, and quickly reaches the top.

At this time, by the will of the virus, genes awake in the caterpillar cells, which start an avalanche-like production of enzymes that dissolve the body, so that it literally flows to the branches located below and irrigates them with myriads of new copies of the virus, ready to move into the next victims.

Kelly Hoover and David Hughes of the University of Pennsylvania interpret this behavior of the caterpillar as the clearest illustration of the concept of “extended phenotype.” To test the Dawkins hypothesis, they undertook a detailed study of the baculovirus genes to establish those that are responsible for the movement of the insect.

They found that when the egt gene is turned off, the virus continues to multiply among healthy cells, eventually turning the caterpillar into jelly, but diseased larvae no longer climb up. Control over the behavior of an animal using only one gene is rather an exception to the rule. In most parasites, whole gene networks are responsible for this. And what secret does the rider Dinocampus coccinellae hide with its ladybug controlled by its larva?

Fanny Maur and colleagues from the University of Montreal managed to find out that the rider who turns the victim into an obedient bodyguard is probably an “extended phenotype” of another organism: along with the eggs, the rider introduces into the ladybird’s abdomen a mixture in which there is a virus living in his ovaries . Apparently, it was he who immobilizes the beetle and turns it into a defender of the larva.

In this case, the evolutionary tasks of the virus and the rider coincide: under the guardianship of ladybugs, more riders are hatched, which means that the habitat of the virus is expanding. Therefore, their genes joined forces to zombify a common victim. However, the real puppeteer in this case is not the rider – his wings are neatly suspended from thin strings, for which the more powerful gentleman, safely hidden from our eyes, cleverly pulls, is a virus.


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