Zombie techniques in wildlife
While some people frighten with zombies through TV and conspiracy theories, and others half-jokingly call the fetus in the womb a parasite that manipulates her, these manipulating parasites successfully subdue their masters, turning them into zombies. And these schemes have been working for tens of millions of years, and not some miserable tens of centuries, while the historical period of Homo Sapiens lasts. Other biological "zombies" - "the walking dead" - lurked on the branches of the evolutionary tree and even inside our cells. Still others walk the streets and think that everyone around is already dead, which means that you can commit crimes, because they don't care anymore. So who do scientists call zombies and where to find them? Let's figure it out.
Zombie under the microscope
Let's start with the fact that not only the whole organism, but also individual cells can be made to rise from the dead and bend to one's will. Mycobacterium tuberculosis knows very well how to do this. This bacterium, which causes tuberculosis, multiplies inside the cells of the human body. It also settles inside macrophages - immune cells that eat dangerous "outsiders". After infection, the membrane of macrophages becomes leaky, which almost always means certain death for a cell. But mycobacteria do not want to lose their "incubators" so easily, so they do not allow already practically dead cells to die to the end, turning them into zombie factories that can produce new tubercle bacilli and hide from antibacterial therapy.
If you shrink further and look even deeper, zombies can be found in all of us. Moreover, without them we could not have been born. At least that's what the authors of the "zombie centrioles" hypothesis, published in the scientific journal Frontiers in Cell and Developmental Biology, think. The centriole is nine triplets (triplets) of microtubules. Centrioles "orchestrate" cell division, forming a division spindle, which pulls chromosomes (or their halves, depending on the type of division) to the poles of the cell. It has been established that in most animals, during sexual reproduction, centrioles are inherited from the father, but in humans and other mammals, as well as insects, the centrioles of the sperm are modified (or absent altogether). Where, then, do the two centrioles in a fertilized egg come from? In insects, for example, there is only one centriole in the sperm cell. In humans, nothing is clear at all: it seems like some scientists observed in a fertilized egg as many as three instead of the normal two, and in a mouse, centrioles were not seen at all until the cell divides into 32 or 64.
The clue, according to the authors of the article, is simple: there are two centrioles in the sperm cell, but one of them is modified, degraded. However, the "dead" centriole can divide and function without being restored to its normal form. “Due to the fact that they are simultaneously degraded (“dead”) and functional (“alive”), we called them zombie centrioles," the authors of the work write. When centrioles are fertilized into a zygote, this zombie centriole can form a daughter normal centriole, so there are three centrioles.
An even stranger thing than zombie centrioles (but definitely existing in reality) are zombie squirrels. Scientists from England, Australia and Norway wrote a whole review about them, complaining that the "zombie proteins" devoid of enzymatic activity, to put it mildly, are not favored by their colleagues.
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And completely in vain: they are present in all kingdoms of the living and occupy 10-15% of the genome. If they weren't needed, natural selection would slowly clean them up (it's also a pleasure - wasting energy on synthesizing useless junk), but this is not happening. And if we begin to understand that even non-coding, "junk" DNA actually does not just take a place in the genome for a reason, then the coding genes from which even non-functional proteins are produced should certainly make sense. Such "zombies" can influence their working prototype, switch its activity, attach it to the right place in the cell - that is, lead a completely life after its "functional death".
And the zombie sits on the branches
What about plants? We're not going to undeservedly bypass them in our story. Moreover, they can also become zombies. Due to phytoplasm bacteria, zombie plants grow leaves on twigs instead of flowers. They can no longer reproduce and, like macrophages colonized by tubercle bacilli, become factories for the production of bacteria.
Single-celled plants make plants weak-willed slaves with the help of the SAP54 protein, which interacts with the proteins of the MADS domain (no, they are not responsible for insanity, but for the formation of a flower). In the process of evolution, the bacterial protein SAP54 learned to resemble the K-domain protein, which normally interacts with MADS - so it hacked into the system of access to plant control.
But not only the branches of plants can become "zombies". Scientists can also call the branches of the evolutionary tree that. Much debate is going on around the model of mammalian development after the Cretaceous-Paleogene extinction, which destroyed dinosaurs and many other animals and plants. There are two versions of these events. According to the first "explosive" model with the "long fuse explosive model", the ancestors of different orders first diverged within the mammalian class, and then the species within each order appeared. The short fuse explosive model assumes that these events happened almost simultaneously. Recently, a “soft explosion” model was proposed, according to which some lines arose in the Cretaceous period, and after the extinction, the main “explosion” occurred.
The problems and weaknesses of the latter model reveal the ghost branches and zombie branches that have arisen on the evolutionary tree. The first ones appear due to the fact that the fossil record is incomplete: no ancestors were found, and the descendants were in place. The latter are more problematic because they occur before molecular data predicted the divergence of their ancestral groups.
The authors of the Molecular Phylogenetic and Evolution article analyze all three models and argue for a “long fuse cord,” showing that division at the order level most likely began in the Cretaceous. This concept allows us to defeat the zombie branches and refine the evolutionary tree of mammals.
Terrorist worms inside crayfish and ants
In the course of evolution, parasites have acquired an extensive set of zombie technologies. All means are used here: manipulation, belief in the falsity of the signals of one's own sense organs, wars with immunity, and counterfeit hormones.
"Neuroparasitology is a science on the verge of science fiction," writes Michael Dickinson, co-editor of The Journal of Experimental Biology, of the University of Washington. This phrase is seasoned with a mixture of horror and admiration: from the request "don't eat me, I'll still be useful to you," directed to someone else's immune system, to insidious manipulation and zombification of the owners - one step. And you can go through it by forging molecules that are used to communicate by the host's cells.
Sometimes it doesn't seem to make much sense. There are gammarus crustaceans, which belong to the order of amphipods (they are called that because they literally swim sideways). But when they are attacked by parasites, the crustaceans lose control of themselves and begin to move like all other crustaceans, and even swim to the light, instead of hiding from it.
Gammarus crustacean. WithJulie / Flickr
As a result, they are more likely to be eaten by fish, which the parasite needs to get into. Scientists were able to influence the movement of gammarus with serotonin and showed that the parasites most likely affect the immune response, which causes the nervous system in crustaceans to become inflamed. As a result, the work of the neurotransmitter serotonin, an intermediary molecule that transmits signals between nerve cells, as well as between nerve and other cells, is disrupted. The crustacean does not correctly receive visual and olfactory signals and swims towards its death.
It happens that parasites have to spend "childhood" in one organism, "adolescence" in another, and grow up in a third. This is how the fluke worm, the lanceolate fluke Dicrocoelium dendriticum, goes through its life.
Its eggs should lie in the soil, future flukes are sent to the "kindergarten" inside the snail. At the next stage, they swim free for some time in the form of cercariae larvae with tails, then they are swallowed by an ant, after which they become metacercariae.
But not all cercariae will go through the ant "school": one of them is destined to die in order to allow the rest to grow up. It crawls into the subopharyngeal ganglion of the ant, the lower half of the prototype of the brain that insects have. There, the cercaria settles down more comfortably, surrounding itself with a thin shell, and sits down at the “control panel”. From now on, the ant works during the day, like all its brothers, but at night it turns into a weak-willed zombie. It leaves the anthill, climbs onto a blade of grass, grabs it with its teeth and waits until it is swallowed by a mammal: a zombie ant is just a six-legged transport hijacked by worms to achieve its cherished goal, which becomes the liver or bile ducts of a sheep or cow. There, the worms will thrive and can multiply up to tens of thousands of individuals.
Development Cycle / Dicrocoelium dendriticum Wikimedia Commons
Other parasites, hairworms, send grasshoppers and cockroaches to certain death, forcing them to jump into the water, where the fish, the next host of the parasite, must eat the insects.
Cats and mice and flowers of evil on a spaceship
But if zombifying an ant or crustacean is not so difficult, then “hacking” the brain of a large and complex vertebrate is a task for the most advanced “hackers”. Therefore, the most famous and even canonical example of zombification is, of course, toxoplasmosis. This disease is caused by the simplest Toxoplasma gondii. Toxoplasma, unlike the larvae of worms, consists of only one cell, but can subjugate trillions of strangers.
The ultimate destination of the Toxoplasma route, their promised land, is the cat. Immediately, insidious unicellular parasites can rarely get into it, so most often the path to the Toxoplasma paradise lies through death. True, not their own: to get to the cat, they use smaller vehicles - rats, mice or birds.
In these animals, Toxoplasma undergoes various transformations. Once in the brain, they produce tyrosine hydroxylase, a protein that regulates the rate at which the hormone dopamine is synthesized. This hormone, associated with pleasure and trust, is a key link in the motivation and reward system. Having succumbed to the promise of happiness, that very inner carrot on a fishing rod of any motivation, rodents or birds become fearless and themselves begin to look for meetings with the clawed paws of a fluffy beast.
Tyrosine hydroxylase molecules look like harmless flowers, but in the hands of Toxoplasma they turn into flowers of evil, and not in the Baudelaire sense, but in the truest sense / Gla086 / Wikimedia Commons
But even the most beautiful mechanisms can fail. Imagine that you have to steal a car or a helicopter to get to, for example, a paradise island, where a serene life awaits you. But then the unexpected happens: your next trooper by mistake ends up in a spaceship. You have learned to hijack cars and planes, you know how to steer them, hide from guards - the immune system - and multiply, populating different tissues, you have made your victims more attractive to the opposite sex, you even threaten the populations of Hawaiian geese, "hijacking" in some places before 48% of them. But you have not learned how to fly spacecraft. In general, your ship flies to Mars on autopilot. Not only is there no paradise island now, but you also don’t have a spacesuit with you. Of course, the spaceship has buttons inside, similar to thosewhich you are used to clicking - but sometimes they work in a completely different way.
In addition, the victim's immune system can also affect the nervous system (for example, through inflammation, like in the same gammarus) and change their behavior - and not always in the direction in which Toxoplasma is beneficial.
Zombifying People: Echoes of Mouse Love and the Apocalypse in New York
Probably, this is how Toxoplasma would feel if it could assess its chances of success by infecting a person. The cat is unlikely to eat it, and it reacts to the stimuli of Toxoplasma in its own way. For example, he has an increase in schizophrenia or an increased risk of its manifestation. Also, surveys of infected people show that Toxoplasma victims become slower and more passive in the presence of danger, the instinct for self-preservation becomes weaker, and although in general patients are more irritable, they do not want to fight for anything. In frightening situations - in a dark forest, in an empty house at night - they remain calm.
People can also have dramatic changes in their level of suspicion and sociability. Women become friendlier and more altruistic, while men, on the contrary, do not want to share anything. The suspicion of the infected is influenced not only by gender, but also by place of residence: urban men and village women become more trusting due to Toxoplasma, while village men and urban women, on the contrary, look for a catch in everything.
Once again, we see the parasite's “dopamine” handwriting, reminding that this hormone does affect male and female mammals differently. Experiments with prairie voles, models of love and fidelity in the rodent world, have shown that dopamine is needed not only for attachment to your soul mate, but also for aggression towards strangers. Such influences can affect males and females in different ways: some need to protect the territory, others need to become attached and be friendly. Perhaps this response is also felt in the differences in the behavior of men and women infected with Toxoplasma - although, of course, people are not mice, and the interface for Toxoplasma is not so convenient.
But Toxoplasma does not learn from their mistakes and continues to encroach on spaceships: in regions where sanitation is not very popular, and a lot of raw meat gets into food, more than half of the people form antibodies to Toxoplasma. This means that they are sick or, at least, met with these parasites in the past. It is not easy to cure this infection with antibiotics: somewhere, a dormant form of the parasite - a cyst, which is always ready to multiply again, survives somewhere. And again a man is no longer exactly he, but a Toxoplasm spaceship controlled from within, striving inside an unattainable cat.
But there are other scenarios for a zombie apocalypse. Plants can also be not only victims, because drugs obtained from them can intoxicate people. The story of how chemists, while researching cannabinoid receptors (which react not only to narcotic cannabinoids, but also to many of the body's internal neurotransmitters), accidentally provoked the appearance of "zombies" on the streets of New York, is not the plot of a film about the apocalypse, but the theme of the real scientific article published in The New England Journal of Medicine in January 2017.
On July 12 last year, rapid response units were sent to find and capture sinister people behaving exactly like zombies. In Brooklyn, New York, a total of 33 people were found roaming the streets mechanically with blank, uncomprehending eyes. People slowly, like somnambulists, moved their hands and growled like real zombies. Eight newly minted "zombies" were tested for urine and blood. After an investigation that lasted 17 days, scientists were able to establish that the culprit was a cannabinoid recently synthesized in order to search for drugs, which people bought under the guise of the herbal drug AK-47 24 Karat Gold, which affected the first type of cannabinoid receptors.
Read the continuation here.
Ekaterina Mishchenko
