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Introversion: randomness within the margin of error, or evolutionary mechanism of species conservation?



“A small and defenseless calf, fighting off the herd, risks becoming the prey of a hungry predator” - a similar phrase can often be heard in documentaries about wildlife. Its essence is clear - for some species it is much easier to survive in a team than alone. It is difficult to argue with such a straightforward logic of nature, because social species are social and that live and survive in groups. However, in any society, whether people or simple organisms, there are individuals who go against general social goals and act in their own way. For us, these are introverts - single people who do not particularly like crowds, try to stay away from parties, concerts and other things. Such behavior is most often associated with psychology and personality traits of a person, but in the wild, such behavior is due to something more.A group of scientists from Princeton University (USA) conducted a study involving amoeba D. discoideum, who usually live alone, but in some situations form groups, which allows us to study aspects of introversion and socialization. What is so unusual about the behavior of amoebas, how does it affect the development and conservation of a species, and how important are loners for social species? The answers to these questions await us in the report of scientists. Go.

Study basis


Given the recent alarming (to put it mildly) events taking place in the world, many of us have become introverts to one degree or another, albeit forcedly. However, even in calm times, and even in the most ideal society, where everyone loves and respects each other, there have always been and will be single people who would rather spend the evening under a blanket with a book rather than go to some social event. We regard this behavior in terms of psychology and psychiatry. Introversion can be caused both by the character of a person, and by some event of the past, which led to self-isolation.

Given the evolution of not only humans, but also technology, our species has ceased to need socialization at the level of survival. In other words, a single person can very well survive without communicating with other people, although such an existence will be to some extent difficult again from the point of view of psychology. However, many social species, such as wildebeests or herring, exploit their full socialization. A large accumulation of individuals confuses predators, allows you to quickly notice danger or food (more individuals - more eyes, as they say), etc.

However, there is nothing perfect. Even among a huge herd of wildebeests, which during migration seem to be subject to a single mind, there are individuals who prefer to act independently.

Researchers themselves cite the locusts as an example. These insects have two phases of social behavior - single and herd. At the beginning of their lives, locust individuals survive one by one, and upon reaching a certain age they begin to gather in numerous groups (up to millions and even billions of individuals).


At the moment, one of the largest invasion of locusts is occurring, affecting several countries in Africa and the Middle East.

Studies have shown that some individuals do not join a swarm upon transition to mature individuals. Even during the experiments, when these loners were deliberately planted with groups of relatives, they did not acquire the need for swarming.

Even among plants, a similar trend is observed. For example, bamboo, which usually blooms massively and is very synchronized, is not without loners who bloom according to their schedule, so to speak.

All these observations make one wonder whether the introversion is accidental or is there some kind of hidden meaning? Perhaps loners are the inevitable statistical error of large-scale aggregations of individuals, and perhaps deliberate programmed behavior, providing a backup plan of survival for the whole species.

Scientists note that earlier, very little attention was paid to the study of this question, because no exact answers to these questions have yet been given. Those few works that have nevertheless been carried out claim that loners are nothing more than accidents. Naturally, the authors of the work we are considering today do not agree with this, since often the same theory cannot be applied to different types of creatures.

In their work, scientists focused on studying the behavior of amoebae of the dictiostelium species (Dictyostelium discoideum ). This organism, like the locust, has two phases of life: single and herd. These phases are due to the presence or absence of food. D. discoideum

lives in soil and litter (fallen leaves) of mixed forests of the temperate climatic zone, and feeds on bacteria. If there is enough food, unicellular individuals of D. discoideum keep their distance from each other. If the food becomes critically small, they begin to connect with each other, forming the Megazordmigratory "flock". During this period, the expression of glycoproteins and adenylate cyclase occurs in the cells. Glycoproteins provide cell-cell adhesion (cell adhesion to form a multicellular system), and adenylate cyclase synthesizes cAMP (cyclic adenosine monophosphate), which coordinates cell adhesion, which is a signal of cell hunger.


Individuals of D. discoideum form a migratory slug.

Further, the formed system begins to rise up, and then lies to one side and forms a migrating mucus - a mobile pseudoplasmodium 2-4 mm long. The body moves where there is more light, heat and drier air. Having reached the optimal location, the slug goes into the “sombrero” stage, when the front and rear pseudo plasmodium cells change places. The anterior cells form the fruiting body with spores at the apex that dissipate after maturation.

All this amazing process of rebirth has been studied over the past seventy years, but very little attention has been paid to those individuals who for some reason did not participate in the formation of a single multicellular body. Of course, in conditions of prolonged hunger, such loners died, but if there was little food, then they continued to exist and share, forming new individuals that could later form a multicellular system.

According to scientists, introverted amoeba can be quite conscious participants in the life cycle of their species, i.e. an important part of survival strategies.

To confirm this theory, the researchers observed the behavior of various strains of D. discoideum , and modeling was also carried out using the data obtained.

Research results


To determine whether loner behavior is hereditary and thus whether there is potential for natural selection, an experimental protocol has been developed to identify and quantify single cells ( 1A and 1B ). This made it possible to characterize their spatial distribution ( 1C ), quantify their density ( 1D and 1E ) and establish hard boundaries for possible measurement errors.


Image No. 1

Three natural strains were used in the study, which were collected in the same place. This ensured that the observed behavior of individual strains was not a laboratory breeding error.

The first observations showed that single cells in higher density were at the boundaries of the formed multicellular system than at the center of aggregation ( 1C ).

In repeated experiments under controlled conditions, the density of loners was constantly within the same distribution (allocated to 1E ). In addition, the distribution of lone individuals in some strains varied significantly in their mean value and variance (compare strains NC28.1 and NC85.2 by 1D ). This confirms that the behavior of the separation of individuals into aggregators and loners is inherited.

In order to characterize the process of self-organization that underlies the separation, it is first necessary to determine whether the decision of an individual to participate in aggregation or to remain alone from external factors.

If such a “decision-making” on the part of the cell does not depend on the context (i.e., without external signals), then the density of individuals should increase linearly with the density of the originally clad cells (growing layer by layer during the formation of a multicellular system).

However, observations showed the opposite situation: at low initial densities, the cells were too rare (far from each other), therefore, aggregation did not occur, and all cells remained single. When the density reached above the level necessary for aggregation, this process proceeded with increasing efficiency, and the density of loners decreased. Surprisingly, at high initial cell densities, the densities of loners were equalized (they became constant, a plateau on 1D ).

Next, the efficiency of strain aggregation was determined as the value of this plateau: it was found that one strain is a better aggregator than another if the first plateau has a lower value than the second. If a strain is called the best aggregator, this means that most of its cells are involved in aggregation; if the strain is called the worst, then among its cells there are a lot of loners.

Experiments with different porosity of the substrate, where the experimental individuals lived, showed that in a less porous substrate, the density of single individuals is higher ( 1E ). Porosity also influenced the dispersion of single individuals.

Observation data indicate that the inherited aggregator / loner separation depends on external stimuli, in particular, on abiotic factors * .
Abiotic factors * - a set of direct or indirect effects of an inorganic environment on living organisms.
To determine the degree of influence of external factors on the formation of the aggregator / loner separation, a model was created with variable conditions of the intended environment.

In accordance with the experimental plan, the simulation began with a population of cells immediately after the depletion of food. These cells are in a pre-aggregating state (P). Given experimental data that show a decrease in the motility of vegetative cells in numerous populations, it has been suggested that P cells do not move. P cells emit extracellular signaling molecules at strain-specific speed Îł.

The signal is scattered with a diffusion coefficient D and is used to determine the quorum (minimum population of cells) that regulates the stochastic transition to the state of aggregation (A): when the signal perceived by the cell exceeds the sensitivity threshold, the model assumes that the cell has a strain-specific probability of one time λ to become an aggregating A-cell.

A-cells continue to emit a signal and move to the center of aggregation with a constant specific velocity ν. In the center, the cells become a multicellular system (M-state), and also stop moving and emit a signal.

Scientists note that their model was intentionally simplified, as it focused on studying the distribution of cell populations. That is, some aspects of the real aggregation process were intentionally missed.

For example, before aggregation, cells must starve for a certain time, become excitable (signal for aggregation) due to cyclic adenosine monophosphate (cAMP) and go into chemotaxis (cell movement in response to a chemical stimulus). Some of these steps (e.g., cell starvation) are irreversible. But the distribution of cAMP and chemotaxis are reversible processes. The simplification of the model consists in linking all stages into one irreversible process of transition from the pre-aggregating to the aggregating state (PA transition). Because of this simplification, non-deterministic factors that are associated with all of these processes and which may affect the PA transition time are included in λ.

As a result of stochasticity, the periods in which P-cells transition to the A-state follow a distribution whose width determines the degree of synchronization in the process of aggregation. Higher λ values ​​result in narrower distributions and, therefore, more synchronized PA transitions, while lower λ values ​​lead to wider distributions and a more asynchronous process.

Despite the forgiveness applied, the resulting model is still able to repeat most of the properties of the observed population distribution ( 1F ).

The total density of loners depends on how quickly P-cells transition to the A-state with respect to how quickly they remain without a quorum (λ / ν), as well as how easy it is to maintain a quorum. Thus, the higher the rate of the λ PA transition, the fewer single cells remain, since the P cells that receive the quorum transition into the A state more synchronously.

There is an opposite situation with the aggregation rate ν: the higher the aggregation rate ν, the more loners remain because A-cells move away faster and narrow the time window in which P-cells support quorum. In other words, loners do not have time to join the remaining cells involved in aggregation.


Image No. 2: model aggregation scheme. AND- at a high initial density, all P cells reach the necessary quorum for initiating aggregation; B - some P-cells go into the A-state; C - during A-cell aggregation, some P-cells remain without a quorum and become single; D - at the end of the process, the P-cells farthest from the center of aggregation remain without a quorum and remain single.

Therefore, the farther the cell is from the center of aggregation, the sooner it will remain without a quorum, and the more likely it will become a loner.

These results indicate that lone D. discoideumcan be the result of the interaction between the degree of synchronization of the PA transition and the time during which the quorum persists before the A-cells enter the multicellular phase.

It was also found that lower diffusion leads to a higher density of solitary cells, since the signal remains highly concentrated around the emitters, and the cells must be more densely packed to maintain quorum. Moreover, a decrease in the diffusion coefficient affects the worst and best aggregators ( 1G ), since the diffusion rate and signal propagation are not linearly related.

If diffusion is indeed the regulator of the whole process, then at least one molecule from the cells participating in the quorum-dependent distribution should stand out from the rest. That is, certain signals from cells participating in the aggregator / loner distribution process should be the driving force of this process (autocrine signaling, when the cells respond to substances released by the same cells). These can be PSF (pre-starvation factor), CMF (environmental factor), or PDE (phosphodiesterase) signals.

It is important to note that PSF is released during the growth phase, and CMF is released at the early stage of fasting. Accordingly, signals in the early stages of the distribution process can play the most important role in how it flows. To test this theory, two different experiments were carried out.

The first experiment was similar to the one shown in image No. 1, with the exception of adding a thin layer of water on top of the cultured cells. The aqueous layer evaporated over 4 hours, but until then it could facilitate the diffusion of signaling molecules.

If the limited diffusion of molecules secreted during these 4 hours of fasting promotes the formation of single cells, then it is assumed that the aqueous layer will reduce the number of observed single cells.


Image 3: The effect of signal changes on lone formation.

In a second experiment, cells were allowed to grow in a bacterial suspension until the resources were depleted, after which they were placed on an agar gel. With this option, the initial responses to resource depletion occurred in a well-mixed medium (i.e., with very high diffusion of signals), and any signal molecules secreted at this stage should have uniformly reached all cells, thereby increasing their behavioral coordination. Therefore, it was expected that among the cells that went into the state of aggregation, there would be a minimum of loners.

All the expected results were confirmed in practice, and this suggests that in the distribution process, the aggregator / loner plays an important role not only in the late stages of fasting, but also in the early stages of fasting, and even before it begins. Such behavior even further separates the process of formation of loners from the epithet “random”, confirming the theory that this process is programmed from the beginning and should proceed under any conditions.


Image No. 4: cell interaction during the joint aggregation of different strains.

Another important factor affecting the behavior of single cells is the origin of neighboring cells, i.e. the process of joint aggregation of cells from different strains ( 4A ).

During the next experiment, two strains were used: NC28.1 (the best aggregator) and NC85.2 (the worst aggregator), which developed together under fasting conditions.

It was found that the total density of single individuals in mixed strains strongly deviated from the linear combination (when the strain is only one), displaying a sigmoid curve in graph 4B .

Thus, when the best aggregator was more common in the mixture (25%: 75%), the total number of loners was less than predicted by a linear combination. If the worst aggregator was more common (75%: 25%), then there were more single individuals.

Therefore, cells of different strains interact with each other, affecting the aggregation and formation of loners. Unfortunately, it was impossible to experimentally determine the origin of each cell (to determine which strain it belongs to), however, this could be done theoretically through modeling.

Calculations showed that the behavior of the strains depended on each other's properties: the best aggregator became even better in the presence of the worst aggregator and vice versa, which increased the difference between these interacting strains.

In practice, the spatial distribution of single strains of mixed strains allows us to evaluate their potential composition ( 4C and 4D) As soon as the worst aggregator becomes part of the mixture, the spatial distribution of the mixed singles in the mixture is almost identical to that of the worst aggregator and is very different from the distribution of the best strain. This suggests that in the mixture most of the loners are represented precisely by cells from the worst aggregator.

If joint aggregation has such a significant effect on the cellular distribution of aggregators and loners, then the formation of the mucus should be equally powerful. And this influence can be manifested not only in the visual differences of the mucus from one strain or a mixture of several, but also in how the cells inside this system interact with each other.

In nature, when there is a joint aggregation of two strains, they can go through not only one stage of fasting, but several, i.e. go through several complete life cycles together, constantly uniting.

Simulation of such a situation showed two possible developments: the strains participating in joint aggregation become more different from each other ( 5A ) or more similar to each other ( 5B ).


Image No. 5: consequences of the interaction of two strains in the development process.

In order to evaluate how the process of joint aggregation can proceed and what its consequences will be for the participating strains, an additional “variable” was added to the existing model — the mutual competition of strains for resources during successive cycles of fasting and growth.

Simulations were performed for two pairs of strains. For each pair, the results of mixing and joint aggregation of the two strains were compared with a hypothetical scenario in which the two strains ideally segregated and avoided joint aggregation.

Between the two periods of fasting, the differential survival of spores and loners, combined with a delay in spores in the consumption of resources, led to changes in the relative number of strains (i.e., the difference in fitness of the strains). Regardless of whether joint aggregation occurred in any environment, there was a competitive exception. Consequently, the outcome of competition was determined by two factors: which of the strains prevails and how long it took for the extinction of the losing strain.

It was found that strains with more single individuals are more competitive, and vice versa, fewer single individuals are less competitive.

In deterministic media, the winner strain was also deterministic and did not change as a result of joint aggregation (insert at 5C and5D ); however, co-aggregation changed the time until the loss of the losing strain (insert at 5E and 5F ).

In contrast, in stochastic media, there are a number of media in which the winner strain is uncertain, and this range changes dramatically as a result of joint aggregation ( 5C and 5D ). As in deterministic media, co-aggregation also affected the time until the disappearance of the losing strain ( 5E and 5F ).

From this it follows that the percentage distribution of cells of different strains during joint aggregation directly affects how the competition for resources between these strains proceeds.

In severe conditions, when there is little food, the winner in the competition is always obvious: when pairs of strains compete for resources over many growth-starvation cycles, the worst aggregator always loses. However, due to the fact that the strains change their behavior when interacting with each other, the time until the disappearance of the worst aggregator changes.

In cases where a pair of strains segregate, the worst aggregator becomes even worse and therefore produces even less spores. From this, the best aggregator becomes even better, which leads to a reduction in time until the worst aggregator disappears.

In cases where a pair of strains are combined, the worst aggregator becomes better and, thus, the production of its spores increases, which leads to an increase in the number of its cells in the subsequent growth cycle and, consequently, to an increase in the disappearance time of the worst aggregator.

For a more detailed familiarization with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it .

Epilogue


In this work, we studied the behavior of single cells of D. discoideum , which are in no hurry to unite with their relatives to form a multicellular system. Such behavior can be called destructive and even dangerous for its initiators themselves, but the results of experiments and modeling suggest the opposite.

If all cells follow the same path, which in theory can lead to death, then all of them will die together. If some part of the cells is left behind, they will have a chance to reproduce and create their own colony. This is nothing more than risk diversification.

A similar situation is observed among more developed creatures. If an infectious viral disease begins to spread among a large herd of wildebeest, their collective behavior will only help the infection spread, killing more and more individuals. At the same time, if a certain number of individuals stay further away from the main herd, they will survive.

Of course, in collective behavior there are many advantages that cannot just be excluded from the equation, but there are risks that also cannot be ignored.

Talking about this study, it is difficult not to think about the terrible events that are happening now around the world. Our society has reached tremendous heights in many directions, but the presence of a developed central nervous system and millennia of history cannot be a shield against all possible threats. A microscopic organism that does not have a brain or grasping hand decided to remind humanity that being on top of evolution does not mean being invulnerable.

However, the struggle continues. And now, referring to the study we examined today, I would like to invite everyone (if possible, of course) to collectively become introverts, because in this way we can not only protect ourselves, but also greatly simplify the already hard work of doctors, scientists and researchers who fighting the virus.

Thank you for your attention, remain curious, have a good working week, take care of yourself and your loved ones.

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