Once, single-cell life dominated the earth.

For about 3 billion years, single-celled generations of food have only eaten, grown, and reproduced among each other, forming complex and dynamic ecosystems in every niche on the planet. It wasn’t until about 600 million years ago that some unicellular organisms crossed the boundaries of multicellular organisms.

Today, single-celled organisms are synonymous with primitive and simple. However, a recent study suggests that the capabilities of these single-celled organisms may far exceed the imagination of their distant relatives.

Harvard Medical School’s systems biologists have repeated an experiment conducted more than a century ago and now have compelling evidence that at least one single-celled organism called Stentor roeselii has complex and diverse behavioral strategies .

Researchers say that repeated exposure to the same stimuli can actually “change your mind” about how to respond, suggesting that they have the ability to make relatively complex decision-making processes. The findings were published online December 5 in Current Biology.

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S. roeselii is shrinking (Source: Bill Porter / Harvard Medical School)

“Our findings indicate that a single cell is much more complex than we usually think,” said Jeremy Gunawardena, author of the study and associate professor of systems biology at the Blavatnik Institute at HMS. Sexually makes sense.

” Creatures like S. roeselii were predators at the top of the food chain before the emergence of multicellular organisms. They were extremely widespread in many different aquatic environments. They must be clever to figure out what to avoid, where to eat, and the organism Everything else the body has to do in order to survive. I think it’s clear that they can do it in a complex way. “Gunawardena said.

I. Experiments after a century

More than 100 years ago, a famous American zoologist named Herbert Spencer Jennings described a complex and diverse avoidance behavior of single-cell freshwater protists called Stentor roeseli . But subsequent experiments failed to reproduce what he saw, and his claims were questioned and set aside.

Ten years ago, in a speech by British biologist Dennis Bray, Herbert Spencer Jennings published a special experiment in Behaviour of Lower Organisms in 1906 that caught Gunawardena’s attention.

Jennings was studying S. roeselii, a widely distributed freshwater protozoa . These single-celled organisms are known for their relatively large size and unique trumpet-shaped body. The creature’s surface and horn “bell” are covered with hair-like protrusions called cilia, which can be used to swim and create swirls in the surrounding liquid, sweeping food into their “mouths.” At the other end of their bodies, they hide a gripper that holds them to their surroundings, keeping them still while eating.

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S. roeselii (Source: Current Biology)

Jennings carefully recorded S. roeselii ‘s behavior when exposed to environmental stimuli (some powders) using a microscope, pipette, and stable gripper . Jennings observes a series of ordered behaviors. Normally, S. roeselii bends his body repeatedly to avoid the powder. If the irritation persists, it reverses the movement of the cilia and expels these powders with his mouth. If this method also fails, it shrinks and quickly tightens itself, like some invertebrates retract into their shells. In the end, if all the previous efforts failed, S. roeselii would break free and move quickly.

These behaviors constitute an orderly strategy in which single-celled organisms constantly change the way they respond. This observation suggests that this monocyte organism has some of the most complex behaviors known.

This experiment caused widespread interest, but subsequent repeated experiments, especially a study published in 1967, were unsuccessful. This also caused Jennings’ discovery to be largely questioned and forgotten by modern science.

2. Successful “part-time” projects

Just like a creature that originally lived in a completely livable puddle, the sudden powder irritation made Gunawardena uncomfortable, so he decided to follow the 1967 study.

The result surprised him that he found that those researchers who later repeated the Jennings experiment, because they could not find S. roeselii , used a different species, Stentor couleus , and this single-celled organism preferred to float around, and Not actively adsorbing food.

According to Gunawardena, it is not surprising that they failed to reproduce the experimental results. So he tried to accurately replicate Jennings’s experiment. But as a mathematician who manages a medical laboratory focused on molecular information processing, he finds it difficult to convince those around him.

“I’ve been coming up with this idea at my lab group meeting, saying it tells us something about single-cell capabilities. We no longer think about how cells work this way, as expected, no one is interested. This It’s ancient history, it’s descriptive biology, and all these young, smart students don’t want to touch. “Gunawardena said.

About eight years ago, Joseph Dexter, an undergraduate intern, was also attracted by the idea. He later became a doctoral student at Gunawardena and is now a researcher at the Neukom Institute for Computational Sciences at the University of Dartmouth. Gunawardena’s insistence eventually attracted Dexter’s colleague Sudhakaran Prabakaran, now the research team leader of the University of Cambridge in the United Kingdom, to the discovery.

Unable to curb their curiosity and sense of history, the three of them started a sideline project that lasted several years without formal funding.

Dexter and Prabakaran designed and experimented, and their first challenge was to find S. roeselii . They searched everywhere and even ran to search in the local pond. Eventually, they found a supplier in the UK who sourced organisms from a golf course pond and shipped them across the Atlantic.

The research team built an experimental device equipped with a video microscope and micro-positioning system to accurately deliver the stimulus near S. roeselii ‘s “mouth”. They initially used carmine powder but had little reaction. After repeated experiments, they found that micro plastic beads were effective.

To their delight, the three successfully reproduced all the behavior that Jennings had described a century ago.

However, they did not see the level of proficient and orderly behavior recorded by Jennings. On the contrary, there seems to be a considerable difference between these subjects, one sample may bend and change its cilia before contraction, another sample may only contract repeatedly, and the other will bend and contract alternately.

To further explore the reasons behind the phenomenon, the three rely on their core expertise as quantitative biologists to develop a method that encodes the different behaviors they see into a series of symbols, and then uses statistical analysis to find patterns.

The failure of the naked eye, but accidentally found through the mathematical knowledge of these three people, analysis shows that these single-celled organisms indeed have a progressive behavior strategy . When facing irritants, S. roeselii usually bends and changes its cilia at the same time. If the stimulus persists, it will shrink or break free, and then swim away. The latter behavior almost always occurs after the former behavior, and these creatures never break free without contracting first, which indicates that there is a priority order of behavior.

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In the face of irritants, S. roeselii first bends down, changes the ciliary beat, and expels the particles with the mouth (Source: Dexter et al, 2019)

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If bending and cilia are not altered enough, S. roeselii will contract, break free and swim (Source: Dexter et al, 2019)

“They do simple things first, but if you keep stimulating, they” decide “to try something else. S. roeselii has no brain, but there seems to be some mechanism, in fact, once the stimulus feels too long it will make it ‘change “The idea ‘.” Gunawardena said, “This hierarchical structure gives people a sense of image, making people feel that some kind of relatively complex decision-making calculations are being performed inside this single-celled organism, weighing which one action is performed versus the other. better.”

Third, new discoveries

By successfully repeating Jennings ‘experiments and making new quantitative observations of S. roeselii ‘s behavioral capabilities, the research team hopes it has resolved historical confusion about the accuracy of Jennings’ findings.

But the results now raise many new questions.

Analysis shows that for an individual S. roeselii , choosing to contract or separate has almost an equal probability. This is a particularly tempting clue for scientists studying how cells process information at the molecular level.

The researchers said that the decision between the two behaviors was consistent, with each organism independently tossing an unbiased coin regardless of previous behavior.

“In part, it’s a decision based on fair coin tossing at the molecular level, and I can’t think of any known mechanism that would allow them to do that. This is incredibly fascinating, Jennings It was never observed because it required quantitative measurements to reveal it, “Gunawardena said.

Video Stentor roeselii ‘s coping strategies in the face of external stimuli (Source: James Weiss / Harvard Medical School)

Observing that a single cell can perform complex behaviors is also of great value to other areas of biology. For example, in developmental biology or cancer research, the processes that cells go through are often called programs, Gunawardena says, which indicates that cells are “programmed” to do what they do. “But cells that exist in a very complex ecosystem that communicate and negotiate with each other to some extent, respond to signals and make decisions.”

According to Gunawardena, this experiment forces us to think about the existence of some form of cellular “cognition” in which a single cell can process complex information and make corresponding decisions. All life has the same foundation, and the latest findings provide us with at least one evidence of why we should broaden our horizons and incorporate this thinking into modern biological research.

“It also shows that sometimes we tend to ignore things, not because they don’t exist, but because we don’t think it’s necessary to focus on them. I think that’s why this research is so interesting,” Gunawardena said.

Orignal link:https://news.cnblogs.com/n/651598/