Secondone
(Is there a common rule for a single amoeba transforming into a multicellular organism, birds flying beautifully in formation, fish swimming together, ants moving as a whole, humans walking orderly on sidewalks, and the smooth flow of vehicles?)
Malcolm W. Brown
When times are good, when there is plenty to eat, the bacterium Dictyostelium discoideum lives as a single cell. It lives by consuming any food that comes near its surroundings.
But if a famine or disaster occurs, something amazing happens. These amoebas release certain chemicals and connect with other amoebas nearby. The amoebas extend their pseudopods to approach and adhere to each other. As a result of this adhesion, the amoebas form a multicellular organism that can sense light and heat, and travel at a speed greater than that of a single amoeba.
These aggregate cells split into two distinct forms in the animal’s body: as rods and as protected seeds. In this famine-stricken Dictyostelium, ready to sow its dormant seeds into the air, awaiting favorable conditions for survival.
This has been known for many years. But a team of scientists at the University of California, San Diego, have discovered a new way to study this amoeba. They have also developed a mathematical model that calculates the behavior of the amoeba.
The ability to switch between single-celled and multicellular life is not unique to Dictyostelium amoebas. Mixobacteria, yeast, and marine sponges possess this ability. But the Dictyostelium amoeba has a special attraction: it is slightly easier to study. Studying this amoeba provides fundamental knowledge about other multicellular organisms (including humans), says Dr. William Loomis of the University of California.
In a recent article published in journal Physical Review Letters, Dr. Loomis and Dr. Herbert Levine jointly discuss how this amoeba survives and overcomes famine, hunger, and difficult times with efficient strategies.
Dr. Loomis says in an interview: “Once you start learning about this organism, your desire to learn more ignites. Its life is easy to know. When I started in 1970, I thought I would know everything within 10-15 years.
In 1990, I realized that there was still much to learn.”
Many scientists are eagerly studying how a single organism changes into a multicellular organism. For example, how does a single cell, without caring about other organisms of the same species, suddenly join together to form a new multicellular organism like higher organisms (birds, humans), and become self-aware?
Is there a common rule for this single amoeba transforming into a multicellular organism, birds flying beautifully in formation, fish swimming together, ants moving as a whole, humans walking orderly on sidewalks, and the smooth flow of vehicles?
This group from San Diego realized that studying these amoebas in their natural habitat presented a significant challenge. Therefore, they created a two-dimensional environment for the amoebas to live in – between two glass slides. They ensured no external food sources reached them by sealing with agarose.
The result was a beautiful window into the life of *Dictyostelium* amoeba.
Within a short period, these food-deprived *Dictyostelium* amoebas began to transform their single-celled biology. They changed into stalk cells and seed cells.
“Stalk cells extend like tiny towers from the ground, scattering their seeds into the air,” says Loomis. “By differentiating in this way, they perform two different functions. The stalk-like cells propel the globe-like cells in various directions.”
But Loomis discovered even more surprises when looking through the two-dimensional window. These independent cells had joined together to form a thin pancake-like multicellular organism.
After the pancake formed, another surprise occurred. It began to rotate on its own, spinning for about 10 hours.
“The remarkable thing is that this rotation happens without any external control,” Loomis said. And in the middle of the pancake was a hole, like a bullseye.
“These holes are interesting. You see similar holes among flocks of birds and schools of fish. Perhaps there’s some common principle at play,” Loomis explained.
A few simple rules can explain this phenomenon. If one fish gets too close to another, it slightly veers away. Many fish doing the same thing create a swirling motion in the school. Because of a limit on how much a fish’s spine can bend, the circle doesn’t shrink beyond a certain point. When it can’t grow any smaller, a hole forms in the middle of the group.
Similar principles explain the movement of different organisms with similar structures. Scientists are attempting to explain these behaviors using mathematical models.
Essentially, *Dictyostelium* single cells (and similar organisms) are constantly changing shape. They use pseudopods to move and position themselves as needed. They secrete sticky proteins to adhere to each other.
If one cell tries to cross another, both cells adjust and merge. As this process happens repeatedly, it begins to swirl. This action helps determine which of its cells needs to change in what way, Loomis believes.
But the cells on the outer edge of the pancake couldn’t keep up with the rotation of the inner cells, falling behind and forming a swirling star-like pattern when viewed under a microscope.
Using computer simulations, they could replicate the behavior of changing cells, compressing them together, causing them to transform and move randomly. After 100 to 1000 iterations, they formed a pancake shape and began to rotate.
Although the mechanisms for forming star systems and amoebic aggregates might seem similar, scale differences make this comparison unlikely. “You can’t really compare what happens at micron scales with what happens over billions of years,” Loomis says.
But the study of *Dictyostelium* has become a powerful tool in biological research.”This helps to study the common characteristics among all unicellular organisms,” says Loomis. “Dictyostelium is very helpful in a well-ordered physical analysis. It has engaged me for 30 years and still sparks interest.”
-Translation: Ko Rajarama, Tukaram Gopalrao