2023-07-09 20:00:00
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“If there is one thing that the history of evolution has taught us, it is that life has no barriers. Life expands into new territories and crashes into barriers painfully, perhaps even dangerously, but… life always finds a path”. With these words the doctor defined life Ian Malcolmthe talented mathematician specializing in chaos theory played by the actor Jeff Goldblum in Jurassic Park.
In the real world; in the laboratory of the evolutionary biologist Jay T. Lennon, professor in the Department of Biology in the College of Arts and Sciences at Indiana University Bloomington, there are no velociraptors. Neither are tyrannosaurs or other dreamy dinosaurs. However, what Lennon and his team have just shown is that, as Malcolm expressed in the film, life always finds a way, no matter how surprising or complicated it may seem.
What Lennon’s research team has been studying are the so-called minimal cells, a purely theoretical concept of which we cannot find examples in nature, but fundamental in synthetic biology, and which can be defined as a cell endowed with the minimum number of genes for it to function in the unlimited presence of nutrients. In other words: a cell artificially reduced to its minimum functional genetic expression.
Tom Deerinck & Mark Ellisman / National Center for Imaging and Microscopy Research / UC San Diego.
The simplified bacterium M.Mycoides contains less than 500 genes.
However, what is exceptional about this type of cell, and what Lennon’s team has just discovered, is that can evolve as much or faster than a normal cell, demonstrating, however simple they may be, the adaptability of organisms, even those with a non-natural genome that would apparently provide little flexibility. The results of it are collected in a study recently published in the journal Nature under the title Evolution of a minimal cell.
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“There seems to be something in life that is really solid,” Lennon declares. “We can simplify it down to the essentials, but that doesn’t stop evolution from working.” To reach this conclusion, the team led by the scientist studied the synthetic organism Mycoplasma mycoides JCVI-syn3B, a minified version of the parasitic bacterium M. mycoides, which is commonly found in the entrails of goats and other ruminants.
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Over millennia, this bacterium, like other parasitic organisms, has naturally lost many of its core genes as it evolved to depend on its host for nutrition.
Researchers at the J. Craig Venter Institute in California decided to go a step further, and in 2016 removed 45% of the 901 genes from the natural genome of M. mycoides, narrowing it down to the smallest set of genes required for autonomous cell life. With 493 genesthe minimal genome of M. mycoides JCVI-syn3B is the smallest of all known free-living organisms. A fairly simple one if we take into account that many animal and plant genomes contain more than 20,000 genes.
In principle, an organism as simple as Mycoplasma mycoides JCVI-syn3B would possess only the minimum number of genes essential for life. This would have the hypothetical disadvantage that any mutation in said organism could be lethal to the cells, assuming restrictions on evolution. “Organisms with optimized genomes have fewer targets for positive selection to act on, limiting opportunities for adaptation,” Lennon explains. “In the genome of M. mycoides JCVI-syn3B each gene is essential,” he continues. “One might assume that there is no leeway for mutations, which could limit their potential to evolve.”
With 493 genes, the minimal genome of M. mycoides JCVI-syn3B is the smallest of all known free-living organisms.
M. mycoides JCVI-syn3B, like other minimal cells, can grow and divide under laboratory conditions efficiently; However, Lennon and his colleagues wanted to know how a minimal cell would respond to the forces of evolution in an environment with limited raw materials and on which natural selection could operate.
To find out, the researchers grew the strain of M. mycoides JCVI-syn3 in the lab and allowed it to evolve freely for 300 days, the equivalent of 2,000 bacterial generations or about 40,000 years of human evolution. They then compared the results with cultures of M. mycoides original, finding that the minimal bacterium that had evolved for 300 days fared much better, regaining all the fitness it had lost due to genome rationalization.
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The researchers then identified the genes that had changed the most during evolution. Some of these genes were involved in the construction of the cell surface, while the functions of several others remain unknown. Research by Lennon and his team demonstrates the power of natural selection to rapidly optimize fitness in the simplest autonomous organism and its implications for the evolution of cellular complexity. In other words, he proves that life always finds a way.
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