How did life evolve a billion years ago?

Until now, no one knew exactly how evolution into more advanced cells containing 'organs' (organelles) and plant cells occurred including the use of sunlight. Now, for the first time, Swiss scientists have created a model of how this happened a billion years ago. Using a nanoneedle and a bicycle pump, they created fungal cells containing symbiotic bacteria.

“This feat could help scientists understand the beginning of the symbiosis that gave rise to certain organelles such as mitochondria (“powerhouses in cells,” note) and chloroplasts (structures that use sunlight, note) more than a billion years ago,” he said. . The results of the research were presented in the scientific journal “Nature” last week.

The basis for this result was the scientific work of microbiologist Gabriel Geiger from the Swiss Federal Institute of Technology (ETH) in Zurich and his co-authors under the leadership of Julia Voorholt. They published their study on “Creating a new endosymbiont by transplanting bacteria into fungi” in the journal Nature.

In biology, “communities” composed of different organisms are well known and occur relatively frequently. For example, there is endosymbiosis between bacteria and insects. “Endosymbiotic associations, where a microbial partner lives harmoniously inside the cells of another organism, are present in many life forms, including insects and fungi,” Nature said.

However, such events played a decisive role in the emergence of life on Earth. “Scientists are convinced that mitochondria, the organelles responsible for producing energy in cells, descend from bacteria that transplanted themselves into the precursors of eukaryotic cells. Chloroplasts (the structures that contain chlorophyll for using sunlight; note)” evolved as plant precursors evolved, the scientific journal wrote: “The cells absorbed microorganisms that use photosynthesis.” During evolution on Earth, endosymbiosis events can lead to new biochemical capabilities of organisms, leading to species diversity and innovation in biology.

These theses are very reasonable. However, it has not yet been possible to understand how processes lead to the establishment of such endosymbiosis. After all, cells containing mitochondria and plant cells appeared on Earth more than a billion years ago. The organisms that lived at that time have long since disappeared as a result of evolution.

The results of microbiologists at ETH are becoming clearer (https://doi.org/10.1038/s41586-024-08010-x). Using the finest needles (500 to 1,000 billionths of a meter thick), they injected the bacterium Mycetohabtans rhizoxinica into fungal cells of the species Rhizopus microsporus. Then they observed the emergence of endosymbiosis. The experiments continued for years. In order to introduce up to 30 bacterial cells into fungal cells per injection, scientists had to use several tricks. To overcome the internal pressure of the fungal cells, this was done first with the help of a bicycle pump, and then with the force of a compressor.

The bacteria are classified so that they fluoresce under a certain light. At first there was little success. Some fungal cells died because the microbes inside multiplied too quickly and killed the host organism. Other experiments failed because the microbes could not survive. They turn off the fungal cells' immune system.

Finally, scientists succeeded in filtering and propagating the cell structures of symbiotic fungal microbes. After ten rounds of replication, 75% of these fungal cells were as “viable” as bacteria-free fungal cells. The proportion of additional generations of fungal cells generated by spores with bacteria increased from 0.01% to 24.1%.

This means that fungi must somehow reconcile bacteria with their symbiotic “partners,” just as the ancestors of plant cells likely did with the spores and chlorophyll they absorbed. Finally, scientists investigated the question of how fungal cells and bacteria adapt to each other. Interestingly, according to the experts: “We did not find any mutations in bacteria in our evolution experiment.” However, nine genetic mutations have been discovered in the genome of spore-tolerant fungal cells, which appear to be related to the evolution of endosymbiosis. Its role is still unclear.

“Overall, the (gene) sequence analyzes confirmed that genetic changes during this adaptive evolution experiment occurred on the host organism side and not at the endosymbiont level,” Geiger and his co-authors wrote. It would therefore be entirely possible that the precursors of all photosynthetic plant cells evolved endurance after spores combined with chlorophyll so they could use this mechanism to produce energy. It is possible that something similar happened with absorption by organisms that gave rise to the efficient power plants of higher cells more than a billion years ago.