Endosymbiosis is an interesting biological phenomenon through which one organism lives inside one other. Such an unusual relationship is commonly useful to each parties. Even in our bodies, we discover remnants of such symbiosis: mitochondria, the powerhouses of our cells, evolved from an ancient endosymbiosis. Long ago, bacteria entered and took up residence in other cells. This coexistence formed the premise of mitochondria and thus the cells of plants, animals, and fungi.
What is just not yet well understood, nevertheless, is how an endosymbiosis as a life-style actually arises. A bacterium that roughly unintentionally leads to a very different host cell normally has a tough time. It must survive, grow and pass on to the subsequent generation. Otherwise, it should end. And so as to not harm the host, it must not claim too many nutrients for itself and grow too quickly. In other words, if the host and its resident cannot get along, the connection ends.
To study the beginnings of such a special relationship between two organisms, a team of researchers led by Julia Verholt, professor of microbiology at ETH Zurich, began such partnerships within the laboratory. Scientists observed exactly what happens firstly of a possible endosymbiosis. They have just published their study in a scientific journal.
Enforcing Coherence
For this work, Gabriel Geiger, a doctoral student in Voorholt's laboratory, first developed a technique to inject bacteria into the fungus cells without destroying them. He used bacteria on the one hand and bacteria on the opposite. The latter are natural endosymbionts of one other fungus. For the experiment, nevertheless, the researchers used a strain that doesn’t form endosymbiosis in nature. Geiger then observed what happened to the forced accommodation under the microscope.
After the injection of the bacteria, each the fungus and the bacteria continued to grow, eventually so rapidly that the fungus developed an immune response against the bacteria. The fungus protected itself by encapsulating the bacteria. This prevented the bacteria from being passed on to the subsequent generation of bacteria.
Bacteria enter the seeds.
This was not the case with the injected bacteria: while the fungus was forming spores, some bacteria managed to enter them and thus be passed on to the subsequent generation. “The fact that bacteria are actually transmitted to the next generation of fungi through spores was a breakthrough in our research,” Geiger says.
When the doctoral student allowed spores to grow with the resident bacteria, he found that they germinated less often and the young fungi grew more slowly than without them. “Endosymbiosis initially reduced the general fitness of the infected fungus,” he explains. Geiger continued to experiment on several species of fungi, deliberately choosing fungi that had bacteria of their spores. This enabled the fungus to get better and produce more dense but viable spores. As the researchers were capable of show through genetic analyses, the fungus modified and adapted to its host throughout the experiment.
The researchers also found that the host co-produced biologically lively molecules with its host that might help the host obtain nutrients and defend itself against predators reminiscent of nematodes or amoeba. “An initial disadvantage can thus become an advantage,” stresses Vorholt.
Critical system
In their study, the researchers show how fragile early endosybiotic systems are. “The fact that the initial decline in host fitness could mean an early death of such a system under natural conditions,” Geiger says. “For a new endosymbiosis to develop and stabilize, there must be an advantage to living together,” says Verholt. The condition for that is that potential residents bring with them characteristics that favor endosymbiosis. For the host, it's a possibility to accumulate latest traits by incorporating one other organism in a single fell swoop, even when it requires adaptation. “In evolution, endosymbioses have shown how successful they can eventually be,” emphasizes the ETH professor.