Endosymbiosis is an interesting biological phenomenon wherein one organism lives inside one other. Such an unusual relationship is usually 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 idea of mitochondria and thus the cells of plants, animals, and fungi.
What shouldn’t be yet well understood, nevertheless, is how an endosymbiosis as a life-style actually arises. A bacterium that kind of by accident leads to a totally different host cell often has a tough time. It must survive, grow and pass on to the following generation. Otherwise, it would 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 initially 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 way 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 following 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 following 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 recuperate and produce more dense but viable spores. As the researchers were in a position to show through genetic analyses, the fungus modified and adapted to its host through the experiment.
The researchers also found that the host co-produced biologically energetic molecules with its host that would help the host obtain nutrients and defend itself against predators equivalent to 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 chance to amass recent 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.