Unraveling the Mysteries of Endosymbiotic Relationships

Endosymbiosis refers to a remarkable biological process where one organism resides within another. This unique arrangement is frequently advantageous for both organisms involved. Evidence of this relationship exists even within human bodies: mitochondria, known as the energy producers of cells, are the result of an ancient endosymbiotic event.

Bacteria entered host cells and remained there. This mutual association eventually led to the development of mitochondria, which are now essential components of cells in plants, animals, and fungi.

What remains unclear is how endosymbiosis as a way of life initially develops. When a bacterium unintentionally finds itself inside a foreign host cell, it typically faces significant challenges. It must survive, reproduce, and be transmitted to future generations. Failing this, it will die.

Moreover, to avoid harming the host, the bacterium must refrain from consuming too many resources or growing too rapidly. Essentially, if the host and its internal partner cannot coexist harmoniously, the relationship will not endure.

To investigate the early stages of this unique relationship between two organisms, a research team led by Julia Vorholt, Professor of Microbiology at ETH Zurich, initiated similar partnerships in a controlled laboratory setting. The researchers closely observed the initial processes involved in the development of a potential endosymbiosis. Their findings were recently published in the scientific journal Nature.

Enforcing Cohabitation

For this study, Gabriel Giger, a Doctoral Student in Vorholt's lab, devised a technique to introduce bacteria into the cells of the fungus Rhizopus microsporus without damaging them. He worked with E. coli bacteria as well as bacteria from the genus Mycetohabitans, which are natural endosymbionts of a different Rhizopus species. However, for the experiment, the researchers selected a strain that does not naturally form an endosymbiosis. Giger then used microscopy to observe the outcomes of this artificially induced cohabitation.

After the injection of E. coli bacteria, both the fungus and the bacteria continued to grow. However, the bacteria eventually grew so quickly that the fungus initiated an immune response to defend itself. The fungus encapsulated the bacteria, effectively preventing them from being passed on to future generations of the fungus.

Bacteria Enter the Spores

This outcome differed from that of the injected Mycetohabitans bacteria. As the fungus formed spores, some of these bacteria were able to enter the spores, allowing them to be transmitted to the next generation of the fungus.

The fact that the bacteria are actually transmitted to the next generation of fungi via the spores was a breakthrough in our research.”

Gabriel Giger, Doctoral Student, ETH Zurich

When the doctoral student allowed the spores containing the resident bacteria to germinate, he observed that they germinated less frequently, and the young fungi grew more slowly compared to those without the bacteria.

The endosymbiosis initially lowered the general fitness of the affected fungi.”

Gabriel Giger, Doctoral Student, ETH Zurich

Giger extended the experiment over several generations of fungi, intentionally selecting those whose spores contained bacteria. Over time, this allowed the fungus to recover and produce more spores that housed bacteria while remaining viable. Genetic analyses revealed that, throughout the experiment, the fungus underwent changes and adapted to coexist with its bacterial resident.

The researchers also discovered that the resident bacteria, in conjunction with their host, produced biologically active molecules that could aid the host in acquiring nutrients and defending itself against predators like nematodes or amoebae. “The initial disadvantage can thus become an advantage,” notes Vorholt.

Fragile Systems

In their study, the researchers demonstrate the fragility of early endosymbiotic systems. “The fact that the host's fitness initially declines could mean the early demise of such a system under natural conditions,” adds Giger.

For new endosymbioses to arise and stabilize, there needs to be an advantage to living together.”

Julia Vorholt, Professor, Microbiology, ETH Zurich

A key requirement for this process is that the potential resident possesses traits that promote endosymbiosis. For the host, this presents a chance to gain new characteristics quickly by integrating another organism, even if it necessitates some adaptations. “In evolution, endosymbioses have shown how successful they ultimately can become,” adds the ETH professor.

Video Credit: ETH Zurich/from Giger G.H., et al, Nature 2024.