Endosymbiosis is an intriguing biological process in which one organism lives inside another. The relations between them are often symbiotic.
One such relation exists in our very bodies: mitochondria, the powerhouses of our cells. Originated from an ancient endosymbiotic relationship whereby bacteria invaded and colonized other cells. Giving rise to mitochondria, which are found in plants, animals, and fungi.
However, researchers don’t understand ways in which such endosymbiotic lifestyles arise . Whenever the host cell take a bacterium in, the fate of the bacteria is always uncertain.
To explore how these special relationships begin, a research team led by Julia Vorholt, Professor of Microbiology at ETH Zurich, formed such partnerships in the lab. They reported their findings in the scientific journal “Nature”.
Gabriel Giger, a PhD student in Vorholt’s laboratory, found a way to inject bacteria into the cells of the fungus “Rhizopus microsporus” without destroying them.
He used two types of bacteria: “E. coli” and “Mycetohabitans”.
Whereas “Mycetohabitans” are natural endosymbionts of another “Rhizopus” fungus, the researchers chose a strain that does not form endosymbiosis in nature. Giger then observed what happened during this forced cohabitation with the microscope.
Further growth of both the fungus and bacteria occurred after the injection with E. coli. However, this rapid growth of E. coli triggered an immune response in the fungus. The fungi encapsulated the bacteria, preventing their transfer to the next generation.
Injected Mycetohabitans bacteria, on the other hand, behaved quite otherwise. The fungus produced spores, but some of the bacteria could also pervade them and manage to progress into the next generation. Giger adds, “The fact that the bacteria are actually transmitted via the spores was a breakthrough in our research.”
When Giger let the spores with the resident bacteria germinate, he found they did so less often, and the young fungi grew more slowly than bacteria-free fungi.
“At first, endosymbiosis lowered the overall fitness of the fungi,” he said. Generation after generation, Giger tried selecting fungi whose spores had bacteria inside- the only way the fungus could recover and spew out more viable spores.
The scientists discovered that the resident bacteria and their host produce biologically active molecules. That allow the host to receive nutrition and protect themselves against predators. “This initial disadvantage can become an advantage,” Vorholt said.
Their work underlines the fragility of early endosymbiotic systems. Giger said, “The decline in host fitness may explain their frequent failure under natural conditions.”
If novel endosymbioses are to thrive, then a benefit must be derived from the partnership. Vorholt said, “A successful endosymbiosis can bring about enormous evolutionary advantages.”
ANI
