Unveiling Endosymbiosis: The Key to the Evolution of Complex Life

By Jenny Tze Wai Chen,

Biology and Chemistry Columnist; The Lawrenceville School, NJ  

The main component of the termite diet is wood, yet they cannot digest it on their own, at least not exactly. In fact, they have what is known as an endosymbiotic relationship with bacteria in their gut that help them with the digestion process. These termites and their symbionts have relied on each other for the past 250 million years, and one cannot live without the other.

Endosymbiosis, a fascinating alliance in nature where the symbiotic organism lives inside of the host organism, explains how modern eukaryotic cells were once formed. Free-living aerobic prokaryotic bacteria were engulfed by eukaryotes, developing an endosymbiotic relationship, and gradually formed the mitochondria that provides energy to the cell. By that same process, chloroplasts were developed because a certain photosynthetic prokaryote entered an endosymbiotic relationship with its host.

Forming a conjoined metabolism system, primitive endosymbiosis provided the foundation for plant and animal cells, as well as the course of evolution, as the partnership faces natural selection as an entity. The symbiont cannot multiply too fast as it will have to battle the host’s immune system, but it also has to ensure that the generations after are germinated, whilst maintaining synchronized growth. Once a stable endosymbiosis is established, both parties benefit from one another, with the host cell unlocking new sources of energy or biochemical capabilities and the symbiont having a stable supply of nutrients.

But how did such a relationship even first begin a billion years ago? In October of 2024, microbiologist Julia A. Vorholt and her team at Swiss Federal Institute of Technology in Zurich sought to find an answer. The experiment consisted of bacterial cells being injected into host cells using a 500–1,000-nanometre-wide needle to allow the passage of bacteria while minimizing wound size (Giger et al.). Although initially the symbionts divided too rapidly which killed the host cells, the team soon found that Rhizopus microsporus, a fungal plant pathogen, was receptive to Mycetohabitans rhizoxinica, an endofungal bacterium which produces rhizoxin, a type of mycotoxin (toxins produced by fungi) that inhabits plant mold.

To combat the thick cell walls and the difference of pressure inside the cell, an enzyme mixture in osmoprotective solution was used to soften the host cell’s walls and an air compressor was used to up the pressure to 6.5 bar.

The scientists injected about 1-30 bacteria labelled with green fluorescent protein into each host cell, then cultured the germlings on agar plates until they began to produce spores. Through fluorescence-activated cell sorting (FACS), the scientists found that a portion of spores produced by implanted host cells were indeed colonized by fluorescent bacteria.

The findings of Vorholt and her team have strong implications for endosymbiosis theory, while also inciting new areas of research. First of all, the experiment showed that M. rhizoxinica may be carried on to new generations of R. microsporus spores; more broadly, a vertically transmitted bacterium to a host, a newly created endosymbiosis, can be passed onto the next generation, and the next. Next, an adaptive laboratory evolution experiment was performed using FACS technology – only selecting fluorescent spores to propagate (positive selection), by the 10th generation of R. microsporus, “the germination success of the [spores that contained bacteria] had increased to 75%” and “was no longer distinguishable from that of spores without bacteria” (Giger et al.).

Thus, this example of a successful single-cell implantation shows us that artificially induced endosymbiosis can potentially be used to engineer purposeful cell to cell partnerships. The experiment also clarifies that factors like adaptive evolution are key to the evolution of stable endosymbiogenesis.

With this new understanding of endosymbiosis, designing organisms with desired traits, such as ones that may benefit from consuming carbon dioxide or nitrogen, can be the just the right strategy for environmental restoration. “That’s the idea,” says Vorholt, “to bring in new traits that an organism doesn’t have, and that would be difficult to implement otherwise.”

Works Cited

Callaway, Ewen. “Bacteria Implanted into Fungi Offer Clues to the Origins of Complex Life.” Nature, Nature Portfolio, Oct. 2024, https://doi.org/10.1038/d41586-024-03224-5. Accessed 16 Feb. 2025.

Giger, G.H., Ernst, C., Richter, I. et al. “Inducing Novel Endosymbioses by Implanting Bacteria in Fungi.” Nature, 635, 415–422, Springer Science and Business Media LLC, Oct. 2024, https://doi.org/10.1038/s41586-024-08010-x.

rubenbristian.com. “Termite Symbiotic Protists & Prokaryotes – Stage One Study. | Protists.” Protozoa.com.au, 2021, protozoa.com.au/termite-symbiotic-protists-prokaryotes/. Accessed 16 Feb. 2025.