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A Billion Years Later: The Merging of Two Distinct Species
Last month marked a significant event from an evolutionary biology standpoint, one not witnessed in over a billion years, as claimed by experts. In a laboratory setting, two distinct species, an algae and a bacterium, merged to form a single organism –a phenomenon termed endosymbiosis. It is believed that the last occurrence of such an event took place when plants began to proliferate across the Earth. The findings of this research were recently published in the journals Cell and Science.
What exactly is symbiosis?
Symbiotic relationships, or symbiosis, denote enduring biological associations of paramount significance between two different species. The term stems from the Greek words “syn,” meaning “together,” and “biosis,” meaning “living.” In these associations, one party may benefit at the expense of the other (parasitism), both parties may mutually benefit (mutualism, as exemplified by ants protecting aphids in exchange for sap), or one may benefit while the other remains unaffected (commensalism, such as plant seeds like burrs hitching rides on animals for dispersal). The participants involved in such relationships are called symbionts (and hosts in some cases). Such relationships may be vital for some species to survive, or may arise incidentally when species happen to encounter each other.
The definition of symbiosis has been a subject of debate for over a century. German biologist Albert Bernhard Frank, who first coined the term mycorrhiza in 1877, used it to describe lichens. In 1878, German mycologist Heinrich Anton de Bary defined symbiosis as the “co-living of different organisms.” The term may carry varied interpretations among scientists; some contend it should exclusively describe exploitative relationships, while others argue it encompasses all three types of associations mentioned earlier.
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Symbiotic relationships are also categorised based on the physical proximity of the involved parties. Ectosymbiosis, derived from the Greek “ecto,” meaning “external,” denotes associations where one organism resides on the host’s body, such as on its skin or the surface of its internal organs (for instance, remora fish that feed on leftovers on sharks). Endosymbiosis, from the Greek “endo,” meaning “internal,” pertains to instances where one party dwells inside or between the host’s cells.
What is endosymbiosis?
The new discovery is an example of endosymbiosis. In this type of association, the host organism, serving as the carrier, is unable to access certain nutrients provided by the symbiont. Consequently, it produces specialised cells within its body to foster a suitable environment for the symbiont and modifies its genetic makeup to ensure this trait is passed on to subsequent generations. For the symbiont, this transformation is even more profound. It loses most of its genes related to functions like digestion and DNA repair, drastically reducing its genome size. Nevertheless, it retains genes associated with genetic transcription (the creation of messenger RNA using DNA as the template), replication, and protein synthesis. Endosymbiosis is regarded as a pivotal biological advancement that has shaped the course of evolution, facilitating the emergence of complex cellular structures and multicellular life forms. We’ll delve into “why” shortly.
All living organisms on Earth are categorised into two groups based on whether their nucleus components are enveloped within membranes: prokaryotes and eukaryotes. Prokaryotes, derived from the Greek “pros,” meaning “first,” and “karyon,” meaning “nucleus,” are typically single-celled, considered primitive organisms, and lack a membrane-bound nucleus (examples include viruses, bacteria, and blue-green algae). Eukaryotes, from the Greek “eu,” meaning “good,” and “karyon,” possess a distinct cell nucleus enclosed with a membrane, along with other membrane-bound organelles. Eukaryotes are not necessarily limited to a single cell as their cells can differentiate into various structures serving diverse functions, such as muscle, nerve, and reproductive cells, thereby facilitating the existence of complex multicellular organisms with organs, systems, and metabolic processes (examples include plants, animals, and fungi).
*The term “cell” was derived from the Latin “cellula,” meaning “small room,” coined by the naturalist Robert Hooke in 1665.
With this background information, let’s discuss why the theory of endosymbiosis is significant from an evolutionary perspective. To our knowledge, such leaps have occurred only three times in the past, each heralding breakthroughs in evolution. The initial instance unfolded approximately 2.2 billion years ago when a prokaryote engulfed a bacterium. Over countless generations of evolutionary progression, the bacterium integrated into the host, metamorphosing into what we now recognise as mitochondrium within the cell. Mitochondria, as you might recall from high school biology lessons, are organelles responsible for generating energy (ATP – Adenosine Triphosphate) in cells. They share striking resemblances with bacteria in terms of size, shape, ability to divide and multiply, and possession of their own ribosomes, DNA, and RNA. These features support the endosymbiotic theory, and the similarities between mitochondria and bacteria has captivated scientists since the late 1800s.
This event is now called the primary endosymbiosis. As one might imagine, such an event is exceedingly rare even over billions of years of evolutionary history. According to Tyler Coale from the University of California, “the first occurrence of this event enabled the emergence of all complex life forms.” When it occurred again approximately a billion years ago, it resulted in the chloroplast organelle, which facilitated photosynthesis, thereby enabling the existence of plants -known as secondary endosymbiosis. In this process, it is believed that a complex organism assimilated a single-celled organism called cyanobacterium (blue-green algae). Cyanobacteria are able to perform photosynthesis, so when they became part of a more complex organism, they contributed their photosynthetic ability. This led to the emergence of plants, which rendered our planet habitable for all of us.
The first person to propose the theory of endosymbiosis was Lynn Margulis, a scientist whose ideas were initially considered radical but later gained widespread acceptance. In 1970, she published her book “Origin of Eukaryotic Cells,” wherein she introduced the theory of endosymbiosis, which faced considerable criticism at the time. However, the theory gained acceptance over time and is now widely recognized in scientific circles. Margulis, likely accustomed to criticism by then, further elaborated on her theory in her 1981 book “Symbiosis in Cell Evolution,” positing that another endosymbiotic event occurred afterward; a bacterium called spirochete evolved into the transportation system of the nucleated cell. Throughout her career, she developed similar endosymbiotic theories, collectively known as the Serial Endosymbiotic Theory (SET).
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The endosymbiotic event mentioned in the recently published article involved the merging of an algae and a cyanobacterium. The algae (Braarudosphaera bigelowii), thanks to this relationship, can convert atmospheric nitrogen into ammonia for use in various cellular processes. Researchers have suspected since 1998 that there was a symbiotic relationship between B. bigelowii and a bacterium named UCYN-A, wherein the bacterium provided nitrogen to the algae, and the algae protected the bacterium. However, by observing parallel growth between the two organisms, they discovered a tighter metabolic relationship, suggesting that UCYN-A might be an organelle. Using advanced X-ray imaging technology, they found that the replication and cell division processes of the host algae and UCYN-A were synchronized as well, providing evidence that these two were engaged in an endosymbiotic process. Additionally, by isolating and comparing the proteins produced by both organisms, the researchers found that the bacterium could only produce half of the proteins it needed, relying on the host’s proteins for survival.
The research team claims that UCYN-A has fully transformed into an organelle, which they now call a nitroplast. They note that the process began at least 100 million years ago, reminding us that this is still a relatively short period on an evolutionary scale. Despite many questions remaining about the relationship between these two organisms, it appears that this development is recognized as significant for evolutionary biology by the entire scientific community. However, beyond theoretical knowledge, the nitroplasts’ ability to convert atmospheric nitrogen into ammonia holds potential for revolutionary developments in agriculture.
REFERENCES
- 1. https://www.popsci.com/science/two-lifeforms-merged-into-one/
- 2. https://www.livescience.com/planet-earth/microbiology/scientists-discover-1st-of-its-kind-cell-part-born-from-a-swallowed-microbe
- 3. https://acikders.ankara.edu.tr/pluginfile.php/80732/mod_resource/content/0/Ders%202%20H%C3%9CCRE.pdf