Rutgers researcher makes major evolutionary finding


endosymbiotic
Photo by Michael Makmur |

Researchers discovered further evidence supporting the endosymbiotic theory, which states that cells consumed cyanobacteria in the distant past, which led to some of the organelles modern cells use to function.


Rutgers researchers recently made a breakthrough regarding an aspect of evolution known as the endosymbiotic theory.

The endosymbiotic theory states that about 1.5 billion years ago, an ingesting host cell absorbed a bacterium and both organisms relied on each other for survival, according to the University of Utah website.

The term “symbiosis” refers to two different species developing a system of mutual benefit. Endosymbiosis refers to a similar relationship, but in which one species physically engulfs the other in the process, according to the site. 

This explains why energy-creating organelles like mitochondria and chloroplasts are found only inside cells. Both the ingested bacterium or endosymbiont and the host cell benefit one another through biochemical processes.

Dana Price, an associate research professor in the Department of Plant Biology and Pathology, is involved in studies that observe the endosymbiotic theory. Price found the original host cell involved in endosymbiosis.

“In the initial stages of organelle establishment, the host will provide the endosymbiont with a sheltered environment, and the two will exchange metabolites and other carbohydrates,” Price said.

There are more detailed biochemical processes that contribute to the change of endosymbionts into organelles, such as the exchange of genetic information from the engulfed cell to the host, he said.

“Over the course of hundreds of million years, genes that encode proteins that are critical for the endosymbionts’ survival transferred from its genome to the host nuclear genome in a process called endosymbiotic gene transfer,” he said. “Eventually, the host ‘takes control’ of the endosymbiont from a metabolic standpoint; at that point, you have an organelle.”

The endosymbiotic theory, like any other theory, is supported by substantial evidence. An example of this is chloroplasts, organelles involved in the process of photosynthesis for plants, he said.

If chloroplasts were taken out of plants and had their genomic DNA sequenced, the sequences would be highly similar to the genomes of free-living cyanobacteria that also turn light into energy. A similar concept also applies to mitochondria, Price said.

The sequencing of genomic DNA suggests today’s chloroplasts and free-living cyanobacteria share a common ancestor which shows how a cell could have engulfed a cyanobacterium in the past. A more recent and specific case of this can be illustrated by the freshwater amoeboid — Paulinella chromatophora, he said.

Between 80 to 100 million years ago, the amoeboid engulfed a cyanobacterium. Researchers are aware of this because close relatives of the species also consume cyanobacteria, he said.

“When we sequence their DNA we can still find fragments of the cyanobacterial prey in their food vacuole,” he said. “But in this case, P. chromatophora did not digest its food. It instead converted it to a plastid just like the event that took place 1.5 billion years ago.”

The ancestral relationship between chloroplasts and cyanobacteria creates the idea that the endosymbiotic theory was responsible for chloroplasts, and explains why they are encapsulated in cells as opposed to free-living in nature like today’s cyanobacteria, he said.

The symbiotic relationship between the host cell and the endosymbiont exists in humans as well with the help of successive stages within endosymbiosis. These successive stages are known as primary and secondary endosymbiosis, Price said.

Primary endosymbiosis, which occurred 1.5 years ago, is when one cell engulfs another cell. Secondary endosymbiosis occurs when a host cell that went through primary endosymbiosis becomes engulfed, he said.

“One event (of secondary endosymbiosis) created an immense group of photosynthetic ocean protists known as diatoms that produce large quantities of atmospheric oxygen,” Price said. “These events were paramount to the entire trajectory of Earth’s evolution and helped give rise to organisms such as mammals.”

With such complex interactions occurring between these microbes over the course of many years, scientists became curious as to why humans are unable to accomplish such a feat. Germ lines offer an explanation as to why these interactions do not persist through generations of humans, he said.

The human germlines include egg and sperm cells. Because the only two cells that matter when a zygote is formed are the egg and the sperm, human germlines are completely sequestered from non-sex cells, he said.

Any mutations or genomic aberrations that happen to cellular DNA in non-sex cells, like cancer, will not affect the sequestered germline, which means that it would not get passed onto the next generation. This is why the endosymbiotic theory applies mainly to single-celled eukaryotes, he said.

Although humans are unable to directly grab bacterial genes from the environment for our own benefit, the endosymbiotic theory does help us to understand the medical implications of certain diseases or infections, Price said.

Fighting diseases is difficult when there are multiple sets of DNA in a cell, said Vasudha Kumar, a first-year student in the School of Environmental and Biological Sciences.

“The mitochondria and chloroplasts have their own DNA. That’s important because when there’s an abnormal or infected cell, not only do you have to kill the bacterial DNA but you also have to kill the mitochondrial DNA when you’re using antibiotics,” Kumar said.

Krupali Kothari, a School of Arts and Sciences first-year student, said he believes this information will impact, not only the past, but the future of scientific research. 

“We can use this knowledge of cells absorbing organelles in order possibly manipulate cells in the future so we don’t die of certain illnesses,” Kothari said. “Life is very complex, so the way it came about fascinates me on how I am today. Because of interactions between these microbes, I am here today.”


Akhil Gumidyala

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