Friday, June 18, 2021

Symbiosis of 4, not 3


Cicada (Univ of Montana)

An evolutionary event, the development of a new species of bacteria, has been discovered and documented within the cells of the cicada, a press release out of the University of Montana, Missoula, trumpets.

It was previously known that two bacteria, Hodgkinia and Sulcia, lived inside the cells of a cicada. Because cicadas eat only tree sap, the bacteria work to produce the nutrients needed for survival. In exchange for their hard work, the bacteria enjoy a comfortable life and get to reproduce actively inside the cicada’s cells. Fun, right?

But a recent discovery in a UM research lab, published in the Aug 28 issue of the journal Cell, shows that three, not just two, separate species of bacteria live in a symbiotic relationship with the cicada. Each of the four organisms depends on the other three, since each one performs a part of the nutritional tasks.

A team led by UM microbiologist John McCutcheon took a deeper look at the bacterial genomes and found the Sulcia still inside the cicada’s cells, as they had predicted. But in addition, they found two different kinds of Hodgkinia.

“When we looked at the genes, they were clearly closely related to each other,” he said in a press release. “If there was a broken gene in one version of Hodgkinia, it would be complete and functional on the other and vice versa. So, the functional genes in each, when working together, seem to operate as one.”

Dr McCutcheon theorizes that the evolutionary development of a new bacteria species was the result of “slop and chance.” However, this accidental evolution may answer some questions about how other organisms have evolved and become more sophisticated over time.

In the case of the cicada, Hodgkinia’s speciation event added a new member to the symbiotic team, but it didn’t add any new functionality to the symbiosis. “This process parallels what is observed in some organelles, where massive genome expansions result in little if any observable increase in function,” Dr McCutcheon explained in the research paper, which is entitled “Sympatric speciation in a bacterial endosymbiont results in two genomes with the functionality of one.”


Three types of symbiosis are part of the science learning standards in most states: mutualism, parasitism, and commensalism.

In mutualistic symbiosis, all of the organisms involved get something positive out of the relationship. The most commonly cited example of mutualism is the relationship between bees and flowering plants. Bees get nectar to meet their nutritional needs, and plants get their pollen distributed so they can reproduce and propagate the species.

The symbiotic relationship between the cicadas and bacteria is mutualistic in nature, since all organisms benefit from the relationship and no organisms are harmed.

Parasitism, on the other hand, is a “symbiosis” in which one organism is harmed in the relationship and the other benefits. For example, fleas on a dog benefit, but the dog is harmed. Another example of parasitism is tapeworm inside a host.

You may have heard some people suggest parasitism isn’t really a form of symbiosis; they say it’s closer to a predator-prey relationship, although predators don’t actually live inside their prey. These people are basically incorrect: symbiosis just refers to organisms that live together, even if one is harming or killing the other in the process.

Plus, many mutualistic symbioses evolved from parasitic relationships. “Vertical transmission, defined as the direct transfer of infection from a parent organism to its progeny, has been suggested as a key factor causing reduction of symbiont virulence and evolution of mutualism,” wrote Motomi Genkai-Kato and Norio Yamamura of the Center for Ecological Reeearch at Kyoto University. That says, basically, that as parasites get transferred from host parent to host offspring, what was once a parasitic relationship evolves into a mutualistic one. Since the parasite gains no selective advantage by causing the death of its host, there’s evolutionary pressure on both organisms to make the symbiosis mutualistic.

Commensalism is kind of in between mutualism and parasitism. In a commensalistic symbiosis, one organism benefits and the other is neither harmed nor helped. One of the most common examples of commensalism is when mosses grow on trees. The moss benefits, and the tree is unaffected by the moss, which neither causes the tree harm nor improves its existence.

(A fourth type of symbiotic relationship is one in which one organism is unaffected and the other harmed or inhibited in some way. Known as amensalism, it’s perhaps the saddest interaction. “Streamside vegetation, for example, frequently is trampled by animals coming to drink,” writes the University of Texas, El Paso. “Those animals gain nothing from the trampling, but pity the poor plant under the hoof of an 800-pound cow!”)

Aside (Next Generation Science Standards)

Searches for “parasite,” “symbiosis,” and “mutualism,” using the search tool provided by the developers of the Next Generation Science Standards, here, found no matches.

It is my fervent hope that, as schools in Maryland and Illinois move forward with the implementation of the NGSS, they maintain instruction about these relationships that are vital to life on Earth.

What’s the coolest example of symbiosis you have learned about?

Paul Katula
Paul Katula is the executive editor of the Voxitatis Research Foundation, which publishes this blog. For more information, see the About page.

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