#holobiont

As biologist Scott Gilbert and his colleagues write, “Almost all development may be codevelopment. By codevelopment we refer to the ability of the cells of one species to assist the normal construction of the body of another species.” This insight changes the unit of evolution. Some biologists have begun to speak of the “hologenome theory of evolution,” referring to the complex of organisms and their symbionts as an evolutionary unit: the “holobiont.” They find, for example, that associations between particular bacteria and fruit flies influence fruit fly mating choice, thus shaping the road to the development of a new species. To add the importance of development, Gilbert and his colleagues use the term “symbiopoiesis,” the codevelopment of the holobiont. The term contrasts their findings with an earlier focus on life as internally self-organizing systems, self-formed through “autopoiesis.” “More and more,” they write, “symbiosis appears to be the ‘rule,’ not the exception. . . . Nature may be selecting ‘relationships’ rather than individuals or genomes.”

I'm fascinated by microbiomes, the communities of bacteria, unicellular fungi, and other tiny living beings that populate various parts of animal bodies. We literally can't function without them; our gut micriobiome, for example, is crucial to our ability to digest food.

It's long been thought that the brain was kept sterile by the blood-brain barrier. However, this recent study shows that fish have a brain microbiome, so it's not entirely out of the realm of possibility we might, too. It's tough to study, because accessing the brain opens up the risk of outside contamination confounding the results of the study.

But if it is true, the implications could be huge. There's already a growing body of evidence that our gut microbiome has a significant impact on our mental health and mood through what's known as the Gut-Brain Axis. This is a series of connections between these parts of the body that includes the nervous, endocrine, immune, and other systems. Given there's about one microbial cell for every human cell in our bodies, it wouldn't be at all surprising that our microbial communities have a bigger influence on our lives than we think.

Y'all, I am SO excited about this!!!! I've long been fascinated with holobionts, which are basically miniature ecosystems that revolve around one relatively large being essentially acting as a habitat for a whole host of tinier ones in a generally symbiotic relationship. A lichen is (at least to our current understanding) a relatively simple version thereof, with a mycobiont (fungus) creating structure for photobionts (algae and/or cyanobacteria) and, more recently, unicellular yeast--and possibly others we haven't discovered yet.

But our body's microbiomes also make us holobionts! Our skin, guts, and vaginal flora are all a part of our bodies as much as our cells, and often outnumber the human cells in our systems. We are walking around with veritable nations of microscopic beings living out generation after generation in and on our bodies, and without them we would cease to exist. (Though they would outlast us a while in the event of our death.)

What's really cool about this study is that it demonstrates how it's not just our own human genes that shape our evolution, but the ever-changing genomes of our microbiome residents as well. It really pokes holes in the reductionist idea that we are biological islands that grow and develop independently of other beings, and adds a whole new set of layers to us being part of wider, interconnected ecological systems.

Holobiont in biology refers to “an organism plus its persistent communities of symbionts” (Gilbert 2017: M73) and was used for decades by scientists focusing on plants.

How to be a good Holobiont? Relating during the COVID-19 pandemic by Sabine Biedermann | EASST Review 39(1) 2020