from AEON Website
Photo courtesy Wikimedia.
Cicadas might be a pest, but they're special in a few respects.
For one, these droning insects have a habit of emerging after a prime number of years (7, 13, or 17). They also feed exclusively on plant sap, which is strikingly low in nutrients.
To make up for this deficiency, cicadas depend on two different strains of bacteria that they keep cloistered within special cells, and that provide them with additional amino acids.
All three partners - the
cicadas and the two types of microbes - have evolved in concert, and
none could survive on its own.
The concept (Holobionts and the ecology of organisms - Multi-species communities or integrated individuals?) has taken off within biology in the past 10 years, as we've discovered more and more plants and animals that are accompanied by a jostling menagerie of internal and external fellow-travelers.
Some of the microorganisms kill each other with toxins, while others leak or release enzymes and nutrients to the benefit of their neighbors. As they compete for space and food, cohabiting microbes have been found to affect the nutrition, development, immune system and behavior of their hosts.
The hosts, for their
part, can often manipulate their
resident microbiota in many ways, usually via the immune
...and might even be carrying larger organisms such as worms and fungi as well.
The future direction of
medical science could very well hinge on the answer.
Margulis thought that
there was a tight analogy between an egg and a sperm
coming together to form a new organism, and the coming together of
two species to form a new symbiotic consortium, which she called a
But instead of sending
out tiny cells to reproduce, holobionts send out individual
organisms of different species.
But the word took on a very different cast in the hands of the American coral reef biologist Forest Rohwer and his colleagues, who defined a holobiont as,
Two protagonists just aren't enough when it comes to explaining the evolutionary success of corals.
They are made up of clusters of polyps, tiny wiggling things that get by with just a few tentacles and a toothless maw. Coral polyps reproduce by cloning themselves, and then sticking together to form large colonies, supported by a jointly fashioned skeleton.
The most spectacular corals work hand-in-hand with photosynthetic algae that they host within their own cells.
The algae provide nutrients via photosynthesis, while the coral gives the algae both food and protection. And those simple little polyps don't end their symbiotic relationships there.
Corals don't possess a complicated immune system to fend off pathogens; instead, they seem to selectively cultivate helpful or benign bacteria, which crowd out the harmful microbes.
Corals also produce mucus
that appears to be able to trap phages, viruses that infect and kill
only bacteria. An enemy of an enemy is a friend, after all.
For a time, the ecological account prevailed.
But Margulis's physiological conception of holobionts was revitalized in the late 2000s as part of a new theory:
Advocates merged both versions of holobiont into something a bit more conceptually loaded.
On this view, the ecological notion of holobiont (the host and all its resident microbes) is given additional properties. It's an entity that's coherent enough to have its own hologenome, made up of the host genome plus all the microbial genomes.
A major implication of this theory is that natural selection doesn't just act on the genome of individual organisms:
Today, researchers engage in fierce debate over which forces shape holobionts and host-microbiome systems.
They can be roughly split into two factions,
On the ecological side, holobionts are seen as complex and dynamic ecosystems, in constant flux shaped by individual interactions from the bottom up. So you are part of a holobiont.
stands in opposition to the
evolutionary account, which casts holobionts as higher-level
entities akin to organisms or units of selection, and believes that
they are shaped as a whole from the top down. On this view, you are
Evolutionary theory predicts that the parts of a unit of selection will tend to cooperate:
Ecological theory, by contrast, predicts competition and exploitation:
Think of the differences
between an ant colony and a motley assortment of
insects fighting over scarce resources.
If the ecological account of holobionts is true, a human host is more like a habitat to be managed, with the right balance and competition between different kinds of microbes being an important consideration.
What counts as healthy can depend on what kinds of services we want out of our attendant ecosystem. If the microbes in a holobiont are more like ants in a colony, or genes in a genome, they are parts of a larger integrated whole.
So we might expect stable
co-adapted partners living in concert across holobiont generations.
framework might also provide some justification for the calls for a
return to a palaeo-microbiome that existed before the modern diet -
for that would literally help to return a missing part of ourselves.
Most of the partners come together anew each generation, and don't interact in the ways that are necessary for higher-level integration into organismic wholes.
The theoretical bar for making that transition is high, and getting over it is going to be rare. But it potentially varies from holobiont to holobiont.
There is still a long and
exciting scientific road ahead, as researchers begin to unravel the
secret lives and complex effects of microbes on the development,
behavior and evolution of their hosts.