Holobiontics for Dummies

I have seldom thought of myself as particularly fluid. Oh, I change my mind and stuff, but I am neither a Jell-O pudding nor, for that matter, a lump of room temperature Play Doh (I even had to look up the spelling). So it is with some consternation that I am forced to confront the possibility that the microbiome that I’ve carted around with me all these years, might be considering a takeover. A palace coup.

Perhaps I’m being overly sensitive, but I’ve always considered me as plinth-worthy and the others just tagalongs -useful, no doubt, but subordinate. I am outnumbered, of course, but so is a queen by her subjects, a priest by his congregants. They may not all work for the betterment of the whole, but at least they know where the buck stops. Who to write to with their complaints.

But, it has come to my attention that the parade ground is restive. Socialism, it would seem, has come to the body. ‘Things fall apart; the center cannot hold; mere anarchy is loosed upon the word,’ in the unsuspecting words of Yeats -or am I being a bit thespian?

I have to admit, I thought it was pretty well settled: once upon a time we were alone, and then, word spread that there was free room-and-board available for little tiny things who wouldn’t mind living in the dark, no questions asked.

I don’t want to say that I really mind them rooming with me. I mean, they’re not heavy to carry around or anything -a bit messy I suppose, and you’ve got to be careful they stay in their rooms, but otherwise they’re relatively quiet neighbours. Also, not to seem overly speciesist I think they maybe could use a little direction on the stuff they cook in there -you know, close the windows, or something. But, overall, I have been happy with them. Until, that is, I heard the rumours. I first caught wind (sorry) of them in a delightful article in Aeon by Derek J. Skillings, a biologist and philosopher of science who is currently a lecturer in philosophy at the University of Pennsylvania: https://aeon.co/ideas/i-holobiont-are-you-and-your-microbes-a-community-or-a-single-entity

He writes about something I should have thought about before -that there’s no free lunch. Well, okay he doesn’t exactly put it like that, but I’m extemporizing for him.

The crux of the issue seems to be in deciding just what they’re doing in the neighbourhood: were they invited, or did they just barge in willy-nilly like hoodies at a teenage party, as the word spread? In other words, now that they’ve mingled with the locals, do they remain expats, or become voters? Are they for us, or for themselves? I worry about a Fifth column, eh?

As Skillings puts it, though, ‘These organisms together make up what’s known as a holobiont: a combination of a host, plus all of the resident microbes that live in it and on it.’ Holobiont: you  have to love the word! ‘Some of the microorganisms kill each other with toxins, while others leak or release enzymes and nutrients to the benefit of their neighbours. As they compete for space and food, cohabiting microbes have been found to affect the nutrition, development, immune system and behaviour of their hosts. The hosts, for their part, can often manipulate their resident microbiota in many ways, usually via the immune system.’

So, a standoff? Nothing’s easy when you’re dealing with the great unwashed. ‘So are you a holobiont, or are you just part of one? Are you a multispecies entity, made up of some human bits and some microbial bits – or are you just the human bits, with an admittedly fuzzy boundary between yourself and your tiny companions?’ Can I trust them, in other words? Or, worse, do they even care -and if so, for what? Does a finger care about other fingers? Do clothes care about their wearers? Am I Cassandra?

The article does a nice job of separating the competing views. ‘They can be roughly split into two factions, the ecological and the evolutionary. 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. But this 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 a holobiont.’

Frankly, I’m not sure I am reassured. There is still stuff in there that seems to have entirely too much say about what’s going on. Too much democracy, if you ask me. I mean, Skillings evens admits that hostages could be taken: ‘The ecological and evolutionary views make for very different predictions about how a holobiont will change over time. Evolutionary theory predicts that the parts of a unit of selection will tend to cooperate: to sacrifice their own interests for the good of the whole. Ecological theory, by contrast, predicts competition and exploitation: parts will cooperate only insofar as it benefits them.’ Great!

So where do I fit in all this? Viewed from either side, it seems like sedition: I am a pawn -or at most, a figurehead, grudgingly acknowledged for political purposes. ‘But in an ecosystem, there are no bad guys, just species playing different roles. 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… So we might expect stable co-adapted partners living in concert across holobiont generations. However, the evolutionary version of holobionts gives us reason to stick to an expanded version of the ‘us versus them’ picture of medicine. It’s just that now we have a few more allies on our side that we need to take care of.’

Come on! Make up your mind, eh? Is it just a matter of choosing sides, of putting your chips on a number, or whatever they do in casinos? Or is there even a choice? The author tries to father me gently into the night with reassuring bedtime stories: ‘As things stand, the evidence leans heavily towards a more ecological interpretation of holobionts. 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.’

I suppose Skillings means well, but for some reason I’m left a little unsettled.  Of course, no matter how my father told the story and no matter how much he smiled, Hansel and Gretel still gave me nightmares when he turned off the light.


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