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The Tiniest Farmers in the World

Fuligo septica, the “dog vomit” slime mould

Just when you thought slime molds couldn’t get any more bizzare, some researchers at Rice University have caught them farming.

If you are already familiar with slime molds, you can skip over the next couple of paragraphs.  If not, get ready to have your mind blown.

Life, you see, is quite fond of befuddling our attempts to understand it.  While you may have a pretty good idea of what I’m talking about if I say “plant” or “animal” or “fungus” or even “protozoan,” slime molds don’t fit neatly into any of these categories.  In fact, the things commonly lumped together under the title of “slime mould” exhibit a plethora of shapes and behaviours, ranging from things that look more or less like mushrooms (but aren’t) to things that look more or less like dog vomit (see illustration).  Modern DNA analysis shows that different species of slime mould aren’t even that closely related to each other, let alone anything else.

At any rate, the thing I’m talking about (Dictyostelium discoideum, or D. Disco to use its gangsta name) is hard to categorize because its life stages are so different from each other.  It begins as a more or less typical amoeba, chowing down on the bacteria that grow in decaying piles of leaf litter.  But, when the food runs out, things get wacky.  Tens of thousands of amoebas start to gather together, drawn in by a chemical signal.  They pile up, squish together, and eventually assemble, Voltron™-like, into a multicellular slug 2-4 mm long, big enough to see without a microscope.

A magnified picture of the D. Disco slug. Credit: Dirk Dormann, University of Dundee

This slug then moves forward in single-minded pursuit of light, heat, and humidity.  This is usually no more than a couple of centimetres, but it’s enough to get it to the top of the pile of litter (although grad students are fond of teasing the things across petri dishes using flashlights.)

Once it reaches its destination, it transforms again.  About 20% of the cells die and become a stalk, raising the rest up to become a “Mexican Hat” (this, to my knowledge, is the technical term).  The cells in the hat turn into spores and float away on the breeze, just like true mushrooms do.  When they land, the spores hatch and start the process over again.

D. Disco fruiting body. Credit: Strassmann and Queller Group, Rice University

Now, if that wasn’t weird enough, these things have just been shown to exhibit another behaviour: farming.  It’s certainly not the only example of farming outside of humans; fungus-growing ants and termites have been doing it for centuries.  It’s also not terribly complex, and some would argue that it’s better described as husbandry.  Still, it’s impressive enough for something that doesn’t remotely have a brain.

What happens in some D. Disco clones is that the amoebas stop eating before the food is exhausted.  They then take some bacteria into the slug with them.  The bacteria become incorporated in the spores and are carried along to the new location.  In this way, the slime mold is seeding its next habitat to ensure that there is a plentiful supply of food.

The molds aren’t picky about what bacteria they take; the researchers noted they pick up many species, some of which aren’t even particularly good food sources.  Nevertheless, it’s clear that this behaviour is purposeful and inherited: when different varieties of the mold were cleaned of bacteria and then re-introduced to them, only the ones that had previously farmed continued the practice.

The decision to stop eating before the food is gone has a clear evolutionary cost.  On the other hand, seeding your new home with bacteria is advantageous if you end up somewhere where the food supply is low or nonexistent.  Both strategies coexist in D. Disco, with about a third of the wild clones acting as farmers and the rest as gatherer-hunters.  This is also true of humans (co-existence of agricultural and nomadic populations) but not of other farming animals, such as ants and termites; an entire species either farms or it doesn’t.  However, it is interesting to note that all animals that farm are inherently social, living in groups with specialized roles.

I take two things away from this latest discovery:

a) It’s so cool that the same strategies that made our species so successful are equally valid on the microscopic scale
b) Nature will never, ever cease to present us with interesting surprises

Link to the Nature Article

1 ping

  1. Trans-Canada Slimeways » Scientific Canadian

    […] readers of this blog (I flatter myself that such people exist) will know I’m keen on slime moulds, a form of life that defies easy description. So the publication this week of a paper that show how […]

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