the-altruism-of-slime

[Before I begin I’d like to make it clear that I’m not poo-pooing on the use of social media for activism, but rather the passive (see: slacktivist) use of it to feel like one has made a difference even though they have just hit the “share” button on a viral video.]

If you’ve never taken a close look at a rotting log or a mossy patch of dirt, you’re missing out. I spent most of my childhood taking Calvinesque walks through the woods behind our house. Every now and then, my brother and I would follow the creek that ran through it until the “No Tresspassing” signs got too scary, but the best stuff was always within a short distance, hiding under a rock.

Look at any damp section of nature and you’ll probably find slime mold. You’ve probably accidentally touched it before, recoiling at the cold wet mass attached to a tree. They come in a wide variety of forms, from cellular slime molds (showcased above) to the plasmodial variety  for which the creature likely got its less than pleasant nickname. I call it a creature, because while they often are mistaken for lichen or mushrooms—and at one point they were considered fungi. But like the N. Fowleri that I featured a while back, they are protists. Like most protists, they are weird and awesome.

Protists give us a window into ancient life on Earth, and D. purpureum is no exception. People who have problems understanding evolution tend to just have trouble visualizing the transitional steps. Whether it’s the formation of the modern eye from rudimentary light-sensitive cells, to the journey from single-to-multi-cellular existence. With D. purpureum (and other dictyostelids) you have single-celled organisms that can band together to form one multi-cellular organism, complete with locomotion*. It was in this fashion that colonies of previously lone organisms at some point banded together, finding strength in numbers and efficiency in the delegation of functions. For this reason they’re also known as “social amoebae.”

For D. purpureum and other dictyostelids, this cooperation is temporary and borne out of necessity as a survival mechanism. With good reason, as it comes with a cost: The amoebae who are assigned to become the stalk die in order to form the sturdy, cellulose structure. This may seem counterproductive, but think of it in movie terms: When our heroes face certain doom, undoubtedly one or more will sacrifice themselves so that the rest of the group can live. The stalk gives the spores a higher vantage point in which to fall from, increasing their spread. It also increases the chances that it will be picked off by wandering predators and spread in their feces. I used the hero analogy, but it’s flawed in the sense that the choice to become a stalk is hardly mandatory. Apparently it’s just a matter whether you’re at the front or the back of the “slug*” when it forms. Their fates are sealed from the beginning.

But there’s a benefit to laying down your life for the good of the swarm! This form of sacrificial altruism, where an organism will put aside its own survival instinct for the good of a related group, is known as kin selection. Let’s bring it back into a tangible, real life situation:

Two of your siblings are in danger of falling in a lake of magma (what?). Saving them requires you to walk out into the lake** and activate the alien mechanism that would save them. While your death is inevitable, saving your siblings is not only a truly noble act (your heroism will be remembered, and Starfleet will miss you dearly) but it’s also really smart from an evolutionary standpoint. You share half of your DNA with each sibling, and you allow your genes to pass on through them. This sort of cost/benefit, good-of-the-many thinking governs even the simplest of organisms, and I find that truly amazing.

We’ll be taking a closer look at dictyostelid altruistic behavior (including cheating!) as well as some other interesting features of slime molds in future comics.

UPDATE: Alex Wild over at Scientific American posted some PHENOMENAL photos of these fascinating little creatures. Go now! Look at them!

 

 

This post was brought to you by sporange. An older form of the term sporangium, and one of the only words that rhymes with “orange.” Want another? The Blorenge. A prominent hill in Southeast Wales. Thanks to Noah for that one.

*I left out the “slug” phase of the process in the drawing above to focus on the sorocarp aspect. Not to be confused with the gastropod mollusc, but rather named for the shape the dictyotelids take when moving as one organism, just before taking root and spreading spores. You can read all about the development of D. discoideum, including instructions on how to harvest and grow them yourself over at the obviously-named Dictybase.org website.

**Because molten rock is much denser than water.