I for one welcome our new amoeba overlords…
Sci is aware that the world freakin covered this paper last week. I don’t get access to press releases and so couldn’t get the paper until Friday, and well, time. But I really wanted to read it myself (yes, sometimes I read scientific papers for fun. What?!?!). And dang, this paper is COMPLETELY FASCINATING. I normally don’t say that about stuff outside my field. But WHOA.
Amoebas. They navigate mazes. They go from single, solitary little beings to huge conglomerates of cells in times of trouble. And now, they’re farming. Next, the WORLD. After all, they already figured out the transport system of Tokyo.
Brock, et al. “Primitive agriculture in a social amoeba” Nature, 2011.
Today’s amoeba is the charming, yet humble, Dictyostelium discoideum. May I call
Dicty is an amoeba which inhabits various regions of the world, wherever there is soil and leaf litter (it's not picky), including temperate zones like Virginia and Minnesota (where the data for this paper was taken). Amoebas in their normal lives move along the forest floor, feeding happily on soil bacteria. In times of stress, when there are few bacteria to be found, Dicty gathers to gether with all its friends (also named Dicty) and forms a large conglomerate known as a slug (yup, that’s the technical term). This slug then crawls along in search of food. When things get too lean, the Dictys act collaboratively, allowing about 20% of the cells to die in order to produce a long stalk known as a fruiting body. The fruiting body releases the remaining cells, now known as spores, and they get carried on the wind to new locations where there may be more food.
But what if there ISN’T any more food in those far off parts? Well, you might think an amoeba would be pretty screwed. But you’d be wrong.
It turns out they can farm!
The authors of this paper apparently were studying Dicty, and noticed that a bunch of their “bacteria free” amoebas were NOT remaining bacteria free. Rather than thinking something was wrong and throwing it out, they took a closer look, and found that some of the amoeba were CARRYING bacteria with them from dish to
dish. About 39% of the amoebas observed from two different locations (Virginia and Minnesota) were able to pick up bacteria and carry it with them, and can carry more than 43 species of bacteria at a time (it could be more than that, but they can’t detect everything).
But how do they PROVE that the amoebas are carrying bacteria around? You can’t just observe it, this is science after all. So they gathered both amoebas that they saw “farming” and amoebas that they didn’t see with bacteria, and placed them both on clean dishes and gave them both antibiotics, to make them bacteria free. They then put them on plates of dead bacteria so they had a little something to eat. Both sets of Dicty were still bacteria free. But when they transferred the amoebas to dishes that had LIVE bacteria around for them to eat, the “farming” amoebas picked the bacteria right up again, while the non-farming ones just ate it all.
And they proved the usefulness when transferred to new places that had no bacteria, the “famers” could grow their own, and ended up doing better and producing more spores and larger fruiting bodies during reproduction than the non farming amoebas. Up there in A you can see that, when the amoebas were put on a clean plate with no bacteria, the farmers produced way more spores, while the non-farmers suffered on a sudden starvation diet.
Pause here to note: it’s not REALLY “farming” in the strictest sense, because it doesn’t involve cultivation of the tastiest bacteria, and they aren’t really RAISING the bacteria by giving them food or anything. But still, for an amoeba to carry around its food is a dang impressive behavior. This has been seen before in ants, termites, beetles, and of course humans, but never before in something as simple as an amoeba.
Now, farming amoebas! This sounds like a pretty sweet deal, you’d really think that more than 36% of the population would have acquired this rather desirable trait. But farming comes with some tradeoffs. In this case, it appears that overall, farming amoebas are more “cautious” about reproduction than their non-farming brothers (sisters?). When provided with bacterial abundance and more food than they could handle, the non-farmer Dictys went whole hog and produced LOTS of spores for reproduction (which you can see above in B). The farmers produced less spores. And it turns out that this may be because the farming amoebas eat less, and make sure to save leftovers for later.
On the left you can see the amoebas hanging around in a solitary state, like they would under conditions where they have a lot of food. They scientists then lowered the amount of bacteria available in the dish, causing the amoebas to aggregate into their social slugs. When they gave the resulting slugs bacteria, they saw that (on the right) the non farmer amoebas ate a LOT more than their farming friends, who left almost 50% of their food for later.
They also showed another way in which farming amoebas are “cautious”. Fruiting bodies and spores are something that social amoebas form only in times of need, when all the available food has been exhausted and they need to seek new pastures. The non-farming amoebas acted true to form and consumed the majority of the bacteria before they formed fruiting bodies (the panel up there on the right). But the farmer amoebas started forming fruiting bodies when they still had bacteria to burn, presumably to save some of it to spread to new places (panel on the left).
And finally, being bacteria free makes non-farming amoebas travel lighter. You can see above that non-farmers, when they released their spores, got significantly more mileage than the farming amoebas did. This makes sense, when you think about it. If you are a Dicty, and you don’t farm, and food is scarce, you want to get as far as possible in the hopes of finding something better further away. But if you’re a farming Dicty, it doesn’t really matter if you stay closer to home, you’ve got your own bacteria with you that you can seed and eat, and so distance doesn’t matter as much. Have food, why travel?
So you can see that, while amoebas get some benefit from farming, there are some growth costs too, with farming amoebas producing less spores during feasts, but more during famine. This provides a decent explanation of why there are both farmers and non-farmers still present in the population. The farmers have the advantage sometimes, but farming also comes at significant costs.
But this does raise one really important question to me. The farming amoebas and the non-farming grouped together during times of stress. HOW?! Do farming amoebas ALWAYS group with other farmers? And if they do, how can they TELL?
Ah, the amazing amoeba, forming slugs, navigating mazes, and now farming its food.
Next stop: THE WORLD.
Brock DA, Douglas TE, Queller DC, & Strassmann JE (2011). Primitive agriculture in a social amoeba. Nature, 469 (7330), 393-6 PMID: 21248849