The Social World of Slime

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Social behaviour in animals is not uncommon, and we are rarely surprised to observe cooperation in nature. However, most explanations for cooperative behaviour rely upon a certain level of cognitive ability. Cooperating willy-nilly leaves individuals open to cheaters, so successful and long-term cooperation between individuals often relies upon individual recognition. Many social groups are composed of relatives. This makes a lot of sense, as helping relatives yields benefits without the need for reciprocation in the future, because relatives share genes. But still, you might expect that even this requires basic intelligence – you need to be able to recognise who are your relatives.

What IS surprising is that it isn’t just big, brainy animals that are cooperating. Many bacteria cooperate at a basic level, producing and sharing beneficial chemicals within a community and finding ingenious ways to punish cheaters. But one single-celled amoeba has taken things to a whole new level.

Dictyostelium is a social amoeba. At least when times are hard. When food is plentiful, Dicty, as it is affectionately known, is quite happy to be as antisocial as any other amoeba. Single cells move about their environment, engulfing food, excreting waste, and having very little to do with each other. But, when food starts to become scarce, the amoebae stick together. Literally. Hungry Dictycells will start to release a chemical called cAMP, which attracts nearby ameobae and causes them to group together, forming clumps. And then, something really quite special happens. When there are enough cells grouped together, the multi-cellular slime mould transforms into a slug, which crawls to a suitable spot and forms a structure known as a fruiting body. The fruiting body is made up of two parts; a stalk that supports a ball of spores at the top. The stalk lifts the spores into the air where they are carried away by the wind, hopefully to a new, more favourable environment. When the spores land, they revert to their single-celled way of life until the next crisis comes.

SocialSlime003

What is remarkable about this is that it is an example of altruism in an organism with no brain. Cells that form the stalk give up their lives to enable the spore cells to live. It’s quite touching, really. But this kind of system is surely susceptible to cheating? If all the cells in the fruiting body are genetically identical then it’s fine – giving up your own life to save a clone of you is a no-brainer. But what if there are unrelated cells in your fruiting body? Any cell that could bias its chances of being part of the spore cells would benefit, and slowly the stalk-forming cells would die out.

SocialSlime002But Dicty has solved this problem too. When given a choice, Dicty prefers to fruit with relatives. This discrimination is based upon cell adhesion molecules (tgrC and tgrB) that create incompatibility between unrelated amoebae – they simply won’t stick together. This allows Dictyto preferentially pair up with relatives, and reduces the chances of cheaters taking advantage of the sacrifice of unrelated stalk cells.

That’s not to say there isn’t conflict in Dicty, and despite its best efforts to avoid aggregating with non-relatives, it does happen. Individuals can bias their chances of becoming spore cells through changing their response to a signalling molecule, DIF-1. This allows them to cheat the system, and amoebae that can ignore the DIF-1 signal to become stalk tend to ‘win’ against those that can’t. However, DIF-1 mutants (cheaters) pay a price; they can’t form fruiting bodies on their own, because no stalk is formed. Thus they are totally reliant on their altruistic cousins. This prevents cheaters from wiping out non-cheaters in the population at large, although they may enjoy short-term benefits. The altruists are not just powerless victims either; some strains have evolved mechanisms to combat cheating. Together, these mechanisms balance the situation, and overall cooperation prevails. Despite the lack of a nervous system or a brain, Dicty continues to be social.

Want to Know More?

  • Gilbert, Strassmann and Queller (2012) High relatedness in a social amoeba: the role of kin-discriminatory segregation. Proceedings of the Royal Society B: Biological Sciences
  • Strassmann and Queller (2011) Evolution of cooperation and control of cheating in a social microbe. Proceedings of the National Academy of Sciences 108, 10855 – 10862
  • Strassmann and Queller (2011) How social evolution theory impacts our understanding of development in the social amoeba Dictyostelium. Development, Growth and Differentiation 53(4), 597 – 607
  • Hirose et al(2011) Self-recognition in social ameobae is mediated by allelic pairs of tiger genes. Science 333(6041), 467 – 470
  • Mehdiabadi et al(2006) Social evolution: kin preferene in a social microbe. Nature 442(7105), 881 – 882

Featured image is in the public domain.

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