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We cooperators need to stick together

Experiments on the evolution of cooperation can be simpler with microbes than with animals. Microbes do sometimes cooperate. For example, some amoebae get together and form a stalk (consisting of hundreds of individual cells) to elevate spores (many more individual cells) above the soil. Bacteria may collectively produce and release enough extracellular enzymes to make food available for all, when none of them could make enough enzyme alone. But why should an amoeba volunteer to be in the stalk (an evolutionary dead end) rather than becoming a spore? And why not save the cost of making expensive extracellular enzymes, by free-loading on enzyme production by others? Out of the goodness of their hearts? They don't have hearts.

Cooperation is easiest to understand among microbes that share the same allele (one of several alternative versions of a gene) for cooperation. Kin-selection theory says that an allele leading to some individually costly activity may spread if it preferentially benefits others with the same allele, relative to those with different alleles. Cooperation among relatives, who are more likely to share a given allele, is therefore easy to understand.

But can microbes recognize kin, i.e., whether another microbe has the same alleles? Sometimes, apparently. In this week's paper, Elizabeth Ostrowski and colleagues report in PLoS Biology that Kin discrimination increases with genetic distance in a social amoeba.

PLoS papers are open access and Robin Meadows has already written a nice summary of this paper, so I won't go into a lot of detail. Basically, they mixed a fluorescent lab strain with each of 14 wild strains. They used molecular methods to measure how genetically similar (related) each wild strain was to the lab strain.

When they mixed two closely related strains, neither strain took advantage of the other. That is, they made a fruiting body in which the two strains shared rewards (spore production) about equally. But when less-related strains were mixed, one or the other usually dominated spore production, at the expense of the other. In effect, these amoeba recognized close kin and cooperated with them. Recognition was apparently at the level of adhesion: related cells stuck together better than unrelated cells.

There are probably other cases where sticking together, literally, is the key to cooperation. For example, some bacteria stick together and form mats on liquid media, benefiting from additional oxygen at the surface.

But what about microbial cooperation without physical contact between cells? How does a bacterium investing in expensive extracellular enzymes know (to the extent that a cell with no brain can know anything) that is surrounded mostly by relatives? Or what about restraint in consumption of resources, when a cell becomes a metabolically inactive "persister"? This can free up resources for nearby cells, but what if those neighbors aren't relatives?

We just had a grant proposal to study persisters rejected by NSF, leaving us with no money to run our lab this year, so I've been thinking about this topic a lot. We assume that bacteria are often surrounded by relatives, because of how they reproduce. "Cooperate with neighbors" can be equivalent to "cooperate with kin." (It's a little more complicated than that, of course.) So we designed experiments in which spatial structure could favor cooperation among kin.

One of the reviewers of our grant pointed out that kin recognition might work just as well. But is kin recognition among bacteria actually possible? This week's paper suggests that it might be, if they're touching. That might sometimes happen in biofilms. But suppose a cell is touching one clonemate and one unrelated cell. Should it make extracellular enzymes, or not? If resources are scarce, should it restrain its own resource consumption, freeing resources for its neighbors? And what if the cells aren't quite touching? Bacteria produce and respond to various chemical signals. If each strain produces different signals, it's conceivable that they could assess their average relatedness to their neighbors. We have some ideas about that, but testing them will take money. I am hoping President Obama's efforts to stimulate the economy will include more funding for research.

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