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More talks from Evolution 2008

I’m done with two grant proposals, revising a book chapter, and checking the final version of a review article. I still have a pile of interesting reading and writing to do before I can get back into the lab – actually, I did help Ryoko set up an experiment yesterday – but no more looming deadlines for awhile. So, here are two more summaries of talks from Evolution 2008.

Do I know you?

The ability to tell other individuals apart by their faces is presumably maintained by natural selection, so you can recognize and avoid bad guys. But is there also selection for looking different enough to be recognizable? Or is it better to blend in with the crowd, so you can get away with stuff?

Michael Sheehan and Elizabeth Tibbetts are studying individual recognition in wasps (Tibbetts and Dale, 2007). Their hypothesis is that distinctive-looking individuals benefit, because they get in fewer fights over dominance.
4fuscatus.jpg

It’s better to be the top wasp, of course, but the worst situation would be having to fight to establish dominance, every time you ran into another wasp. With individual recognition, you only have to fight each wasp once; then you know which is dominant and don’t have to fight again. That was their hypothesis, anyway. To test it, they painted wasp faces, so that three in a group of four looked the same and one was different. Consistent with their hypothesis, the different one was attacked less by the others. Differences among individuals in appearance (or in calls, in birds) are more common in species that have more social interactions. Things get more complicated if you need to recognize individuals but also membership in some group (fellow colony members, close relatives, etc.). Then two or more different kinds of signals may be required.

Tibbetts, E.A., Dale, J., 2007. Individual recognition: it is good to be different. Trends in Ecology & Evolution 22, 529-537.

Harcombe1.jpg

Your local food cooperative

Members of one species often have effects, positive or negative, on nearby members of other species. One example is “cross feeding?, where each species produces something the other needs. Such interactions may be mutually beneficial, but mutual benefit does not guarantee that an interaction will persist. Will Harcombe has developed a nice experimental system to study cross-feeding, using two species of bacteria that can, to varying extents, benefit each other. E. coli benefits Salmonella by breaking down lactose into a form Salmonella can use. E. coli needs methionine but can’t make it. Will started with a Salmonella strain that makes a little methionine, but not enough for maximum growth of E. coli. The Salmonella bacteria would benefit, collectively, if they invested some of their energy into making methionine, because that would stimulate E. coli growth and break down more lactose for them to eat.

But what if a mutant Salmonella pays the cost of making more methionine, but the benefits are shared with Salmonella cells that don’t pay that cost? Which genotype will win? Will hypothesized that, with a more structured environment, the benefits of making more methionine would be recycled locally, leading to more cooperation between Salmonella and E. coli, including more methionine production. To test this hypothesis, he allowed the two species to evolve on culture plates, where cells interact mostly with neighbors, and in mixed liquid culture, where methionine and other resources are equally available to all cells. On culture plates, Salmonella evolved to make more methionine, as predicted. In liquid culture, Salmonella mutants that made no methionine spread, leading to the extinction of E. coli. There was also an affect of crowding. When competition for resources was intense, there was less cooperation.

This system is simple enough that it can be modeled mathematically, for detailed comparison of theoretical and actual results. One simplifying assumption is that the cost of methionine production is constant; that is, making twice as much methionine costs a cell twice as much glucose (or whatever). But Will expects that natural selection could eventually favor mutants that make the same amount of methionine at lower cost, rather than those that make less methionine. I’ll be looking forward to reading about this work in the future.


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