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Evolution of symbiosis

This week's paper is "Experimental Evolution of a Plant Pathogen into a Legume Symbiont" published recently in PLoS Biology by Marta Marchetti and colleagues.

It's not hard to convert a highly beneficial rhizobium, which infects legume roots and provides them with nitrogen, into a slightly harmful one -- just knock out the nitrogen-fixation gene and you get a bacterium that has some cost to its host but provides no benefit in return. But how hard is it for a bacterium that causes disease to evolve into a beneficial bacterium?

One process that sometimes happens in nature is the wholesale "horizontal" transfer of a gene (or many linked genes) from one microbe to another. So the authors of this week's paper started by transferring the symbiotic plasmid (a loop of DNA with genes for infecting roots and then fixing nitrogen) into a plant pathogen.

The resulting microbe couldn't infect roots of the donor rhizobium's host plant. At least not at first. But bacteria mutate. Given enough mutants, there might be some that conferred the ability to infect host plants, forming root nodules. They applied lots of bacteria to 500 seedlings and got three nodules. What new genes had these mutants acquired?

It turned out that they had lost or modified some of the genes that made them pathogenic. In particular, these were genes for the type III secretion system, which harmful bacteria use to inject toxins into their hosts. (This "molecular syringe" is essentially the base of the bacterial flagellum, disproving the "intelligent design" claim that flagella couldn't have evolved in a series of steps, because they claimed that removing even one component of the flagellum makes it useless for anything.)

Modifying one pathogen gene got the bacteria into roots. A second modification got them inside plant cells, where rhizobia normally fix nitrogen. But, so far, the modified bacteria aren't fixing nitrogen, even though the plasmid they started with has most of the genes known to be essential for nitrogen fixation. Instead, they make lots of PHB, a storage molecule that benefits the rhizobia but has some cost to the plant.

I look forward to further research from this lab. Will they be able to activate nitrogen fixation in these bacteria? If so, will they do it by some process similar to natural selection, or by "intelligent design?"

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