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Ford Denison

This isn't quite right. First, the nonbacteroids aren't dormant, they just don't fix nitrogen. They divide and infect new host plant cells as the nodule grows. Second, only nodules with indeterminate growth have these two types of rhizobia. In soybean nodules, for example, N2-fixing bacteroids don't give up the ability to reproduce. Third, given that soybean rhizobia show that it's not essential to give up reproduction to fix N2, the real question is why would natural selection favor giving up reproduction? We know it sometimes does, as in worker bees, but remember that natural selection among rhizobia works for the benefit of the rhizobia, not the legume. References:
Denison, R.F. 2000. Legume sanctions and the evolution of symbiotic cooperation by rhizobia. American Naturalist 156:567-576.
Kiers, E.T., R.A. Rousseau, S.A. West, and R.F. Denison. 2003. Host sanctions and the legume-rhizobium mutualism. Nature 425:78-81.

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