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Individual and kin selection in legume-rhizobium mutualism

OK, I've been critiquing other people's work for a while. Your mission, should you choose to accept it, is to critique something I've written. It's the summary for a grant proposal I'm about to submit. It will be reviewed by ecologists and/or evolutionary biologists, but they're not likely to be specialists in legume-rhizobium symbiosis. So if something isn't clear to an intelligent but nonspecialist audience, you'll let me know, right? If you're not all too busy reading the many interesting evolution articles in today's New York Times, that is. By the way, the great Myxococcus xanthus photo in Carl Zimmer's article is from Supriya Kadam, who did her PhD with Greg Velicer and just finished a year as a postdoc in my lab.

Individual and kin selection in legume-rhizobium mutualism

Intellectual merit: The legume-rhizobium symbiosis is an ideal model system to study the evolution of cooperation. Cooperation is often undermined by short-term self-interest, despite shared long-term interests, as in tragedies of the commons.

By fixing nitrogen inside legume root nodules, rhizobium bacteria are, in effect, cooperating with other rhizobia infecting the same plant. Each rhizobium cell pays a high metabolic cost to fix far more nitrogen than it needs for its own use. The extra nitrogen enhances host plant photosynthesis, which may increase carbon supply to the nitrogen-fixing rhizobia. But what if unrelated rhizobium strains, perhaps in other root nodules on the same plant, benefit equally? These are likely competitors for limited nodulation opportunities in future years. Individual selection will not favor paying an individual cost to obtain benefits that are shared equally with competitors.

If fixing nitrogen is individually costly, it may still be favored by kin selection. Fixing nitrogen helps protect millions of clone-mates in the same nodule from host-imposed sanctions, which our earlier work showed reduce rhizobium numbers in nonfixing nodules. Still, many rhizobia risk sanctions by diverting resources from nitrogen fixation to their own reproduction or that of kin. Those that potentially increase their reproduction, by fixing less nitrogen, can be said to cheat those that fix more.

The role of kin selection in rhizobium cheating is hypothesized to differ qualitatively among host legume species. Some legumes suppress reproduction of bacteroids, the differentiated rhizobium cells that can fix nitrogen, inside their root nodules. These bacteroids are expected to cheat by transferring resources to their reproductive kin, rather than by hoarding resources themselves.

These hypotheses will be tested with experiments that 1) look for host sanctions against individual reproductive bacteroids, which would disprove the hypothesis that only kin selection favors nitrogen fixation; 2) measure resource hoarding by reproductive versus nonreproductive bacteroids, using various host species; and 3) measure fitness effects of carbon transfers to reproductive rhizobia inside nodules and in soil nearby.

Proposed methods have been successfully applied with rhizobia in our lab. These include flow cytometry, to measure resource hoarding by individual cells. Custom-made root- and nodule chambers allow exposing individual nodules, or part of a root system, to different treatments, such as rhizobium strains known to differ in transfer of resources to kin, or nitrogen-free air to prevent nitrogen fixation. Mathematical models, using data from the experiments, will analyze how the evolutionary stability of rhizobium cooperation depends on factors like the percent of root nodules with more than one rhizobium strain.

Broader impacts: This project will include curriculum development, as well as research experience opportunities for graduate, undergraduate, and high school students. The latter will be the target audience for a web site on statistics and hypothesis testing for science fair projects. Outreach via cross-disciplinary talks, reviews, and a weblog will show broader audiences how understanding evolution can help solve societal problems. These expected impacts are consistent with activities under our current NSF grant.

Soybeans and alfalfa, both of which depend on rhizobia for nitrogen, are among the top three US crops, in terms of land area. The proposed research could help in the development of varieties that selectively enrich soils with the best indigenous rhizobia, based on nitrogen actually fixed, rather than easily mimicked recognition signals. Improved crop management may also be possible. For example, the impact of nitrogen fertilizer on rhizobium evolution depends on the extent and nature of host sanctions. Improved biological nitrogen fixation by legume crops can decrease nitrogen fertilizer use on subsequent crops in a rotation, reducing nitrogen runoff and dependence on fossil fuels.

Comments

I found it clear. Naturally I had questions, namely how you would test the hypotheses and how plants sanction rhizobia, but I presume these will be detailed in the actual body of the proposal. Very nice summary and splendid system I might add.

I agree with John, it is well written and I think many of the questions I have would likely be addressed in the full proposal.

Do you have room to expand a bit on this paragraph:

"The role of kin selection in rhizobium cheating is hypothesized to differ qualitatively among host legume species. Some legumes suppress reproduction of bacteroids, the differentiated rhizobium cells that can fix nitrogen, inside their root nodules. These bacteroids are expected to cheat by transferring resources to their reproductive kin, rather than by hoarding resources themselves."

to aid the non-specialist in understanding the specifics of your system.

What are the qualitative differences between legumes? Which legumes? Or is this not known? Would it help to add something about the relationship between differentiation of bacteroids and the persistence of reproductive cells in the nodules? Is there a cost to the plant from suppression of rhizobia differentiation? How does the expected transfer of resources from bacteroids to their reproductive kin effect the plant?

Soo many questions. Very cool system!

Thanks to both of you for your comments. I'll see what I can do within the 1-page limit. John -- I need to get this submitted, along with a couple papers I want to list as such, before responding to your "tag."

This really isn't my field, but it certainly reads very well and everything seems to make sense! Good luck with the submission.

excellent
good

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