This Week in Evolution

Olivia Judson's latest column includes a good summary of work in my lab on cooperation between soybean plants and the rhizobium bacteria that (typically) provide them with nitrogen. As she points out, "cheating" is less likely to evolve in symbiont populations if they are transmitted in eggs or seeds, relative to symbionts that are acquired from the environment. In the former, if the host dies before reproducing, the symbiont dies, too. Symbionts without brains (bacteria, say) can't anticipate the effects of their actions; it's just that those whose genetically programmed behavior increases host survival become more common over generations.

Similarly, low symbiont diversity within an individual host may favor symbiont investment in costly activities that benefit the host. If each host has many different symbionts, on the other hand, then helping the host indirectly benefits competing symbionts sharing that host.

Rhizobium bacteria reach new host plants through soil, not via seeds, and they can do so even if the host dies without reproducing. Furthermore, each individual plant has multiple strains of rhizobia, which should undermine cooperation. Why then, do most rhizobia use their limited energy supply to fix nitrogen, giving most of it to the host plant? Why not use that energy for their own reproduction, instead?

Although there are several rhizobium strains per plant, they are typically segregated into individual root nodules. So, Toby Kiers and I reasoned, if plants monitor individual nodules and do something nasty to those that provide less nitrogen, that would act as a form of natural selection against cheating rhizobia. A computer model by Stuart West came to similar conclusions. To test this hypothesis, we forced some nodules to cheat, by surrounding them with an argon-oxygen atmosphere lacking nitrogen gas. Control nodules on the same plant got normal air, which is 80% nitrogen. Would rhizobia freed from the burden of fixing nitrogen redirect resources into their own reproduction? Would the plant impose sanctions on nonfixing nodules? If the answers to these questions are yes and yes, what would be the overall effect of cheating on rhizobium reproductive success?

Continue reading "Welcome, fellow Dr. Tatiana fans!" »

Posted by R. Ford Denison at 06:09 PM | Permalink | Comments (0)

Semelparous plants and animals are those that reproduce only once, whether after a few months of growth (annual plants, like wheat) or after years (“century plant” or most salmon). Iteroparous species iterate. That is, they reproduce repeatedly. For example, perennial grasses may produce seeds every year for a decade or more.

One reason this difference matters is that perennial crops may have some environmental benefits, relative to annual crops. Plowing, traditionally more common with annual than perennial crops, can greatly increase soil erosion, especially on steep slopes. So there is increasing interest in developing perennial grain crops as an alternative to wheat.

However, perennial plants have lower seed yield than their annual relatives, so we would need to devote more land to agriculture to get the same amount of grain. One reason for the yield difference is that an annual plant can transfer most of the carbon (energy) and nitrogen (needed for protein) from its leaves, stem, and roots into its seeds. It’s going to die anyway, so the next generation gets its accumulated wealth. A perennial plant needs to hold back some carbon and nitrogen for winter survival and spring regrowth. The more resources it puts into this year’s seed production, the less it can carry forward to support reproduction next year.

This week’s paper shows that iteroparous plants face additional costs when they reproduce, namely, ecological costs. “Herbivore-mediated ecological costs of reproduction shape the life history of an iteroparous plant” was written by Tom Miller and colleagues at the University of Nebraska (where I’ll be speaking on Darwinian Agriculture in April) and published in American Naturalist.

Continue reading "Natural enemies complicate reproductive tradeoffs" »

Posted by R. Ford Denison at 11:55 PM | Permalink | Comments (0)

I will be reviewing another recent journal article today or tomorrow, but meanwhile we seem to have convinced someone that an evolutionary perspective is useful in agriculture. A recent book review mentions a chapter we wrote:

There is also food for thought in some of the chapters, particularly the one by R.F. Denison and E.T. Kiers on sustainable crop nutrition. This perceptive analysis raises questions about the simplistic assumptions that often underlie attempts to improve crop mineral-use efficiency and highlights areas where such attempts are likely to be useful and others where they are not. This reviewer certainly changed his thinking as a result of the ideas put forward.

Continue reading "Sustainable Darwinian Agriculture and Organic Tomatoes" »

Posted by R. Ford Denison at 05:31 PM | Permalink | Comments (0)

Older soybean varieties benefit more from mixtures of good and bad symbiotic nitrogen-fixing bacteria than modern soybean varieties do. This work has also been
discussed on the Nature website by Heidi Ledford and on the Agricultural Biodiversity Weblog by Jeremy Cherfas.

"variations… profitable to the individuals of a species… will tend to the preservation of such individuals, and will generally be inherited by the offspring. I have called this principle… natural selection, in order to mark its relation to man's power of selection."
-- (Darwin, 1859)

This week, Toby Kiers, Mark Hutton, and I are reporting an apparent decrease, over the course of 60 years of soybean breeding, in the ability of plants to benefit from rhizobium bacteria. Our paper “Human selection and the relaxation of legume defences against ineffective rhizobia” is published on-line in Proceedings of the Royal Society.

Continue reading "Soybean symbiosis isn't what it used to be" »

Posted by R. Ford Denison at 07:32 PM | Permalink | Comments (5)

Next week I will be meeting with a publisher to talk about the possibility of writing a book on Darwinian Agriculture to be published in 2009, the 150th anniversary of The Origin of Species. (I apologize to one reader who apparently thought it was a done deal.) Here's a short draft of the first chapter, mostly about sustainable agriculture by ants and termites.

Farmers of 50,000 millennia

“We’ve been farming sustainably for three years”, read the email. I was glad to learn that my friend was farming in ways that he hoped could continue indefinitely, but how could he be sure, after only three years?

It might have been a reasonable assumption, if the farming methods he used were similar to those that other farmers have used successfully for a long time. But how similar is similar enough? And what qualifies as “a long time?” As director of “the world’s youngest 100-year experiment”, I often thought about these questions....

Continue reading "Darwinian Agriculture III" »

Posted by R. Ford Denison at 06:50 PM | Permalink | Comments (6)

I have some comments on a recent paper that's only tangentially related to evolution. Actually, it's more relevant to science fair projects, the topic of my last post.

One type of science fair project my fellow judges and I are really sick of is "The effect of X on plants", where X is mouthwash, vinegar, cola, etc. The obvious question, which we always ask, is "how often are plants in the field exposed to high concentrations of mouthwash?" Unfortunately, whoever reviewed this paper in Proceedings of the National Academy of Science, claiming that "Pesticides reduce symbiotic efficiency of nitrogen-fixing rhizobia and host plants" apparently failed to ask this question.

Continue reading "Rhizobia, pesticides, and peer review" »

Posted by R. Ford Denison at 05:20 PM | Permalink | Comments (2)

This week's paper is "Palaeogenetic evidence supports a dual model of Neolithic spreading into Europe" by M.L. Sampietro and others, published online in Proceedings of the Royal Society. The paper is interesting both for its findings and for its methods.

We know that agriculture spread from the Near East -- do people in Asia call this the Near West? -- to western Europe, starting around 10,000 years ago. But did this mostly involve farmers moving, or the spread of agriculture without major movement of people?

People have tried to figure out past population movements using genetic differences among modern populations, but it would help to have genetic information from people who lived thousands of years ago, as well. This is technically challenging, however...

Continue reading "Tracing the spread of agriculture with stone-age human DNA" »

Posted by R. Ford Denison at 04:51 PM | Permalink | Comments (0)

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.

Continue reading "Individual and kin selection in legume-rhizobium mutualism" »

Posted by R. Ford Denison at 12:04 AM | Permalink | Comments (5)

Last week, I was at a meeting in the Netherlands on “Darwinian agriculture: the evolutionary ecology of agricultural symbiosis.” Topics included: the effects of cows on human evolution, the independent invention of “agriculture” by ants and termites, and some disadvantages of diversity. As promised, here are a few highlights.

Continue reading "Darwinian agriculture II" »

Posted by R. Ford Denison at 06:15 PM | Permalink | Comments (5)

Next week, I'm speaking at a one-day symposium on "Darwinian Agriculture: the evolutionary ecology of agricultural symbiosis", in Wageningen, The Netherlands. So, instead of reviewing a recent paper, this week I'm going to discuss some of the not-quite-so-recent papers on which my talk will be based. The following week, I plan to summarize some of the talks I hear at the meeting.

I may do the same thing in August, when my grad students and I speak at the much larger Ecological Society of America meetings in San Jose, California. Feel free to comment if you feel cheated of your weekly paper review, and I might reconsider. The Evolution meetings are in Christchurch, New Zealand, this year, but my grant won't stretch that far.

"Darwinian Agriculture: when can humans find solutions beyond the reach of natural selection?" was the title of a paper that Toby Kiers, Stuart West, and I published in 2003. Our answers to the title question suggested how increased understanding of past and ongoing evolution could improve: 1) breeding of crops and livestock, and 2) design of agricultural ecosystems.

Continue reading "Darwinian agriculture I" »

Posted by R. Ford Denison at 06:08 PM | Permalink | Comments (4)

Answer: they're both semelparous (reproduce once, then die), so evolutionary trade-offs between number and size of offspring are expected to be similar.

This week's paper is "Evolutionary aspects of the trade-off between seed size and number in crops" (Field Crops Research 100:125-138) by Victor Sadras. You can read the abstract on the web for free. For the full version, you can pay $30 to download, visit your nearest agricultural research library (in the U.S., often at a state university), or email the author at: sadras.victor@saugov.sa.gov.au. My discussion is mostly based on a shorter version presented at the Australian Agronomy Conference.

Demand for grain is increasing, to feed growing human and livestock populations and more recently for ethanol production. Unless those trends are reversed, we will either need to expand the land area used for agriculture or increase grain yields per unit area. Grain yield is the product of plants per area, seeds per plant, and weight per seed. Unfortunately, increasing any one of these (by increasing seeding rate, or through plant breeding) tends to decrease the others.

This paper looks at how natural selection (in the wild ancestors of crop plants and in fish) and plant breeding (especially in maize and sunflower) shape trade-offs between seed number per plant and seed size. The similar patterns in plants and fish show that, as predicted by the relevant aspects of evolutionary theory, we are dealing with fundamental constraints that we are unlikely to change.

Continue reading "Evolutionary trade-offs: how are soybeans like salmon?" »

Posted by R. Ford Denison at 05:11 PM | Permalink | Comments (2)