Defective not defecting?
Like me, you may have wondered how Marek Basler learned about my blog post on his Nature article so fast that he was able to comment within hours of the post appearing. After all, he studies Type VI secretion systems in Vibrio, and is probably not an avid follower of a blog about rhizobia. The secret is that he has a Google Alert set up to notify him as soon as anything appears on the web that mentions T6SS. So as soon as I post this blog, he will get to hear about it. Hello, Marek!
Yet another indispensable truc from those people at Google to add to my list! I couldn’t wait to try it out, so I set up an alert for “rhizobium”. Within moments, Google came back and told me that the most important recent information was … my own blog. Very gratifying, but how could I learn something new? Fortunately, further down the list was something much more interesting, and probably more important. It was an Early View of a new article in New Phytologist by Maren Friesen entitled ‘Widespread fitness alignment in the legume–rhizobium symbiosis’ . Now, New Phyt is one of my favourite journals, though mostly because it has become the leading journal for mycorrhizal research. It publishes rhizobium papers less frequently, but this one is definitely worth reading.
Maren Friesen is one of a growing group of researchers who have done interesting work on rhizobia. Ford Denison, Toby Kiers, Ellen Simms, Joel Sachs, Stu West, Katy Heath … these are not people who will be familiar to most rhizobium researchers, yet they are big names in their own field. One of their papers, ‘Host sanctions and the legume-rhizobium mutualism’ was published in Nature and has been cited 274 times. It surely must say something important about rhizobia, but few of the citations are by rhizobium researchers. Other related papers are almost totally ignored in the rhizobium literature. The problem lies in the tribal nature of scientific research. These people use rhizobia in their work, but they are Evolutionary Biologists. They go to evolution conferences and talk earnestly about cheating and sanctions and evolutionarily stable strategies with people who study fig wasps and yucca moths. They seldom attend the jamborees of rhizobium research – the International and European Nitrogen Fixation Congresses and the North American Symbiotic Nitrogen Fixation Conference – where the talk is of sustainable agriculture and molecular plant-microbe interactions. This split is unfortunate, as there is potentially much to be gained by bringing these fields together, as Maren’s paper illustrates.
I used to meet many rhizobium researchers who thought that legumes could select the symbionts that would do the best job for the plant. The rationale was essentially Panglossian – the purpose of the symbiosis was to increase plant yield (to agronomists, this seemed evident), so anything that promoted this was a Good Thing and bound to happen. This overlooked a couple of important points, of course, that were not widely considered in those plant-centred days. Firstly, there was no obvious connection between the external negotiation that led to the initiation of nodulation and the internal bartering that determined benefit once nitrogen fixation began. Secondly, the plant was not the only party that stood to gain from the symbiosis: the rhizobium had an interest, too, and might be competing for its share of the spoils. Eventually, examples of rhizobial strains that were aggressive competitors for nodulation but were poorly effective punctured the naively optimistic view.
When the Evolutionary Biologists came on the scene, they brought a different mindset. Selfishness and cheating were the default states; it was mutualism that needed special explanation and stabilisation by sanctions and policing. Changes that improved the fitness of one partner might well be at the expense of the other partner.
Maren asks a simple question: do mutations that increase or decrease the fitness of the rhizobium in symbiosis also change host fitness in the same direction, or do they change host fitness in the opposite direction? To answer this, she has scoured the rhizobium literature for suitable data. Overall, she found a tendency for the fitness of the rhizobium and the host to move in the same direction. In fact, she did not find any cases of serious ‘cheating’, in which the rhizobium got fitter but left the plant worse off. The most common mutational effect was that the rhizobium’s own fitness declined, but the plant did not gain anything. The rhizobium was defective, not defecting.
An impressive feature of this paper is that Maren is an evolutionary biologist who really knows the relevant rhizobium research literature, going right back to the pioneering work on competition by the likes of Noëlle Amarger and Eden Bromfield. She has compiled a bibliography that we should all take a good look at.
There are plenty of caveats, of course, that stem from using ‘borrowed’ data. Second-hand clothes seldom fit perfectly. In this case, the data were collected for other reasons, and the most appropriate measurements were not always taken. Maren is well aware of the issues and discusses them at some length. We could have much better data if experiments were designed explicitly to test these hypotheses about the evolution of the symbiosis. This provides a challenge and an opportunity for rhizobium researchers looking for worthwhile studies. Who is going to set up the definitive experiment?
 Friesen, M.L. (2012) Widespread fitness alignment in the legume–rhizobium symbiosis. New Phytologist. http://dx.doi.org/10.1111/j.1469-8137.2012.04099.x