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Mindless manipulation

This week I'll discuss a recent paper from our lab. But first, here are links to three other papers that look interesting:

Evidence from the domestication of apple for the maintenance of autumn colours by coevolution Some insect pests avoid trees whose leaves turn red in autumn and do poorly on those trees, but can trees "lie" or is there an unbreakable link between red color and poor quality as a host, perhaps because "aphids grow better on trees that drop their leaves later [because they have enough nitrogen they can risk losing high-N leaves in frost?], which are known to have fewer autumn colours [because, by the time they lose chlorophyll, UV levels are too low to require the protection provided by red anthocyanins?]."?

Functional morphology of the ankle and the likelihood of climbing in early hominins Modern chimps use their ankles, when climbing trees, in ways some early hominins (1-4 million years ago) probably couldn't, based on fossils.

Cooperation and virulence of clinical Pseudomonas aeruginosa populations
Patients with pneumonia are sicker when bacterial cells cooperate by producing individually costly virulence factors, but bacterial populations evolved "cheaters" that don't make these factors within 9 days.

In our paper, "Rhizobitoxine producers gain more poly-3-hydroxybutyrate in symbiosis than do competing rhizobia, but reduce plant growth", published online in The ISME Journal, my PhD student Will Ratcliff describes experiments showing how symbiotic nitrogen-fixing bacteria can manipulate their plant hosts.

Legumes use the plant hormone, ethylene, to control the number of root nodules they make to house these bacteria, known as rhizobia. Some rhizobia make a chemical, rhizobitoxine, that blocks ethylene signaling, resulting in more nodules per plant.

We were not surprised to find that a rhizobitoxine-producing (Rtx+) strain resulted in worse plant growth than an otherwise-similar Rtx- strain. Past natural selection has presumably eliminated plant genes that fail to adjust nodule number appropriately to meet a plant's nitrogen needs without wasting photosynthate. Therefore, a microbe-induced change in the way a plant allocates its resources is unlikely to benefit the plant. (Toby Kiers and I discussed this point recently in Annual Review of Ecology, Evolution, and Systematics.) Similarly, we would expect natural selection to have eliminated the rhizobial Rtx+ gene unless it benefited the rhizobia. (It's risky to assume that a given complex trait will necessarily evolve, just because it would be beneficial, but eliminating a complex but harmful trait is a lot easier. There must be hundreds of simple mutations that would knock out rhizobitoxine production, for example.)

Benefits to Rtx+ rhizobia were harder to detect than we expected, however. Although the Rtx+ strain more than tripled the number of nodules per plant, it then occupied only 42% of those nodules, when competing with an Rtx- strain. So, if anything, rhizobitoxine helped the competing Rtx- strain. When the two strains were inoculated together, nodules containing the Rtx+ strain were similar to Rtx- nodules in rhizobia per nodule, so there was no benefit there, either.

So what was different? In a word, polyhydroxybutyrate (OK, it's a pretty long word, so let's just call it PHB). Rtx+ rhizobia accumulated 47% more of this high-energy lipid than Rtx- rhizobia in other nodules on the same plant. Previously, Will showed that rhizobia can use PHB to power reproduction or to survive starvation, so this is an important benefit, but not one we could have detected just by counting or weighing nodules.

There are at least two possible interpretations of these results:
1) Rhizobitoxine tricked the plant into giving Rtx+ nodules more photosynthate, which they used similarly to Rtx- strains. In other words, they hoarded more PHB, but also fixed more nitrogen. In that case, why did plants infected only with Rtx+ rhizobia grow less?
2) Maybe rhizobitoxine somehow protected Rtx+ from the "host sanctions" that can reduce the reproduction of rhizobia in nodules that fix too little nitrogen. Once protected against sanctions, mutants that divert resources from nitrogen fixation to PHB could spread within populations of Rtx+ rhizobia.

"What natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for the good of another species; and though statements to this effect may be found in works of natural history, I cannot find one case which will bear investigation." -- Darwin

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