This Week in Evolution

Just as I was starting to dip into retirement savings to keep my lab going, we got word that both of the grant proposals we sent to the NSF in the latest round were funded, one of them with money from Obama's stimulus funding. We won't be paying ourselves any billion-dollar bonuses, but I may be able to get two months salary this year after all. Both proposals are resubmissions, significantly improved based on suggestions and criticisms from past reviewers. Both projects will use rhizobia, bacteria best known for providing legume plants with nitrogen, but the second project may have eventual applications in medicine (e.g., curing persistent infections) rather than agriculture. The summaries below are intended for a nonscientific audience, such as members of Congress.

"Suppression of rhizobial reproduction by legumes:
implications for mutualism"
(with Prof. Michael Sadowsky, largely based on ideas and preliminary results from grad student Ryoko Oono -- see this recent review article we wrote with Toby Kiers)

Rhizobia are bacteria that can live in soil, but also symbiotically, inside root nodules on plants like soybean or alfalfa. Although many rhizobia provide their host plants with nitrogen, saving farmers billions in fertilizer costs, less beneficial strains cause problems in some areas. Some hosts, including alfalfa and pea, make rhizobia swell up as they start to provide nitrogen. Unlike the nonswollen rhizobia from soybean or cowpea nodules, swollen rhizobia apparently lose the ability to reproduce, but does rhizobial swelling somehow benefit the plant?

To find out, the investigators will map this trait on the family tree for crops and wild plants that host rhizobia, to see if causing swelling evolved more than once, suggesting a positive benefit to the plants. Three dual-host rhizobia (plus mutants that differ in their ability to hoard resources) will be used to measure effects of rhizobial swelling on costs and benefits to the plants. Plant defenses against rhizobia that provide little or no nitrogen, already demonstrated in soybean, will be tested in species that impose bacterial swelling.

This research will increase understanding of a symbiosis that supplies nitrogen to agricultural and natural ecosystems, with implications for other important symbioses. Results could guide the development of crops that selectively enrich soils with the best rhizobia, decreasing future fertilizer requirements. Educational opportunities will be provided for undergraduates, at least one graduate student, and a postdoctoral researcher. Two female high school students have already won trips to the International Science Fair for research done in the principal investigator's laboratory, where such mentoring will continue to be a priority.

Evolution of persistence in the model bacterium, Sinorhizobium
(with Prof. Michael Travisano, largely based on ideas, preliminary data, and writing by grad student Will Ratcliff, with some ideas from Andy Gardner and colleagues -- see the second paper discussed in this post -- and possible relevance to our work on evolution of aging.)

Some bacteria can enter a nongrowing "persister" state that allows them to survive antibiotics and other treatments that normally kill them. By suspending growth, they may also free resources for their genetically identical clonemates.

Most species form only a few persisters. This makes persisters hard to study, despite their importance in long-term infections. However, certain harmless bacteria from plant roots can form up to 40% persisters. These will be used to determine whether persisters benefit mainly from enhanced stress resistance or by increasing the growth of their clonemates.

Successful completion of this research will provide two main benefits: First, this research will determine the conditions that favor the spread of persister-forming bacterial strains over nonpersister strains, and the genetic basis of persistence. This can provide direct medical benefits by aiding the development of novel management strategies, drug targets, and eventually treatments for patients infected with persister-forming bacteria. Second, some conclusions may apply to other species that are difficult to eradicate because they, too, form dormant, stress-resistant stages. These include many agricultural weeds and some species of mosquito. One key advantage of the proposed approach is speed: experiments that would take decades with weeds or mosquitoes can be conducted in months with bacteria. This research will provide training opportunities and jobs for undergraduates, high school students, and a post doctoral researcher.

I am planning to accept another grad student for autumn 2010.

Posted by R. Ford Denison at 12:04 AM | Permalink | Comments (1)

"Can plants predict the future?" asked one of my Crop Ecology lectures at UC Davis. Yes, they can. Plants use decreasing daylength to predict oncoming winter, and flower early enough to finish seed development before it gets too cold. Some plants detect early signs of drying soil and reduce their own water use, saving water in the soil for later.(Davies & Zhang. 1991, Bano, et al. 1993) Others detect "distress signals" from neighbors under insect attack, turning on chemical defenses in anticipation (Karban, et al. 2004).

But these anticipatory responses do not require learning: a beneficial change in individual behavior in response to individual experience. An alfalfa plant will never learn that the farmer always irrigates before it actually runs out of water. At least, I assume it won't. An evolving alfalfa population is a different story. Over a few generations under irrigation, genotypes that reduce their water use as the soil starts to dry (thereby reducing their growth) will be out-competed by genotypes that keep using water and growing.

Like plants, microbes can predict the future. As in plants, this trait can evolve. As they pass through the gut, bacteria typically see lactose before they see maltose. So they have evolved to "anticipate" maltose availability, turning genes for maltose use on as soon as they are exposed to lactose. After 500 generations of evolution on lactose without maltose, however, the bacteria have lost lose this anticipatory response, so that they turn maltose genes on only when they actually see maltose.(Mitchell, et al. 2009) The title of the news story in Nature about this work asked whether microbes can "learn from history", but this is clearly not an example of individual cells modifying their responses to lactose based on their individual experience.

Individual insects can learn. But is learning always a good thing? Aimee Dunlap, a grad student in my department working with David Stephens, just published a paper in Proceedings of the Royal Society exploring the conditions under which natural selection will favor learning (Dunlap & Stephens. 2009).

Continue reading "Microbes evolve; flies evolve and learn" »

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Bacteria known as pseudomonads produce and release chemicals (defensive toxins) that protect plants from fungi that would otherwise attack their roots. In return, the roots release various organic compounds that serve as food for the bacteria.

The "in return" part has always bothered me. Each root system is associated with millions of bacteria. In a 2003 paper (Cooperation in the rhizosphere and the "free rider" problem. Ecology 84, 838-845), we pointed out that this system is a potential tragedy of the commons. Mutant bacteria that don't make root-protecting chemicals free up resources for their own reproduction, so we might expect them to out-compete more-beneficial strains. If these "cheaters" become common enough, the host plant might be killed by fungi, but that would hurt the beneficial strains around that root system just as much as it hurt the cheaters. We suggested that the bacteria make these toxic chemicals to protect themselves, with protection of roots as a side effect. Research by others, including some of the authors of this week's paper, has provided data consistent with this hypothesis. For example, toxins made by pseudomonads protect them from predators.

More recently (in Annual Review of Ecology, Evolution, and Systematics), Toby Kiers and I suggested that cooperation between microbes and plants is better understood as cooperation among microbes. For example, by providing their host plant with the nitrogen it needs to grow, rhizobia (root-nodule bacteria) help all the other rhizobia infecting the same plant.

This week's paper shows that defensive toxin production by pseudomonads is similar. Toxin production by pseudomonads may benefit the plant and may benefit individual cells, but it also benefits other pseudomonads nearby. "Predators promote defence of rhizosphere bacterial populations by selective feeding on non-toxic cheaters" was published in The ISME Journal by Alexandre Jousset and colleagues in Germany and Switzerland. These pseudomonads produce various toxins, especially when there are many of them in close proximity. This dependence of toxin production on bacterial density is an example of quorum sensing. Mutants "defective" in quorum sensing have been shown to grow (i.e., reproduce by dividing) faster, because they save the cost of toxin production. Can these "cheaters" free-load on defensive toxin producers nearby, essentially hiding behind their chemical defenses?

Continue reading "Whom do cheating bacteria cheat: host plants or other bacteria?" »

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I once heard PZ reply to this popular creationist question by pointing out that, although many Minnesotans are descended from Norwegians, there are still Norwegians. This isn't really a good analogy, however, because Minnesotans and Norwegians aren't separate species. We know this because they can interbreed, producing healthy children. At the end of this post I suggest a better answer, indirectly inspired by this week's paper.

Two of evolutionary biology's central questions are: how do species change over generations? and how does one species split into two? We have many detailed examples of small evolutionary changes occurring over days (in bacteria) or years (in animals and plants), so one would have to be very close-minded to deny major evolutionary change over millions of years. But major evolutionary change is not enough, by itself, to split one species into two. One subpopulation within a species must change, while the rest of the species either stays the same or changes in different ways. This divergence cannot happen if the two subpopulations continue to interbreed at high rates. In other words, speciation requires some reproductive isolation.

Often, reproductive isolation is a byproduct of geography. After a few individuals (or a pregnant female) cross a mountain range or are blown from the mainland to an island, they no longer interbreed with their ancestral population. Over many generations, random genetic drift or nonrandom natural selection can change the isolated population enough that they can no longer produce healthy offspring with the original population, even if they come back into contact.

Sometimes speciation can occur without a major geographic barrier, but reproductive isolation is still required. This week's paper shows that this has happened and is still happening in Europe.

"A continuum of genetic divergence from sympatric host races to species in the pea aphid complex", by Jean Peccoud and others, was just published online in the Proceedings of the National Academy of Science.

Photo by Jean Peccoud

Continue reading ""If evolution is true, why are there still chimps?"" »

Posted by R. Ford Denison at 10:45 PM | Permalink | Comments (8)

This week I will discuss two papers, both dealing with plants and competition, in the context of genetic relatedness that might be expected to moderate competition:
"Growing with siblings: a common ground for cooperation or for fiercer competition among plants?" by Ruben Milla and colleagues (Proceedings of the Royal Society), and
"Do plant parts compete for resources? An evolutionary viewpoint" by Victor Sadras and me (New Phytologist).

Earlier I discussed a paper by Susan Dudley and Amanda File showing that some plants grow less root when interacting with related than with unrelated neighbors. Spending less resources on roots could have freed resources for more seed production, but they didn't measure that. Now Milla and colleagues have.

Continue reading "Sibling rivalry in plants" »

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Suppose, in an average year, that weather is best for survival of young seedlings in early June. If a plant could make only one seed, it should make one targeted to germinate at the beginning of June. (Plants have some control over when their seeds will germinate, based on plant-hormone concentrations, seed-coat thickness, etc.)

Given variability among years, however, a plant that produces many seeds may have more descendants if those seeds germinate at various times, rather than risking everything on one date. On the other hand, too much variation in germination time may be as risky as too little. For example, seeds germinating really late may be killed by frost. How does actual variation in germination timing among seeds of individual wild plants compare with the optimum amount of variation? This week's paper is apparently the first to answer this question.

"Fluctuating natural selection accounts for the evolution of diversification bet-hedging" was published in Proceedings of the Royal Society by Andrew Simons, of Carleton University in Canada.

Continue reading "Optimal bet-hedging?" »

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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.

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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.

Continue reading "Mindless manipulation" »

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Experimental populations of hermaphroditic plants evolved a significant increase in male function in only three generations.

Many plant species are hermaphrodites, with each individual producing both pollen and seeds. Others species have separate sexes, as mammals and birds do, while still others have mixtures of unisexuals and hermaphrodites. Based on the distribution of these traits in the family tree of life, evolutionary transitions among these "lifestyles" appear to have been fairly common. This week's paper shows how hermaphrodites can evolve to be more female or, in this case, more male. Hermaphroditic Sex Allocation Evolves When Mating Opportunities Change was just published in Current Biology by Marcel Dorken and John Pannell.

Continue reading "How fast can sexual traits evolve?" »

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Several of us are meeting for dinner tonight in honor of Darwin's 200th birthday, February 12. Our hostess suggested bringing a favorite Darwin quote. It's hard to choose! Here are some I'm considering:

I have invariably found that our knowledge, imperfect though it be, of variation under domestication, afforded the best and safest clue. I may venture to express my conviction of the high value of such studies, although they have been very commonly neglected by naturalists.

...variations, however slight and from what ever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the offspring. I called this principle, but which each slight variation, if useful, is preserved, by the term Natural Selection, in order to mark its relation to Man's power of selection.

We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man's feeble efforts, as the works of Nature are to those of Art.

If it profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the down in the pods on his cotton-trees.

As more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life. It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for this case there can be no artificial increase of food, and no prudential restraint from marriage.

That climate acts in main part indirectly by favoring other species, we clearly see in the prodigious number of plants which in our gardens can perfectly well endure our climate, but which never become naturalized, for they cannot compete with our native plants nor resist destruction by our native animals.

Look at a plant in the midst of its range, why does it not double or quadruple its numbers? We know it can perfectly well withstand a little more heat or cold, dampness or dryness, for elsewhere it ranges into slightly hotter or colder, damper or drier districts. In this case we can clearly see that if we wish in imagination to give the plant the power of increasing in number, we should have to give it some advantage over its competitors, or over the animals which prey on it.

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.

There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.

Posted by R. Ford Denison at 11:46 PM | Permalink | Comments (5)

Evolutionary theory suggests that cooperation should often be unstable, even when it benefits all concerned. Toby Kiers and I discussed this problem and some possible solutions in a recent review article in Annual Review of Ecology and Evolution. Meanwhile, Jim Bever and colleagues have published some important experimental data in Ecology Letters, helping to explain cooperation between plants and fungi, a little too late to include in our review. Their paper is titled: "Preferential allocation to beneficial symbiont with spatial structure maintains mycorrhizal mutualism."

Many bacteria and fungi associated with plant roots benefit more from healthy plants than from dead or dying ones. If each individual plant were colonized by only one strain of bacteria or fungus, then strains that helped their host plants (by providing them with nitrogen or phosphorus, for example) would indirectly help themselves, gaining an evolutionary edge over their competitors of the same species. These beneficial strains would become more common in each generation. In other words, cooperation would evolve.

The problem is that each plant is typically associated with several strains of each species of bacteria or fungus. Strains that invest less in helping the plant have more resources to spend on their own reproduction. If less-generous strains benefit equally from the contributions of more-generous strains on the same plant, then less-generous strains will become more common over generations. In other words, “cheating� will evolve.

Continue reading "Spatial structure and the evolution of cooperation between microbes and plants" »

Posted by R. Ford Denison at 10:25 PM | Permalink | Comments (0)

Genes with new functions do not magically appear from nowhere, or so most scientists assume. (If I thought that evolution, perhaps especially human evolution, was being guided by some supernatural individual or group, I would be looking for such “genes from nowhere� rather than whining that theories with no evidence should get equal time. Not that they want schools to teach all theories that lack evidence, of course, just ones favored by their particular religion or short-term economic interests.)

Random duplication of existing genes is often a key step, but there are at least two different ways in which gene duplication could facilitate evolutionary innovation. Once there are two copies of a gene, one could evolve a new function without interfering with the old gene’s function. Or, a single gene could evolve two different functions, doing neither of them particularly well. Then, gene duplication would allow the two copies to evolve separately, each being optimized for a different function. This week’s paper shows that evolution has followed this second pathway at least once, and perhaps often. The paper also provides yet another example of how molecular methods are providing new details on how evolution works.

“Escape from adaptive conflict after duplication in an anthocyanin pathway gene� was published in Nature by David L. des Marais – apparently not the the David J. des Marais who has published on evolution of photosynthesis – and Mark Rausher, at Duke University in North Carolina. They looked at genes whose enzyme products make pigments for flower colors.

Continue reading "How does gene duplication allow evolutionary innovation?" »

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This week’s paper (Ratcliff, et al., in press) is from my lab.

The relationship between a legume plant and the rhizobium bacteria living in its root nodules is usually beneficial to both. Rhizobia convert nitrogen from the atmosphere into a form their plant host can use to make proteins. Rhizobia benefit from infecting plants because they reproduce more inside nodules than they would in the soil. They can also acquire certain resources in nodules. In particular, they can accumulate large amounts of energy-rich PHB – up to 50% of their own weight!

But there can also be conflicts of interest between rhizobia and their host plants. Resources used to make PHB could have been used, instead, to acquire more nitrogen for the plant. Therefore, mutant rhizobia that don’t make PHB provide their hosts with more nitrogen (Cevallos, et al., 1996). So why do most rhizobia make PHB? Our hypothesis has been that the rhizobia themselves benefit from having more PHB. This contrasts with an earlier hypothesis that rhizobia store PHB so that they can use the energy for the benefit of the plant. Consistent with our hypothesis, mutants defective in PHB metabolism reproduce less (Cai, et al., 2000). However, mutations can have complex interacting effects, making it hard to be sure that differences in PHB were the only cause of differences in reproduction. So Will Ratcliff decided to compare rhizobia that were genetically identical but had different amounts of PHB per cell.

Continue reading "Would the host want anyone to starve?" »

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About 1500 scientists attended Evolution 2008 here last week. The four-day meeting was filled with 15-minute talks (usually ten at once, in different rooms), plus two evening poster sessions (like a science fair, for grownups, with discussions rather than judging), scenically located on a pedestrian bridge over the Mississippi. Reports that “scientists are abandoning evolution�? appear to be exaggerated.

Here are summaries of some of the talks I enjoyed.

Continue reading "Evolution 2008: sexy plants, battling bacteria, durable cooperation" »

Posted by R. Ford Denison at 8:36 PM | Permalink | Comments (3)

“Under drought conditions,� says Bänziger, CIMMYT’s director for corn research, “the maize plant puts more resources into pollen formation and less into seeds.� From the plant’s point of view this makes sense. Pollen is much cheaper energy-wise for the plant to make, and, if the pollen manages to fertilize another plant’s seed, the drought-afflicted parent will still contribute 50% of its genes to the offspring. But this is of little help to farmers, who sell kernels, not pollen." -- Nature 452:273

Maize plants are hermaphrodites, having both male (pollen-producing) and female (seed-producing) flowers. Other plant and animal species have two sexes, such as males and females. From the title, “Density-dependent regulation of sex ratio in an annual plant�, I assumed that this week’s paper (by Marcel Dorken and John Pannell, published in American Naturalist) would be about how parent plants adjust the male:female ratio in their offspring, a topic I have discussed previously.

But no. Mercurialis annua is stranger than that. Its two “sexes� are male and hermaphrodite.

Continue reading "Regulation of sex ratios in plants" »

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“Every one is familiar with the difference between the ray and central florets of, for instance, the daisy… But with respect to the [two types of] seeds, it seems impossible that their differences in shape…can be in any way beneficial�—Charles Darwin

The theory of evolution is famously linked to the Galapagos Islands, but this week’s paper “Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta,� published in Proceedings of the National Academy of Science, studied much smaller “islands.� In an urban environment dominated by concrete, patches of soil around sidewalk trees (below left) are among the few places where plants can grow.

Photo credits: Gilles Przetak and Eric Imbert.

Members of the daisy or sunflower family (Asteraceae) often produce two types of seeds (above right) on the same disk-shaped composite flower head. Seeds from the center of the disk are light in weight and plumed, so they are easily dispersed by wind. Those from the outer edge of the disk are heavier and not plumed, so they tend to fall near the mother plant. Although Darwin apparently failed to see the benefit of having two types of seeds, this kind of diversity acts as a form of bet-hedging. Wind dispersal of seeds over a wide area decreases the chances that all of a plant’s offspring will be killed.

Then why not disperse all of the seeds? Because, given that the mother plant managed to reproduce -- many plants don't -- conditions near the mother plant may be better than where most wind-blown seeds might land. This was particularly true in the study discussed here. Earlier, Jonathan Silvertown pointed out, in an essay titled “When plants play the field," that the ratio of the two seed types changes in beneficial ways with changes in flower head diameter. The area of a disk increases four-fold as the circumference doubles, giving proportionally more of the wind-dispersed central seeds. So the plant will always drop some seeds in the same place that it managed to reproduce. But if favorable conditions lead to larger flower heads, more seeds will be dispersed by wind over a larger area, where they can compete with other plant's seedlings rather than with each other.

So, without any genetic change, this disk-size dependence adjusts the ratio of dispersing to nondispersing seeds to match current conditions. But what if conditions consistently favor more or less seed dispersal? Can this ratio also evolve, with a genetic change over generations?

Continue reading "Fear of flying -- in plants" »

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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!" »

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Two papers this week describe recently discovered sophisticated adapatations of two different parasites: Gall insects can avoid and alter indirect plant defenses, published in New Phytologist by John Tooker and colleagues, and Parasite-induced fruit mimicry in a tropical canopy ant, published in American Naturalist by Steve Yanoviak and colleagues (if you're in a hurry, skip to the end for amazing photos).

Various plants recruit "bodyguards" when attacked by insects. For example, when caterpillars start munching on corn (maize) plants, the plants (including uninjured leaves) release gaseous chemicals called terpenoids. These terpenoids attract parasitic wasps, which lay their eggs into the caterpillars. This eventually kills the caterpillars, which presumably benefits the plant. But what if the caterpillars could prevent the plant from signaling to the wasps? As far as I know, caterpillars haven’t evolved this trick (yet), but there are apparently some insects – the Hessian fly, Mayetiola destructor (say) – that do not trigger signaling when they feed on wheat plants. There are at least two possible explanations…

Continue reading "Tricky parasites winning the evolutionary arms race" »

Posted by R. Ford Denison at 11:24 PM | Permalink | Comments (1)

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" »

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Science advances by disproving previously-tenable hypotheses. For example, "The earth is <10,000 years old" was disproved by annual sediment layers long before we were able to estimate the actual age. Actually, Tom Kinraide and I argued in "Strong inference -- the way of science" that a hypothesis needs to be explanatory as well as falsifiable. So for a young earth to ever have qualified as a hypothesis, it would first have needed to explain at least some real world observations. Right off hand, I can't think of any actual data that an unbiased person would look at and say, "Well, these data would make sense, but only if we assume the earth is <10,000 years old."

Similarly, if someone wanted to convert "intelligent design" from religious whining into a scientific discipline, we'd need some falsifiable hypotheses. Suppose, for example, we hypothesized that current features of plants and animals (not just their single-celled, distant ancestors) were supernaturally-imposed designs to maximize their success. That hypothesis is consistent with the many examples of sophisticated adaptations (err, "design"), but what can we conclude from the many examples of maladaptation ("bad design")? Maladaptation is predicted by evolutionary theory (when current conditions don't match those under which past selection occurred, for example) but if some design team is continuously intervening in evolution, do maladaptations imply that they had a busy week? If so, should we expect the problem to instantly disappear, once they get around to it?

This week's paper is another example of the pattern we see repeatedly in biology: many sophisticated adaptations, but also serious "design flaws." In particular, Acacia trees can be fooled into feeding and housing ants that are harming them.

Breakdown of an Ant-Plant Mutualism Follows the Loss of Large Herbivores from an African Savanna was published this week in Science by Todd Palmer and five coauthors, three of whom I know from my years at UC Davis.

Continue reading "Ants en't ents" »

Posted by R. Ford Denison at 11:26 PM | Permalink | Comments (2)

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)

This week's paper is "Kin recognition in an annual plant", by Susan Dudley and Amanda File of McMaster University, just published online in Biology Letters.

Researchers in several countries have recently shown that roots respond differently to another root from the same plant than they do to a root from a different plant. Typically, they grow more aggressively towards a neighbor's root than towards one of their own. But what if the neighbor is a close relative?

Continue reading "Can plants recognize kin?" »

Posted by R. Ford Denison at 11:35 PM | Permalink | Comments (2)

Two species coevolve when changes in either lead to changes in the other. This includes “arms races� between species that compete with each other, but also interactions that benefit both species. “Gene flow� is the movement of genes from one population into another, of the same or related species. For example, some genes in modern cows seem to have come from mating with wild aurochs, before they went extinct. Gene flow often provides new genes; some may be useful to the recipient population. For example, pollen from transgenic sugar beets could transfer herbicide resistance (along with other crop genes) to related weed beets. More often, genes that were useful in the source environment may be harmful to the recipient population. Natural selection will tend to eliminate these, unless gene flow rates are too high. For example, if plants growing on toxic soil around an old mine are outnumbered by neighbors on nontoxic soil nearby, gene flow may swamp natural selection, preventing evolution of tolerance to toxic soil.

This week I’ll discuss a review article on coevolution and then an experimental paper showing how gene flow can affect coevolution. The review is “Variable evolution� by Elizabeth Pennisi, published in the May 4 issue of Science. It discusses coevolution of wild parsnip with the webworms that eat them and coevolution of pine trees with birds and squirrels, among other topics.

Continue reading "Coevolution and gene flow" »

Posted by R. Ford Denison at 3:31 AM | Permalink | Comments (4)

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 6:08 PM | Permalink | Comments (4)