This Week in Evolution

Ants that grow fungi for food have to control other fungi that attack their gardens, but what about fungi that attack the ants themselves? Two papers published recently reveal surprising sophistication in both ants and fungi.

Sandra Anderson and colleagues discuss "The life of a dead ant: the expression of an adaptive extended phenotype" in American Naturalist. Richard Dawkins coined the term "extended phenotype" to refer to a consistent effect of a gene inside an individual on something outside that individual. For example, it might be possible to link differences in the shape of webs made by different spiders to genetic differences among those spiders. This week's paper shows that ants infected by certain fungi show complex behavior that benefits the fungi. Ants infected by fungi with different genes would probably not show this behavior, but the genes involved have not yet been identified.

Before the fungus-infected ants die, they attach themselves (by biting) to the underside of leaves that are ideally located for fungal reproduction: on the cooler and moister north side of trees, near (but not on) the ground. The researchers showed that these locations were favorable for fungal reproduction by moving infected ants higher in the canopy or down to the ground. Ants on the ground mostly disappeared, but fungi grew abnormally in those that remained. Fungi were unable to compete their life-cycle on ants moved higher in the canopy.

I can imagine a fungus producing an ant hormone (or perhaps destroying a particular neuron) to make its ant host bite a leaf, but getting ants to bite leaves in a particular humidity and temperature range and then hold on until dying seems pretty sophisticated. It would be easier if the ants spent most of their time in that zone anyway, but the one ant colony they found was much higher, about 15 meters.

The second paper shows greater sophistication on the part of the ants. "Adaptive social immunity in leaf-cutting ants" was published by Tom Walker and William Hughes in Biology Letters. The paper is freely available on-line.

These social ants protect each other from fungal infection by grooming each other, much like meerkats or baboons. Ants exposed to the fungus got groomed about twice as long as ants exposed to a control solution without the fungus, or about three times as long if their nest had been exposed to the same fungus two days before. (Another example of learning in insects.) Ants placed in nests that were previously exposed to the fungus were twice as likely to survive for two weeks after they were inoculated.

Posted by R. Ford Denison at 11:57 PM | Permalink | Comments (3)

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)

I hope to welcome one or possibly two new graduate students in autumn 2010. Here's the summary I wrote for the Ecology, Evolution and Behavior web page:

Research inspired by W.D. Hamilton's ideas, often using microcosms and noncharismatic microfauna: evolution of cooperation and conflict in legume-rhizobium symbiosis (New Phytologist 2009), longevity-vs.-reproduction tradeoff as a possible explanation for hormesis etc. (PLoS One 2009), and agricultural implications of past and ongoing natural selection (Q. Rev. Biol. 2003 and forthcoming book).

Posted by R. Ford Denison at 3:21 PM | Permalink | Comments (1)

"Scientists once thought that wolves chase deer and may even try to eat them, that sharks attack other fish, and that cold weather can kill penguins. But recent research has shown that these so-called threats are actually beneficial, because they encourage togetherness."

OK, I made the above "quotation" up, but the logic is the same as in a recent story in Science about antibiotics:

"there's scant evidence that bacteria or fungi deploy antibiotics to kill or ward off other microbes... These molecules, they assert, may be less weapons for competition or combat than tools of communication... When certain bacteria are exposed to nystatin, the microbes form slimelike communities known as biofilms... this may be just one of the molecule's natural roles."

But why use antibiotics as signals to form biofilms, when bacteria can produce and detect plenty of nontoxic signals? See previous posts on "quorum sensing", but also this discussion.

The most logical reason to form a biofilm in the presence of an antibiotic is to escape from the antibiotic! I am reminded of this quotation about cattle in lion country:

" Yet although the ox has so little affection for, or individual interest in, his fellows, he cannot endure even a momentary severance from his herd. If he be separated from it by strategem or force, he exhibits every sign of mental agony; he strives with all his might to get back again and when he succeeds, he plunges into its middle, to bathe his whole body with the comfort of closest companionship."

The quotation is from Francis Galton, but I got it from Bill Hamilton's 1971 paper, "Geometry for the selfish herd." Other examples in this classic paper -- "Evilutionary Biologist" John Dennehy has written a nice summary -- include reindeer at the edge of a herd suffering much more attack from parasitic insects and gulls nesting at the edge of a colony suffering much more predation.

I am reading a couple of interesting papers relevant to biofilms and may have time to write about them this weekend. As for the real-world role of antibiotics as antibiotics, see this recent post.

Posted by R. Ford Denison at 11:07 PM | Permalink | Comments (0)

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?" »

Posted by R. Ford Denison at 1:09 AM | Permalink | Comments (2)

Where to publish a paper on the genetics of altruism? In an open-access journal, of course! One day after publishing the fossil primate paper that's creating so much excitement -- it's a great fossil, but too old to tell us anything about our recent ancestors, shared with other apes, or the less-recent ones shared with monkeys -- PLoS One published "The Oxytocin Receptor (OXTR) Contributes to Prosocial Fund Allocations in the Dictator Game and the Social Value Orientations Task", by Salomon Israel and colleagues. Like all papers in open-access journals, the full text is available on-line.

These researchers measured altruism in 200 students, based on how each chose to divide a pool of money with another unknown individual. Their hypothesis, based on various past studies, was that the hormone oxytocin is important for social interactions in general and for human altruism in particular. For example, Zak and colleagues showed that sniffing oxytocin made people offer a more generous split when the recipient had the chance to retaliate for a low offer (the "Ultimatum Game"), although not when there was no chance to retaliate, as in the Dictator Game used in the current study.

The researchers tested for statistically significant relations between and different variants of the oxytocin receptor gene, which codes for the protein that responds to this hormone signal in the brain, and "prosocial responses" (generosity) in the Dictator Game and a more-complex version, the SVO. Interestingly, none of the genetic differences they looked at were in the protein-coding part of the gene (orange). Most were in an intron, which would be transcribed from DNA into messenger RNA but then cut out before the mRNA is translated into protein. So I assume these genetic differences could affect how much oxytocin receptor protein is made where and when, but not the structure of the protein itself.

Continue reading "Oxytocin and the genetics of altruism" »

Posted by R. Ford Denison at 6:00 PM | Permalink | Comments (0)

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" »

Posted by R. Ford Denison at 8:10 PM | Permalink | Comments (0)

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.

Continue reading "Mindless manipulation" »

Posted by R. Ford Denison at 7:53 PM | Permalink | Comments (0)

If being infected is bad, is being infected by two different pathogens at once even worse? Not necessarily, as this week's paper shows. "Quorum sensing and the social evolution of bacterial virulence" was published in Current Biology by Kendra Rumbaugh and colleagues. Their results contradict an earlier prediction, although not the fundamental evolutionary principle behind that prediction.

Continue reading "Mixed infections, for better or worse" »

Posted by R. Ford Denison at 4:46 PM | Permalink | Comments (0)

My NSF grant will run out soon, so I get to spend the year in which we celebrate the 200th anniversary of Darwin's birth and the 150th anniversary of The Origin of Species as an amateur scientist, like Darwin himself. I'm not as smart or as rich as he was, but I do have imaginative and hard-working students and much better equipment.

I'm working on two grant proposals and several papers while dreaming of getting back to writing my book, so no detailed paper analysis this week. But Nature is highlighting 15 major papers on evolution they have published in the last few years.

Posted by R. Ford Denison at 5:11 PM | Permalink | Comments (0)

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)

Experiments on the evolution of cooperation can be simpler with microbes than with animals. Microbes do sometimes cooperate. For example, some amoebae get together and form a stalk (consisting of hundreds of individual cells) to elevate spores (many more individual cells) above the soil. Bacteria may collectively produce and release enough extracellular enzymes to make food available for all, when none of them could make enough enzyme alone. But why should an amoeba volunteer to be in the stalk (an evolutionary dead end) rather than becoming a spore? And why not save the cost of making expensive extracellular enzymes, by free-loading on enzyme production by others? Out of the goodness of their hearts? They don't have hearts.

Cooperation is easiest to understand among microbes that share the same allele (one of several alternative versions of a gene) for cooperation. Kin-selection theory says that an allele leading to some individually costly activity may spread if it preferentially benefits others with the same allele, relative to those with different alleles. Cooperation among relatives, who are more likely to share a given allele, is therefore easy to understand.

But can microbes recognize kin, i.e., whether another microbe has the same alleles? Sometimes, apparently. In this week's paper, Elizabeth Ostrowski and colleagues report in PLoS Biology that Kin discrimination increases with genetic distance in a social amoeba.

Continue reading "We cooperators need to stick together" »

Posted by R. Ford Denison at 7:32 PM | Permalink | Comments (0)

Cooperation is widespread in nature, despite theoretical predictions that "cheating" mutants could displace cooperators over just a few generations of evolution. We don't apply human moral standards to other species, of course, but define cheating as contributing less, while benefiting from activities of others. The evolutionary persistence of cooperation is usually attributed to reciprocity (trading resources or services) or to kin selection: cooperation among relatives, such as parental care, can persist even without reciprocity. Fish that clean parasites from other fish are a standard example of reciprocity -- they get to eat the parasites -- whereas nonreproductive worker ants are a standard example of kin selection. I will briefly discuss one recent paper on each of these.

Redouan Bshary and coauthors report in Nature that "Pairs of cooperating cleaner fish provide better service quality than singletons." Cleaner fish often prefer to eat client mucus (yum!) than client parasites, but clients don't like this and tend to leave. When a male and female cleaner work together, the client fish may leave if either of them takes a bite of mucus. Females, in particular, were less likely to do this when cleaning with their male partner rather than alone. The authors also did an experiment to see whether cleaner fish would eat a less-preferred food (fish flakes, perhaps analogous to client parasites) if eating their more-preferred food (prawns, perhaps analogous to client mucus) resulted in the food plate being taken away. They did, especially the females. This may have been because the male often chased her if she ate a prawn, costing them both the rest of their meal. Overall, pairs appear to provide better service to clients, because they are better-behaved together than alone, especially the female.

Shigeto Dobata and coauthors reported on "Cheater genotypes in the parthenogenetic ant Pristomyrmex punctatas" in Proceedings of the Royal Society. Social insects, such as ants and bees, usually have reproductive queens and nonreproductive workers. Worker genes are transmitted to the next generation by the queen, who is typically the workers' mother and therefore shares most of their genes. Pristomyrmex punctatas is different. An individual ant may reproduce (usually when young) and also work. Some individuals are more like queens, however. These are larger, reproduce more, and do little or no work for the colony. If these nonworking ants were close relatives of the workers, this behavior could perhaps be maintained by kin selection. It could be an example of division of labor for mutual benefit, a less-extreme version of the more-familiar worker/queen division. So Dobata and coauthors analyzed the DNA of hundreds of ants to see how they were related. They found that these nonworking ants were much less closely related to workers than queens usually are. Most of the ants (working or not) reproduced parthenogenetically, essentially cloning themselves without sex. Working hard while unrelated individuals profit from your work doesn't usually work out over the long run, but these nonworking ants have been seen in the field for over 25 years. Is this an evolutionary dead end? A similar situation with Cape honey bees, whose colonies are parasitized by unrelated "pseudoqueens" usually leads to colony extinction. I look forward to reading more about this interesting ant species.

Posted by R. Ford Denison at 12:44 AM | Permalink | Comments (0)

I just heard an interesting talk by Joan Silk on lasting friendships among female baboons, in which grooming and mutual support during conflicts are both important. Here’s a link to some of her papers. This week’s paper is on a somewhat-related topic, but in birds rather than apes.

“Duration and outcome of intergroup conflict influences intragroup affiliative behaviour� was just published in Proceedings of the Royal Society by Andrew Radford, of the University of Bristol.

Woodhoopoes are African birds (videos here) that live in small groups, typically a breeding pair and some close relatives. Conflicts over territory with neighboring groups (mostly yelling at each other) are common, often more than once a day. Neighbors rarely take over each other’s territories, but if they win the shouting match they stay and forage for awhile. Do such conflicts and their outcomes affect group solidarity?

Continue reading "Conflict builds cooperation" »

Posted by R. Ford Denison at 7:31 PM | Permalink | Comments (0)

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?" »

Posted by R. Ford Denison at 6:11 PM | Permalink | Comments (1)

I’m done with two grant proposals, revising a book chapter, and checking the final version of a review article. I still have a pile of interesting reading and writing to do before I can get back into the lab – actually, I did help Ryoko set up an experiment yesterday – but no more looming deadlines for awhile. So, here are two more summaries of talks from Evolution 2008.

Do I know you?

The ability to tell other individuals apart by their faces is presumably maintained by natural selection, so you can recognize and avoid bad guys. But is there also selection for looking different enough to be recognizable? Or is it better to blend in with the crowd, so you can get away with stuff?

Michael Sheehan and Elizabeth Tibbetts are studying individual recognition in wasps (Tibbetts and Dale, 2007). Their hypothesis is that distinctive-looking individuals benefit, because they get in fewer fights over dominance.

Continue reading "More talks from Evolution 2008" »

Posted by R. Ford Denison at 9:37 PM | Permalink | Comments (0)

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)

The beehive was an early Mormon icon, symbolizing hard work and cooperation. To an evolutionary biologist, however, a beehive could symbolize reproductive skew, a situation where some individuals reproduce much more than others. Extreme reproductive skew is one of the defining characteristics of eusocial species, of which honey bees are a prime example. Reproductive skew can differ between the sexes. In honey bees, the queen lays most of the eggs, and most females don’t reproduce at all. Polygamous species and groups show the opposite pattern: males vary much more in reproductive success than females do. Maybe an inverted beehive would have been a better symbol. Note that the cells in our bodies behave somewhat like a eusocial bee colony; any children we have are directly descended from a few sex cells, while brain cells and skin cells play the supporting role of worker bees.

This week’s paper, “Ancestral monogamy shows kin selection is key to the evolution of eusociality� was published in Science by William Hughes and others. Like humans, some bees are monogamous, meaning that the queen mates with only one male, so her daughters (the workers) are all sisters. In other bee species, the queen mates with several males, so her daughters are half-sisters. Relatedness generally favors cooperation, although there are some possible complications, discussed below.

This week’s paper asks how mating behavior affects the evolution of eusociality. They reasoned that, if mating system doesn’t matter, then today’s eusocial species could be descended from either monogamous, polygamous, polyandrous (each female has multiple mates), or promiscuous ancestors. Alternatively, eusociality may evolve more easily with one of these mating systems than with the others.

Continue reading "Traditional values in bees" »

Posted by R. Ford Denison at 9:47 PM | Permalink | Comments (1)

(Top) A small leafcutter worker atop a leaf guards her sister against attacks by parasitic flies. Ants carrying leaves cannot use their mandibles for defense, so they carry hitchhikers to ward off the parasites. (Bottom) The fungus garden in a nest of Atta leaf-cutter ants. Notice the diversity of ant sizes within a colony, from the large red soldier ants to the minute orange ants tending to the garden. Atta ants have some of the most sophisticated caste systems among the social insects. -- photos and captions from Alex Wild (mymercos.net)

I’ve never done research on the fungal “farms" of ants and termites, but I’ve been interested in them every since a camera company bought a close-up photo (not Photoshopped like this one) of an ant carrying a leaf along a barbed wire “bridge" on its way back to its nest, from my mycologist father, William Denison. Dad was best known for pioneering research in the tops of tall trees, but never had to fight a shaman, as far as I know.

Continue reading "Pest control for ants" »

Posted by R. Ford Denison at 6:57 PM | Permalink | Comments (6)

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 6:09 PM | Permalink | Comments (1)

This week’s paper is Facultative cheater mutants reveal the genetic complexity of cooperation in social amoebae published in Nature by Lorenzo Santorelli and colleagues at Rice University and Baylor College of Medicine, both in Texas.

The evolution of cooperation is a central problem in the history of life. Darwin explained how sophisticated adaptations -- “the structure of the beetle which dives through the water… the plumed seed which is wafted by the gentlest breeze� -- can evolve in a series of small improvements over generations. But some of the major transitions in evolution are harder to explain, because It seems that they should have been opposed, rather than supported, by natural selection. The origin of multicellular life is a good example. It’s not that hard to imagine independent cells working together in loose groups for mutual benefit – huddling together for defense, say – but why would a cell give up the ability to reproduce, as most of the cells in our bodies have done?

Continue reading "Knowing when not to cheat" »

Posted by R. Ford Denison at 9:41 PM | Permalink | Comments (0)

Punishing cheaters selects against cheating, but what selects for punishing? Are the answers different, depending on whether the species involved have brains? A recent internet experiment suggests that altruistic punishment, perhaps unique to humans, doesn't promote cooperation as effectively as previously thought.

My own research focuses on cooperation in species without brains. We showed that “sanctions� imposed by legume plants limit the evolution of “cheating� rhizobium bacteria (those that divert more plant resources to their own reproduction, relative to other rhizobia, by investing less in fixing the nitrogen needed by the plant). We think individual plants help themselves by imposing sanctions that limit wasteful resource use by less-beneficial rhizobia – they don’t do it for the benefit other legumes.

In theory “altruistic punishment� (paying some cost or taking some risk to punish noncooperators) could help explain why there is more cooperation among unrelated humans than might otherwise be expected. (Cooperation among relatives is explained by kin selection.) But how much are individuals willing to pay to punish noncooperators?
The latest experiments attempting to answer this question were just published on-line in Proceedings of the Royal Society, by Martijn Egas and Arno Riedl: The economics of altruistic punishment and the maintenance of cooperation.

Continue reading "Altruistic punishment? Maybe not." »

Posted by R. Ford Denison at 11:46 PM | Permalink | Comments (0)

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)

Two papers on cooperation this week. If you were trying to help someone, but end up causing problems for them, were you being cooperative? I have no idea, so I like to study cooperation in microbes. Microbes don't have brains, so "intent" isn't a factor. And the only definition of "benefit" that makes sense is an increase in Darwinian fitness or reproductive success, which is often easy to measure in microbes; just count them.
I like these definitions:

Cooperation: a behaviour which provides a benefit to another individual (recipient), and which is selected for because of its beneficial effect on the recipient. [Exhaling CO2 isn't cooperation; it evolved as a side-effect of breathing oxygen, not to benefit plants.]
Cheaters: individuals who do not cooperate (or cooperate less than their fair share), but are potentially able to gain the benefit of others cooperating. ["Equal share" might be less ambiguous.]

Continue reading "Cooperation and cheating in microbes: quorum sensing and persisters" »

Posted by R. Ford Denison at 5:27 PM | Permalink | Comments (0)

The theme of the latest issue of Current Biology is "Biology of Societies." There are reviews on the social life of spiders, crows, hyenas, amoebae, and insects, plus the role of cognition in social interactions among humans. If you are interested in the evolution of cooperation, it might be worth a trip to your nearest university library (if you don't have access via the web) to browse this issue.

I particularly liked "Evolutionary Explanations for Cooperation" by Stuart West, Ashleigh Griffin, and Andy Gardner. Their review reprints figures from several recent papers, so you can see some of the data upon which their generalizations are based. I won't try to summarize the whole thing, just some points that may have been neglected in other reviews of this topic.

Continue reading "Evolution of cooperation reviewed" »

Posted by R. Ford Denison at 10:10 PM | Permalink | Comments (1)

This week, instead of discussing a paper, I will summarize some presentations from the Ecological Society of America meetings last week in San Jose, California (a much more interesting place than I expected, including hands-on transformation of bacteria at the Tech Museum).

I ran into two people who admitted to reading this webblog: Madhu Katti and Don Strong, but didn't get to talk to either of them for long. There were usually several interesting sessions going on at once, from 8 AM to late evening, plus lots of informal discussions, but I will limit my summary to a few of the presentations on the evolution of cooperation, my own area of research.

Continue reading "Cooperation gets complex" »

Posted by R. Ford Denison at 10:15 PM | Permalink | Comments (0)

This week's paper, published in Science (317:679) is "Immune-like phagocyte activity in the social amoeba" by Guokai Chen, Olga Zhuchenko, and Adam Kuspa of the Baylor College of Medicine.

Cells of the social amoeba, Dictyostyleium discoideum forage individually, but eventually group together into a "slug", which crawls through the soil for days before eventually forming a spore-tipped stalk. Previous work with this species has looked at conflicts of interest over which cells have to sacrifice future reproduction (as spores) and become part of the stalk. This week's paper uncovers another example of apparent altruism in Dictyostelium, which may shed light on the evolution of a key part of our immune system.

As a Dictyostelium slug crawls through the soil, some cells are left behind. Are these just random sluggards? Or do they function like human phagocytes, the immune system cells that gobble up bacteria?

Continue reading "Left behind: social amoebae" »

Posted by R. Ford Denison at 6:25 PM | Permalink | Comments (5)

My wife and I have a bird feeder outside our kitchen window. Yesterday I saw an adult male cardinal feeding some of the seed to an immature cardinal not much smaller than he was. I guess it's hard to say "no", but should he have?

This week's paper, "The adaptive value of parental responsiveness to nestling begging" by Uri Gordzinski and Arnon Lotem, published online in Proceedings of the Royal Society, may have answered this question.

Continue reading "Begging: the question" »

Posted by R. Ford Denison at 12:38 AM | Permalink | Comments (4)

This week I will discuss two papers, both of which consider possible benefits of biological diversity. In interpreting the data in the experimental paper, on bees, we need to remember that a given set of data can often be consistent with two or more different hypotheses. This point is reinforced in the review article, which discusses the relationship between diversity and stability of ecosystems.

The experimental paper is "Genetic diversity in honey bee colonies enhances productivity and fitness" by Heather Mattila and Thomas Seeley, of Cornell University (Science 317:362). The review article is "Stability and diversity of ecosystems" by Anthony Ives and Stephen Carpenter, of the University of Wisconsin (Science 317:58).

Continue reading "Diversity, stability, productivity, and policing" »

Posted by R. Ford Denison at 6:34 PM | Permalink | Comments (0)

In the classic cartoon posted on my office door, little Calvin refuses to take a phone message for his father, saying "people always assume you're some kind of altruist." Two papers in the latest PLoS Biology show that some altruistic behaviors can be found in chimps and rats, as well as humans. Should we be surprised?

Continue reading "Low-cost cooperation" »

Posted by R. Ford Denison at 10:53 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)

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)

Given my location halfway between the Twin Cities of Minneapolis and St. Paul, and my childish love of clever acronyms, I sometimes wish I'd named this blog This Week In Natural Selection. But then I suppose I'd have to review a pair of closely related papers each week. I'm going to do that this week, anyway.

This week's twins were both published in PLoS Biology, so both are freely available on-line. Both have new data on bacteria that infect insects. Both help us understand the conditions under which infecting bacteria evolve to be beneficial, rather than harmful. Finally, both disprove, again, the popular idea that any evolutionary change big enough to matter (except antibiotic resistance, which a creationist commenter once claimed always involves "horizontal transfer" of genes among bacteria, even though resistance often evolves in bacteria in a closed container all descended from a single cell) always involves lots of genes and takes millions of years. Evolution is our present and future, not just our past.

Continue reading "Rapid evolution of beneficial infections" »

Posted by R. Ford Denison at 10:29 PM | Permalink | Comments (2)

Paul Rainey has a very interesting essay in the April 5 issue of Nature. Much of what we know about "cheating" in bacteria that form floating mats comes from his research, including collaboration with Michael Travisano, recently hired here at University of Minnesota. See my earlier post, "how disturbed are cheaters", for background on this system. Although cheaters that don't invest in the goop that holds floating mats together can result in mats breaking up and sinking, Rainey's new essay suggests that a similar form of cheating may have contributed to the evolution of multicellular life.

Continue reading "Helpful cheaters?" »

Posted by R. Ford Denison at 4:00 PM | Permalink | Comments (3)

Major transitions in evolution have often involved loss of independence, as discussed last week. Most female bees work to increase their mother’s reproduction, rather than laying eggs themselves. Less extreme examples of helping others reproduce are known in some animals. “Kin selection� favors helping relatives, if the cost of helping is less than the benefit to the one helped, times their relatedness to the helper. This is known as Hamilton’s Rule. As Haldane put it, “I would jump into a river to save two brothers or eight cousins.� “Cost� and “benefit� are measured in number of offspring and “relatedness� is relative to one’s usual competitors. If surrounded by cousins, Hamilton’s Rule would lead to helping only siblings.

For helping behavior to have evolved, there must have been genetic variation in helpfulness. This week’s paper shows that this is still true for western bluebirds in Oregon.

Continue reading "Evolution of babysitting in bluebirds" »

Posted by R. Ford Denison at 6:10 PM | Permalink | Comments (4)

Yesterday, my wife asked, "why are there so many theoretical papers in evolutionary biology?" I suggested one reason may be that evolutionary theory is better developed, in the sense of making accurate predictions, than theory in much of biology. This week's paper, comparing results from an evolution experiment to predictions of a mathematical model, is a good example.

The paper is about the evolution of cooperation. This is a hot topic and also my own area of research. Humans enforce cooperation, to varying extents. For example, we often punish cheaters, those who try to benefit from cooperative activities of others without contributing anything themselves. Human cheaters are mostly pretty stupid -- don't even think about plagiarizing this blog for a term paper! -- but what about cheaters with no brains at all?

Continue reading "How disturbed are most cheaters, really?" »

Posted by R. Ford Denison at 10:16 PM | Permalink | Comments (4)