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January 25, 2013

What's for dinner?

This week's picks all have something to do with food.

Tree climbing and human evolution "aspects of the hominin ankle associated with bipedalism remain compatible with vertical climbing [to collect fruit or honey]"

Earliest evidence for cheese making in the sixth millennium bc in northern Europe" "compelling evidence for the vessels having being used to separate fat-rich milk curds from the lactose-containing whey... in the manufacture of reduced-lactose milk products among lactose-intolerant prehistoric farming communities"

Anatomical enablers and the evolution of C4 photosynthesis in grasses
"when environmental changes promoted C4 evolution, suitable anatomy was present only in members of the PACMAD clade [which doesn't include rice] explaining the clustering of C4 origins in this lineage"

Macropredatory ichthyosaur from the Middle Triassic and the origin of modern trophic networks "recovery from Earth's most severe extinction event at the Permian-Triassic boundary... may have occurred faster [in oceans than on land]"

Sustainable bioenergy production from marginal lands in the US Midwest" "successional herbaceous vegetation, once well established, has a direct GHG emissions mitigation capacity that rivals that of purpose-grown crops "

Extracellular transmission of a DNA mycovirus and its use as a natural fungicide
"Our findings may prompt a reconsideration of the generalization that mycoviruses lack an extracellular phase in their life cycles and stimulate the search for other DNA mycoviruses with potential use as natural fungicides. "

January 11, 2013

Predictability, multiple fitness peaks, fungus-growing ants, pesticide resistance...

Predictability of evolution depends nonmonotonically on population size
"evolutionary predictability based on an experimentally measured eight-locus fitness landscape for the filamentous fungus Aspergillus niger.... entropies display an initial decrease and a subsequent increase with population size N"

Multiple Fitness Peaks on the Adaptive Landscape Drive Adaptive Radiation in the Wild "We measured the adaptive landscape in a nascent adaptive radiation of Cyprinodon pupfishes endemic to San Salvador Island, Bahamas, and found multiple coexisting high-fitness regions driven by increased competition at high densities"

Laccase detoxification mediates the nutritional alliance between leaf-cutting ants and fungus-garden symbionts "laccase activity is highest where new leaf material enters the fungus garden [in ant feces], but where fungal mycelium is too sparse"

A link between host plant adaptation and pesticide resistance in the polyphagous spider mite Tetranychus urticae "selection for the ability to mount a broad response to the diverse defense chemistry of plants predisposes the evolution of pesticide resistance in generalists"

See my Darwinian Agriculture Blog for links to videos of two of my talks.

September 24, 2012

Comments on Forbes article on biomimicry

Steven Kotler, at Forbes, recently posted a story titled "Move Over Genetic-Engineering; Biomimicry Seems The Better Bet For Solving Global Hunger."

The Forbes site said I could comment using my Google identity, if they could just have access to my contact list. No thanks. I"m amazed it's even legal to ask, if it is.

So I'll comment here. Biomimicry is a major theme of my recently published book, Darwinian Agriculture -- but biomimicry of what?

The adaptations of individual plants, animals, and microbes have been improved (by the criterion of Darwinian fitness in past environments) through millions of years of competitive testing against alternatives. But larger-scale patterns we see in nature, such as the total number of species in a forest, or how trees are arranged, haven't been tested competitively. Trees compete against trees, but forests don't compete against forests.

If we copy individual adaptations of trees, we are copying the winners of many past rounds of competition. A forest may have persisted for thousands of years, so it's probably not too dysfunctional. But it hasn't been tested through repeated competition, so there's likely to be plenty of room for improvement.

I would have expected a writer at Forbes -- do they still call themselves "a capitalist tool?" -- to understand how competition is key to improvement, but apparently not.

Now, what about the specific examples in the Forbes post? Spiders compete against spiders and sharks compete against sharks, so it's not surprising that spider silk and shark skin are awesome.

But his "favorite" example is that wildlife corridors that mimic electric circuits work better. I'm not sure what point Kotler is trying to make here -- did he not notice that this is biomimicry in the opposite direction? Although caribou have competed against caribou, curved and straight wildlife migration corridors haven't competed against each other. (OK, maybe they have competed for caribou and their manure, sort of, but the winning corridors don't produce "offspring" with the same degree of curvature.) So this case calls for intelligent design by humans, not mimicry of large-scale patterns seen in nature.

And then there's the endophyte example. Some fungi that live inside plants can provide major benefits to those plants. We will be reading and discussing journal articles about this in a couple weeks, in our graduate seminar.

If there were only one fungus per plant, fungi that benefit their hosts would thereby benefit themselves. But mixed infections seems to be common. With mixed infection, a fungus that invests resources to benefit the host is like someone who pays taxes when nobody else does. Admirable, perhaps, but not likely to be very successful. It's a variation on the classic "tragedy of the commons."

One benefit often provided by endophytes is chemical defense against a plant's enemies. This case isn't too hard to understand. Maybe the various fungi make toxic chemicals to attack each other, since they're competing for the same plant resources, and those same toxins also protect against insects that might otherwise eat the plant.

But how and why do endophytes improve drought tolerance? Unlike mycorrhizal fungi, which extend out into the soil, most endophytes are entirely inside the plant. So it's not as if they can pull more water out of the soil. Sure, they can produce chemicals that mimic plant hormones, thereby manipulating the plant to make more (or fewer) roots or to open (or close) the stomata through which water evaporates from leaves.

But it's hard for me to believe that:
1) a fungus infecting a plant is a better judge of how many roots a plant needs than the plant is
2) that the fungus would put the plant's interests ahead of its own.

It's a mystery. But that's what science does: solve mysteries. Stay tuned.


June 11, 2012

Multicellular Yeast Defeats Rotifer

I really like this movie Will Ratcliff made, showing one of the benefits of being multicellular.

RatcliffEtalMulticellularYeastVsRotifer.mov

The rotifer at the right easily consumes single-cell yeast, but our lab-evolved multicellular yeast was more than it could handle. See our open-access paper in Proceedings of the National Academy of Sciences for scientific details and the Travisano/Dean/Denison Microbial Population Biology website for more videos.

Mike Travisano and Will Ratcliff, principal investigator and super-postdoc, respectively, on the multicellularity project, will have a poster and talk at the upcoming Evolution Meetings in Ottawa.

September 9, 2011

This week's picks

A Gene for an Extended Phenotype "The viral gene that manipulates climbing behavior of the [Gypsy moth] host was identified"

The Foot and Ankle of Australopithecus sediba [hominin fossil from 1.78 and 1.95 million years ago] "may have practiced a unique form of bipedalism and some degree of arboreality"

Assured fitness returns in a social wasp with no worker caste "experimentally orphaned brood... continue to be provisioned by surviving adults... no evidence that naturally orphaned offspring received less food than those that still had mothers in the nest."

The sudden emergence of pathogenicity in insect-fungus symbioses threatens naive forest ecosystems "symbioses between wood-boring insects and fungi... are shifting from non-pathogenic saprotrophy in native ranges to a prolific tree-killing in invaded ranges... when several factors coincide"

Ultra-fast underwater suction traps "this unique trapping mechanism conducts suction in less than a millisecond and therefore ranks among the fastest plant movements known"

The taming of an impossible child - a standardized all-in approach to the phylogeny of Hymenoptera using public database sequences "combines some well-established programs with numerous newly developed software tools"

August 19, 2011

Reciprocity maintains cooperation between plants and mycorrhizal fungi

Mycorrhizal fungi attach to plant roots and trade phosphorus (and sometimes other benefits) for photosynthates. But is it really "trade"? In other words, if one partner defects, will the other continue its contribution? A recent paper in Science, by Toby Kiers and colleagues, shows that the term, "trade" is justified.(Kiers et al. 2011)

Earlier, as my first PhD student, Toby showed that symbiotic rhizobia, living inside soybean root nodules, can't stop supplying the plant with nitrogen without suffering a decrease in their own fitness.(Kiers et al. 2003) We called the plant response "sanctions", although this could be misinterpreted as an attempt by the plant to improve the behavior of the rhizobia. We assume that the behavior of a given rhizobial genotype is programmed by its DNA, but sanctions against less-beneficial strains will decrease their frequency in subsequent generations. More recently, Ryoko Oono demonstrated a subtler form of sanctions against potentially reproductive rhizobia when their nonreproductive clonemates stop fixing nitrogen.(Oono et al. 2011)

Different strains of rhizobia on the same plant are (mostly) segregated in different root nodules, so "shutting down" one nodule preferentially hurts a single strain of rhizobia. But multiple strains of mycorrhizal fungus typically infect the same root. Can plants preferentially allocate resources to the most-beneficial fungi? Jim Bever and colleagues showed that they can, when different strains are attached to different parts of the root, but apparently not when the strains are more mixed.(Bever et al. 2009) Results from Kiers' group, however, suggest that mixing is not a problem.

They grew Medicago truncatula (a model species related to alfalfa) with mixtures of mycorrhizal fungi that they classified as more- or less-beneficial. To track photosynthate allocation, they let plant photosynthesis take up carbon dioxide containing the heavier 13C isotope of carbon. To see which fungi got more of this carbon, they used a clever technique.

They extracted RNA from the fungi and separated it into lighter and heavier (more-recent-plant-carbon) fractions, using a centrifuge. Then they used difference in the RNA base sequences of their different fungi to measure their relative representation in the light and heavy fractions. Fungi with greater representation in the heavy fraction were getting more recently supplied carbon from the plant, so they were benefiting more from symbiosis. A more-beneficial fungal species got more carbon than either of two less-beneficial species. This result is consistent with host sanctions (or, since each fungus interacts with multiple plants, with a "biological market" where individuals trade more with those that offer the best deal).

It's hard to measure all of the benefits a mycorrhizal fungus provides, however, so they also manipulated the exchange of specific resources. The species they classified as more beneficial got more carbon when it had access to more phosphorus, and presumably supplied it to the plant. A less-beneficial species didn't get more carbon when it had access to more phosphorus, perhaps because it didn't give much of the phosphorus to the plant. Similarly, the cooperative strain apparently responded to differences in carbon supply from the plant, allocating more phosphorus to where it was getting more carbon. So sanctions appear to go both ways.

LITERATURE CITED

Bever J. D., S. C. Richardson, B. M. Lawrence, J. Holmes, and M. Watson. 2009. Preferential allocation to beneficial symbiont with spatial structure maintains mycorrhizal mutualism. Ecology Letters 12:13-21.

Kiers E. T., M. Duhamel, Y. Yugandgar, J. A. Mensah, O. Franken, E. Verbruggen, C. R. Felbaum, G. A. Kowalchuk, M. M. Hart, A. Bago, T. M. Palmer, S. A. West, P. Vandenkoornhuyse, J. Jansa, and H. Bücking. 2011. Reciprocal rewards stabilize cooperation in the mycorrhizal symbiosis. Science 333:880-882.

Kiers E. T., R. A. Rousseau, S. A. West, and R. F. Denison. 2003. Host sanctions and the legume-rhizobium mutualism. Nature 425:78-81.

Oono R., C. G. Anderson, and R. F. Denison. 2011. Failure to fix nitrogen by non-reproductive symbiotic rhizobia triggers host sanctions that reduce fitness of their reproductive clonemates. Proceedings of the Royal Society B 278:2698-2703.

August 12, 2011

This week's picks

Reciprocal Rewards Stabilize Cooperation in the Mycorrhizal Symbiosis Toby Kiers, who previously demonstrated host sanctions against cheating rhizobia, now shows that plants give less carbon to less-beneficial mycorrhizal fungi. I hope I can find time to discuss this paper in more detail soon.

Natural variation in Pristionchus pacificus dauer formation reveals cross-preference rather than self-preference of nematode dauer pheromones "strains may have evolved to induce dauer formation precociously in other strains in order to reduce the fitness of these strains"

Nest Inheritance Is the Missing Source of Direct Fitness in a Primitively Eusocial Insect

Polyandrous females benefit by producing sons that achieve high reproductive success in a competitive environment

Kin selection in den sharing develops under limited availability of tree hollows for a forest marsupial

Aging of the cerebral cortex differs between humans and chimpanzees "significant aging effects in humans were... individuals that were older than the maximum longevity of chimpanzees. Thus... brain structure shrinkage in human aging is evolutionarily novel and the result of an extended lifespan"

Bacterial persistence by RNA endonucleases

Host-parasite local adaptation after experimental coevolution of Caenorhabditis elegans and its microparasite Bacillus thuringiensis

Sperm chemotaxis, fluid shear, and the evolution of sexual reproduction

July 23, 2011

Beneficial infections?

Endophytes are microbes (often fungi) that infect plants without causing obvious disease. Some endophytes appear to benefit their plant hosts. How do they do this, and why? I will introduce these questions before discussing this week's paper,(Redman et al. 2011) which shows dramatic benefits to rice from particular endophytes.

How might endophytes benefit plants? Mycorrhizal fungi extend out into the soil, where they can get phosphorus and other resources to give their plant hosts. In contrast, endophytes are typically found completely inside the plant, so any resources they "give" the plant must be modified versions of resources they got from the plant in the first place. Nitrogen is a possible exception -- rhizobia bacteria convert atmospheric nitrogen to forms plants can use, but the oxygen-sensitivity of the key enzyme apparently restricts this process to controlled-oxygen environments, such as legume root nodules.

Although endophytes don't have access to external resources, they can make chemicals the plant can't, such as toxins that protect the plant from being eaten. Or, they might make chemicals that the plant could make itself, but in larger amounts than the plant would otherwise make, at least at a particular time and place. For example, endophytes can make plant hormones, which could stimulate growth.

But what do we mean by "stimulate?" If all of the phosphorus, nitrogen, and carbon in the endophyte comes from the plant, any stimulation must result from the plant using its own resources differently than it would without the endophyte. In other words, the endophyte is manipulating the plant. For whose benefit? That leads to my second question.

Why do endophytes benefit plants? That is, why have endophyte strains that benefit their hosts more sometimes out-competed strains that benefit their hosts less, over the course of endophyte evolution? Mutant endophyte strains that don't make plant-defense toxins or (beneficial?) plant-manipulating hormones must arise. Making these chemicals uses resources the endophyte could otherwise use to reproduce more inside the plant. For strains that make beneficial chemicals to persist over evolution, they must have some advantage that outweighs this cost. I can imagine several ways in which beneficial endophytes might have an advantage.

The first is "group selection", with the group being all the endophytes inside an individual plant. Plants with more-beneficial endophytes grow more than plants with less-beneficial endophytes, and larger plants support more endophytes. If each plant contains only a single genotype of endophyte, this mechanism should work well. But defense of cacao leaves from pathogens was provided by a diverse community of endophytes.(Arnold et al. 2003) If a healthier shared host was the only benefit each endophyte received, wouldn't "free-rider" mutants that invest less in host defense tend to spread?(Denison et al., 2003b; Kiers and Denison 2008)

Maybe the endophytes make antifungal toxins mainly to kill each other, and defense against fungal pathogens is just a valuable side-effect. Such "byproduct mutualism" is an example of the second reason that more-beneficial endophytes may persist.

The third hypothesis is a minor variation on by-product mutualism. Endophyte infection is so common that plants may have evolved to depend on products produced by endophytes, even if the plants (or their ancestors) could produce those products themselves. For example, why might a plant be genetically programmed to make too little of some hormone that would maximize its reproduction? Perhaps because most of its ancestors were infected by endophytes producing that same hormone, so for the plant to make even more of it would have reduced fitness.

Third, maybe individual plants containing multiple strains of endophyte somehow favor the most-beneficial strains, reversing the benefit "free-riders" would otherwise have. Host sanctions against less-beneficial rhizobia(Kiers et al. 2003, Simms et al. 2006, Oono et al. 2011) and mycorrhizal fungi(Bever et al. 2009) have been reported, but is anything similar possible with endophytes?

Fourth, some apparent benefits to plants from endophytes may be misleading. Increased root growth may look like a benefit, but remember that the carbon and nitrogen in that root come from the plant, not the endophyte. So, at least in the short term, increased root growth usually comes at the expense of decreased shoot growth or more rapid depletion of reserves. For example, root-associated microbes that increase root growth of wheat can decrease final yield.(Kapulnik et al. 1987) Even if an endophyte-induced change in resource allocation increases seed production of plants growing individually in pots in a greenhouse, the same change might decrease seed production under competitive conditions in the field.

Now to this week's paper.(Redman et al. 2011) PLoS One is open access, so you can read the whole paper on-line. Regina Redman and colleagues inoculated rice with three different fungal endophytes, which had been isolated from plants growing under salt- or temperature-stress conditions. The fungi produced the plant hormone, IAA (auxin), at least in culture. This is consistent with manipulation of the host as a mechanism, although they didn't detect IAA in the plants themselves.

Although their focus was on stress tolerance, results under nonstress conditions were particularly interesting. Right after germination, infected and noninfected seedlings look similar (their Fig. 2). But after three days, the dry weight of three-day old seedlings not infected with endophytes averaged 60 milligrams (dry weight), while endophyte-infected seedlings averaged 105 mgDW (their Fig. 1). Both weights must include contributions from early photosynthesis, since rice seeds typically weigh only 20-30 mg. Somehow, the endophyte must have increased photosynthesis.

But how could the endophyte increase photosynthesis, if all the resources in the endophyte came from the plant? Actually, right after inoculation, the endophyte would still have some nitrogen and phosphorus acquired during culture. But would they have enough to share to significantly enhance early seedling growth?

Alternatively, the endophyte might have manipulated the plant into using its own resources differently than it would have without the endophyte. (We could call this manipulation "signaling" if the endophyte is providing the plant with useful information for mutual benefit,(Ratcliff and Denison 2011) but what information would an endophyte entirely inside a plant have that the plant itself wouldn't?)

For example, plants can photosynthesize faster if they open the stomatal pores in their leaves more, but that also uses up the soil water around their roots faster, increasing the risk of running out. Could an endophyte-induced increase in stomatal opening over-ride the plant's own water-use strategies? Sure, but what are the chances that the endophyte's strategy is better for the plant? That would seem unlikely, at least under the conditions where the plant evolved. But we wouldn't necessarily expect plant strategies that evolved in the field to be ideal in the greenhouse. A small change in either direction would have a 50% chance of being an improvement. Maybe the endophyte got lucky.

I've suggested increased stomatal opening as one way the endophyte could have increased seedling photosynthesis. Later in growth, however, endophyte-infected plants used less water than those without endophytes and took longer to wilt after watering (their Fig. 3). It's not clear, however, how much of the water use went through stomata, as opposed to evaporating from the soil surface. The endophyte-infected plants were much bigger than the noninfected plants by then, so they would have shaded the soil surface more.

As an alternative to greater stomatal opening, that about allocation to roots? Photos suggest that the noninfected seedlings initially invested more resources in shoots than in roots, whereas the endophyte-infected ones invested more in roots. If the endophytes infect mainly via the root, it's easy to understand why they might manipulate the plant to make more root. But why would plants have evolved to make too little root, when not infected by endophytes? Again, I see two likely possibilities. Maybe the plant's greater allocation to shoot is optimal for the conditions where it evolved(Denison et al. 2003, Denison in press) (germinating underwater in rice fields) but this gives too little root growth under the experimental conditions used in this study. This seems the most likely explanation. Alternatively, rice may be adapted to the presence of endophytes that produce similar hormones to those used in this study, so they have evolved to make hormone amounts that are suboptimal when not infected by endophytes.

Late in growth there were differences between treatments in reactive oxygen species, and the endophyte-infected plants had higher seed yields. The paper also shows beneficial effects on rice growth under low-temperature stress. But I would like to understand the benefits of the endophyte to three-day-old unstressed seedlings before getting into such details.

For practical applications, a more-complete understanding of how endophytes benefit plants would be useful, either to help us identify even better endophytes or to breed crops that get the same benefits with whatever endophytes they usually have now. Understanding endophyte evolution could be equally important. If it is some form of group selection that gives more-beneficial endophytes an edge over free-riders, can we maintain that process in agriculture? If some plant are imposing sanctions on less-beneficial endophytes, we certainly want to preserve that plant trait in our crop-breeding programs, or look for it in wild species and traditional crop cultivars.(Denison et al., 2003a, Denison in press) On the other hand, if endophytes are manipulating their plant hosts in ways that always benefit the endophyte, but which benefit the plant only under certain conditions, then we need to test endophytes under conditions more similar to how the crops will be grown in the field. In any case, this is a very interesting paper which (with related papers from the same group) could lead to exciting new approaches to improving crop production.


LITERATURE CITED

Arnold A. E., L. C. Mejia, D. Kyllo, E. I. Rojas, Z. Maynard, N. Robbins, and E. A. Herre. 2003. Fungal endophytes limit pathogen damage in a tropical tree. Proceedings of the National Academy of Sciences USA 100:15649-15654.

Bever J. D., S. C. Richardson, B. M. Lawrence, J. Holmes, and M. Watson. 2009. Preferential allocation to beneficial symbiont with spatial structure maintains mycorrhizal mutualism. Ecology Letters 12:13-21.

Denison R. F. in press. Darwinian agriculture: how understanding evolution can improve agriculture. Princeton University Press, Princeton.

Denison R. F., E. T. Kiers, and S. A. West. 2003a. Darwinian agriculture: when can humans find solutions beyond the reach of natural selection? Quarterly Review of Biology 78:145-168.

Denison R. F., C. Bledsoe, M. L. Kahn, F. O'Gara, E. L. Simms, and L. S. Thomashow. 2003b. Cooperation in the rhizosphere and the "free rider" problem. Ecology 84:838-845.

Kapulnik Y., Y. Okon, and Y. Henis. 1987. Yield response of spring wheat cultivars (Triticum aestivum and T. turgidum) to inoculation with Azospirillum brasilense under field conditions. Biology and Fertility of Soils 4:27-35.

Kiers E. T., R. F. Denison. 2008. Sanctions, cooperation, and the stability of plant-rhizosphere mutualisms. Annual Review of Ecology, Evolution, and Systematics 39:215-236.

Kiers E. T., R. A. Rousseau, S. A. West, and R. F. Denison. 2003. Host sanctions and the legume-rhizobium mutualism. Nature 425:78-81.

Oono R., C. G. Anderson, and R. F. Denison. 2011. Failure to fix nitrogen by non-reproductive symbiotic rhizobia triggers host sanctions that reduce fitness of their reproductive clonemates. Proceedings of the Royal Society B :doi: 10.1098/rspb.2010.2193.

Ratcliff W. C., R. F. Denison. 2011. Alternative actions for antibiotics. Science 332:547-548.

Redman R. S., Y. O. Kim, C. J. D. A. Woodward, C. Greer, L. Espino, S. L. Doty, and R. J. Rodriguez. 2011. Increased Fitness of Rice Plants to Abiotic Stress Via Habitat Adapted Symbiosis: A Strategy for Mitigating Impacts of Climate Change. PLoS ONE :e14823.

Simms E. L., D. L. Taylor, J. Povich, R. P. Shefferson, J. L. Sachs, M. Urbina, and Y. Tausczik. 2006. An empirical test of partner choice mechanisms in a wild legume-rhizobium interaction. Proceedings of the Royal Society B 273:77-81.


June 30, 2010

Thanks, Olivia!

Olivia Judson is ending her always-interesting weekly posts (often with evolutionary themes) at the NYT. Her penultimate post nominated yeast as Life-form of the Month. I second the nomination and look forward to blogging about some experimental evolution we're doing with yeast, once we publish.

May 15, 2010

Evolution of DNA methylation in animals, plants, and fungi

This week, I will try to explain what DNA methylation is and some of the reasons why it's important, before discussing this week's paper on how DNA methylation has evolved.

The paper is "Genome-Wide Evolutionary Analysis of Eukaryotic DNA Methylation", published in Science by Assaf Zemach and others from the lab of Daniel Zilberman.

DNA methylation usually refers to the attachment of a methyl (CH3) group to a cytosine, one of four DNA bases (C, in DNA's A,T,C,G alphabet). Here's a link showing one way cytidine can get methylated. And this Wikipedia article shows cytosine in place in double-stranded RNA. (DNA would be similar, but with T instead of U.)

The functions of DNA methylation mostly come from the reduced transcription of RNA from methylated stretches of DNA. Surprisingly, when a new DNA copy is made (e.g., when one of our cells divide), methylation patterns are generally copied, too. Together, these two facts explain many of DNA methylation's functions.

First, DNA methylation is key to imprinting, whereby genes inherited from one parent are often shut down, perhaps for life, by methylation. Imprinting often reflects an unconscious battle between male and female parents over whether to maximize growth of this particular offspring, whatever the consequences for the mother's future survival and reproduction, or take or more long-term view. Earlier, I discussed the possible role of imprinting in mental illness.

Continue reading "Evolution of DNA methylation in animals, plants, and fungi" »

December 1, 2009

Better ant fungus farming through chemistry

Leaf-cutter ants feed the leaves to fungi and eat the fungi. Another fungus can parasitize their crop. A few years ago, it was reported that bacteria living on the ants' bodies make antifungal compounds that kill the parasite.

I wondered about this: wouldn't a bacterium that invests resources in antifungal production grow more slowly than a mutant that avoids this costly investment? In the long run, this might hurt ants and bacteria alike, but natural selection has no foresight. So why haven't bacterial "cheaters" that don't make antifungals displaced "altruists" that do? When yeasts (single-cell fungi) were found on the same ants, I suggested that antifungal production might benefit individual bacteria in their war with the yeasts, with activity against the parasitic fungus as a side effect. (Similarly, bacteria that make antibiotics that protect plant roots from fungi have their own selfish reasons.)

Consistent with this hypothesis, it turns out that the antifungal chemicals made by the bacteria aren't active only against the parasitic fungi, and may even harm the fungal crop. But the bacteria presumably benefit the ants more than they harm them, because the ants have specialized structures and secretions whose main function seems to be to support the bacteria. At least, this is true of some fungus-growing ant species. Other species have apparently abandoned use of these bacteria. Instead, they control harmful fungi with antibiotics they make themselves, in special glands. This is an example of a species abandoning one symbiosis (ant/bacteria) when it's no longer beneficial, while retaining a beneficial symbiosis (ant/fungus).
ants.jpg
Black lines shows fungus-growing ant lineages that rely on antibiotics they make themselves, rather than those made by symbiotic bacteria, to control parasitic fungi that attack their fungal crop.
Source: Hermógenes Fernández-Marín, Jess K. Zimmerman, David R. Nash, Jacobus J. Boomsma and William T. Wcislo (2009) Reduced biological control and enhanced chemical pest management in the evolution of fungus farming in ants. Proceedings of the Royal Society B 276:2263-2269.

November 23, 2009

Are ants' fungus gardens a source or sink for nitrogen?

This week's paper, Symbiotic Nitrogen Fixation in the Fungus Gardens of Leaf-Cutter Ants, has already been discussed by Ed Yong, whose blog is among my favorites, and by the always-interesting Susan Milius of Science News. When she interviewed me, I endorsed the main conclusions of the article but expressed skepticism on one point.

The paper clearly shows that the fungus "gardens" cultivated by leaf-cutter ants contain bacteria that extract nitrogen from the air. The part I wondered about was their statement that:

Continue reading "Are ants' fungus gardens a source or sink for nitrogen?" »

August 7, 2009

Ants versus fungi

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.