Category Archives: coevolution

Snake-eating opossums have evolved venom-resistant blood

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The humble Virginia opossum can shrug off snakebites that would kill larger mammals. Photo by TexasEagle.

ResearchBlogging.orgIf you were going to pick the traits of a single animal to confer on a superhero, you probably wouldn’t pick the Virginia opossum. Possums are ubiquitous, scruffy, ratlike marsupials, their toothy grins giving the not entirely inaccurate impression that they don’t have much going on upstairs. Until recently, the nicest thing I could think to say about them is that they eat a lot of ticks.

Blood-sucking Lyme disease vectors are only a small part of the opossum’s eclectic diet, however. They also eat quite a few poisonous snakes, and this has apparently led them to evolve a trait I could call a superpower without exaggeration: opossum blood is resistant to snake venom.

This curious and useful ability was first documented by J.A. Kilmon in a 1976 paper [$a], in which Kilmon reported field observations and laboratory trials showing that opossums tolerate snakebites without visible ill effect. (If animal experimentation makes you queasy, you might want to go read something else about now. Perhaps a nice post about gerbils?)

A natural bite was observed in the field by a 160 cm eastern diamondback on an adult opossum, Didelphis virginiana. The opossum displayed no apparent distress and this suggested a remarkable tolerance by that animal to envenomation. In order to ascertain if an actual envenomation did take place, Mr. Seashole conducted field experiments by manually causing snakes to inflict actual bites on captured opossums. None of the bites caused visible signs of distress to the opossums.

Kilmon brought possums back to the lab, anesthetized them, hooked them up to heart monitors, and “inflicted” bites on them from diamondback and timber rattlesnakes, water moccasins, and at least one cobra. (Kilmon reports he used 15 snakes in total, but doesn’t break that number down by species.) “None of the five opossums,” he wrote, “developed observable local reactions other than trauma attributable to fang penetration and none developed observable systemic effect, exhibiting negligible alteration of heart rate and respiration.”

A timber rattlesnake—no big deal to an opossum. Photo by Tom Sprinker.

Finally Kilmon injected an anesthetized opossum with enough water moccasin venom to kill five fifteen-kilogram dogs, and observed no reaction beyond a brief drop in blood pressure and small spike in pulse rate—when the possum awoke, it was “apparently healthy.” Upon sacrificing and dissecting the animal, Kilmon found no evidence of organ damage.

Kilmon concludes his brief scientific report with a weird aside about the evolutionary history of opossums, which, had he been writing in 2011, would have made me think his research consisted mainly of skimming the Wikipedia entry for Didelphis virginiana. In the course of reporting the opossum’s taxonomic affiliations and known diet, Kilmon notes offhandedly,

This polyprotodont marsupial is a primitive but also very successful mammal. The opossums of varying species are the only marsupials surviving in the placental world, the predominant marsupial and monotreme mammals of Australia having probably survived due to their isolation. The opossum has remained unchanged for millions of years and probably reached his peak of evolutionary specialization several millions of years ago.

I don’t think he could’ve gotten away with that last sentence in an evolutionary biology journal. It’s true that the common ancestor of opossums and placental mammals (i.e., us) diverged quite a long time ago, that opossum-like critters are known from the fossil record going back that far, and that many opossum traits are thought to be shared with early mammals. But that doesn’t mean opossums “remained unchanged for millions of years.” The lineage leading to modern opossums has been evolving exactly as long as the lineage leading to modern humans—and if the opossum’s lifestyle hasn’t led it to such evolutionary heights as the wheel, war, New York and so forth, then it also hasn’t left the opossum unchanged.

As it happens, a pretty good illustration of this point is the paper that led me to Kilmon’s morbid little study in the first place. Mammalogists Sharon Jansa and Robert Voss have just published a study of one blood protein that may underlie opossums’ resistance to venom. The venom of pit vipers like rattlesnakes and water moccasins targets the blood clotting system—one of the unpleasant effects of a snake bite is internal hemorrhage. So Jansa and Voss examined the evolution of a venom-targeted clotting protein called von Willebrand Factor, or vWF, comparing it across the entire family of opossums, the didelphidae.

Photo by Maggie Osterberg.

Since the evolutionary origin of the family, the vWF of opossum species that prey on snakes has accumulated more changes than vWF in non-snake-eating species. That’s circumstantial evidence for the effect of natural selection continuously acting on vWF over millions of years. Jansa and Voss picked out several specific changes that are unique to snake-eating opossums, and found that they’re associated with a region of vWF that is known to bind with one of the toxins in pit viper venom.

The authors suggest that opossums may have been engaged in a evolutionary “arms race” against snake venom toxins since they first developed a taste for rattlesnake. In other words, not only is the opossum not unchanged since the early history of mammals, one of the traits that has changed continuously since then may be the very feature that piqued Kilmon’s interest.

References

Jansa, S., & Voss, R. (2011). Adaptive evolution of the venom-targeted vWF protein in opossums that eat pitvipers. PLoS ONE, 6 (6) DOI: 10.1371/journal.pone.0020997

Kilmon, J., Sr. (1976). High tolerance to snake venom by the Virginia opossum, Didelphis virginiana. Toxicon, 14 (4), 337-40 DOI: 10.1016/0041-0101(76)90032-5

Passive aggression: Parasitic wasp larvae interfere with each other via their host’s host plant

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A large white butterfly caterpillar weaves a cocoon around the wasp larvae infesting its body. Photo by EntomoAgricola.

ResearchBlogging.orgI’m embarrassed to admit that I’ve only just gotten around to picking up Carl Zimmer’s book Parasite Rex. It’s turned out to be a wonderful compendium of all the peculiar ways parasites evade, confound, and resist the defenses of their hosts. Some of the wildest cases Zimmer examines, though, are parasites that manipulate their hosts’ behavior.

One grotesque and well-studied example is the wasp Cotesia glomerata. Female C. glomerata wasps inject their eggs into butterfly caterpillars, and when the eggs hatch, the wasp larvae eat the caterpillar from the inside, saving critical organs so the poor thing stays alive the whole time. Then, when the wasp larvae are ready to burrow out of the caterpillar and form pupae to complete their devlopment, they induce the half-dead caterpillar to spin a web around them and stand guard against predators. (In technical language, this life history makes the wasp a parasitoid, rather than a parasite.) Christie Wilcox has written up a fuller description of the whole grisly process, if you want more detail.

That sounds like a pretty incredible set of manipulations for one clutch of wormy-looking wasp larvae, but they’re not all that Cotesia glomerata can do. New evidence published in Ecology Letters suggests that C. glomerata can somehow make the plants that its host caterpillar feeds on less hospitable [$a] to the larvae of another caterpillar-infesting wasp. In other words, the wasp larvae may manipulate not just their host, but their host’s host.

First off, here’s video of Cotesia glomerata in action. Don’t watch this on your lunch break.

Now, the wasp’s plant manipulations. Lots of plants have what are called induced defenses against herbivores like the butterfly larvae that host C. glomerata larvae. Induced defenses are usually protective toxins that plants produce in response to herbivore damage [PDF]. Erik Poelman and his collaborators reasoned that, since C. glomerata can manipulate it’s host’s behavior, the parasites might change how plants respond to herbivory by infested caterpillars.

To test this, the team first had to induce plant responses. They grew Brassica oleracea—Brussels sprouts—plants in the greenhouse, then infested them with either un-parasitized caterpillars of the cabbage white butterfly Pieris rapae, cabbage white caterpillars infected with Cotesia glomerata, or cabbage white caterpillars infected with larvae of the related wasp C. rubecula. Once the caterpillars had nibbled on the plants enough to induce defensive responses, Poelman et al. removed the caterpillars in preparation for the experiment proper.

The team then introduced parasitoid-free caterpillars and caterpillars infested with one or the other parasitoid species onto host plants that had been through one of the three induction treatments, or that had never been exposed to herbivores. They then tracked the development of the caterpillars, and whether or not the wasp larvae inside them survived.

A healthy cabbage white butterfly caterpillar feeds on a piece of broccoli stem. Photo by Sam Fraser-Smith.

Larvae of C. rubecula fared more-or-less equally well no matter what kind of plant their host caterpillar fed on. But C. glomerata larvae had substantially higher mortality when their hosts fed on plants induced by caterpillars infested with the competitor species. While about 50 percent of C. glomerata larvae died if their hosts fed on plants induced by uninfested caterpillars or caterpillars infested with C. glomerata, almost 75 percent of C. glomerata larvae died when their hosts fed on plants that had previously been occupied by caterpillars infested with C. rubecula.

This impact isn’t because the host caterpillars fared poorly—in fact, caterpillars developed a little faster on plants induced by rubecula-infested caterpillars. So somehow, Cotesia rubecula seems to have influenced its hosts in a way that makes their host plants less hospitable to C. glomerata.

Poelman et al. are scrupulous to point out that this effect might not be anywhere nearly as strong in nature—host plants and host caterpillars might be plentiful enough that Cotesia glomerata can simply avoid the competitor species. On top of that, any natural selection that C. rubecula could be exerting on C. glomerata via induced responses in their shared hosts’ host plants is occurring at multiple removes. The effect Poelman et al. documented is probably not an adaptation for competition with C. glomerata so much as a side effect of C. rubecula‘s effect on its host.

So although this result shows that one parasitoid wasp can reach out and influence another through three other organisms—its own host, that host’s host plant, and the other wasp’s host—it’s not clear how strong that impact has been over the evolutionary history of these two Cotesia species. That said, this is a pretty nifty proof-of-concept.

Reference

Agrawal, A., Conner, J., Johnson, M., & Wallsgrove, R. (2002). Ecological genetics of an induced plant defense against herbivores: Additive genetic variance and costs of phenotypic plasticity. Evolution, 56 (11), 2206-2213 DOI: 10.1111/j.0014-3820.2002.tb00145.x

Poelman, E., Gols, R., Snoeren, T., Muru, D., Smid, H., & Dicke, M. (2011). Indirect plant-mediated interactions among parasitoid larvae. Ecology Letters DOI: 10.1111/j.1461-0248.2011.01629.x

Pesticides and parasites add up to an evolutionary Catch-22

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When Daphnia evolve resistance to pesticides, they become more vulnerable to bacterial parasites. Photo by Chantal Wagner.

ResearchBlogging.orgIf you haven’t read Joseph Heller’s classic Catch-22, cancel your plans for next weekend and spend the time with a copy from the nearest library. It’s a hilarious, bracingly bleak satire of military bureaucracy, as epitomized in the titular clause governing when bomber pilots can be grounded for reason of insanity:

There was only one catch and that was Catch-22, which specified that a concern for one’s safety in the face of dangers that were real and immediate was the process of a rational mind. Orr was crazy and could be grounded. All he had to do was ask; and as soon as he did, he would no longer be crazy and would have to fly more missions.

Heller conceived Catch-22 as a product of malicious middle management, but a similar situation crops up in the natural world when living things are under natural selection from conditions that favor contradictory traits. Biologists most commonly call these tradeoffs.

Over the course of evolution, tradeoffs set up “choices” that natural selection must make—a population can adapt to one alternative set of conditions, or another, or settle on a middle ground. A trivial example is that elephants have long ago “chosen” not to fly (Dumbo notwithstanding) in the course of evolving large, un-aerodynamic bodies suitable for massive-scale herbivory. A more relevant example is a new finding that the evolution of pesticide resistance creates vulnerability to parasites [$a].

The US Environmental Protection Agency estimated [PDF] that in 2006 and 2007 (the latest years for which reports are online) we used upwards of five billion pounds of pesticides to kill unwanted plants, insects, fungi, and other organisms worldwide. Once they’re sprayed, we don’t have much control over where pesticides end up—rain runoff takes them into lakes, ponds, and the ocean. In those bodies of water, critters at the base of the food chain are the first to feel the effects—critters like the tiny, translucent crustacean Daphnia magna.

Of course, those critters may be able to evolve resistance to the pesticides contaminating their environment—but that resistance may come at a cost.

Pesticide application, via the most picturesque method available. Photo by Scott Butner.

Anja Coors and Luc De Meester had already found a hint of this cost [$a] in an experiment using a single clonal line of Daphnia, in which Daphnia exposed to both sublethal concentrations of the widely-used insecticide carbaryl and a parasitic bacterium fared much worse than Daphnia exposed to only carbaryl or bacteria.

In the new study, Coors, De Meester, and three collaborators expand on that initial observation by determining whether Daphnia become more vulnerable to parasites as they evolve resistance to carbaryl, and whether this costly evolution could occur in natural populations. The coauthors took samples of Daphnia from natural populations in four separate lakes and exposed them to carbaryl over several generations—then sampled the resultant evolved populations and tested their vulnerability to the bacterium. Compared to Daphnia left unexposed to carbaryl, the evolved populations were more resistant to the pesticide—and were also more badly hurt by bacterial infection.

It’s hard to say how general this particular result is to the many, many other species that, like Daphnia, must cope with pesticides and other pollutants humans have introduced into the environment. Evolution to resist one pesticide leads to lowered resistance to infection in one aquatic crustacean; in other species, facing different chemicals, maybe such costs are different or lesser or nonexistent. But living things are not infinitely pliable as they evolve in response to the many and rapid changes we’re making in the world. To slow the extinction crisis going on around us, we need to avoid trapping other living things in Catch-22.

References

Coors, A., & De Meester, L. (2008). Synergistic, antagonistic and additive effects of multiple stressors: predation threat, parasitism and pesticide exposure in Daphnia magna.Journal of Applied Ecology, 45 (6), 1820-8 DOI: 10.1111/j.1365-2664.2008.01566.x

Jansen, M., Stoks, R., Coors, A., van Doorslaer, W., & de Meester, L. (2011). Collateral damage: Rapid exposure-induced evolution of pesticide resistance leads to increased susceptibility to parasites. Evolution DOI: 10.1111/j.1558-5646.2011.01331.x

What’s in that dissertation, anyway?

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About to take the plunge. Photo by jby.

So, what with getting my sparrows in a row for my dissertation defense on Friday, I haven’t written any new science post for this week. But! As it happens, I have written about most of the component chapters of my dissertation—so in lieu of something new this week, why not check out those posts?

  • The first chapter of my dissertation is a literature review about the phenomenon ecologists call ecological opportunity, and how it may or may not explain big, rapid evolutionary changes. I’ve also written about this topic for the Scientific American guest blog.
  • The second chapter uses phylogenetic methods to reconstruct what yucca moths were like before they were yucca moths.
  • The third chapter presents a mathematical model of coevolution between two species, and determines what kind of interactions—predation, parasitism, mutualism, competition—can cause those species to evolve greater diversity.
  • The fourth chapter is the latest work on my lab’s big study of Joshua trees and their pollinators. The material I’m including in this chapter hasn’t been reviewed and published yet, but you can read the most recent Joshua tree post to learn what we know so far, and what kinds of questions we still want to answer.

Regular posting resumes next week, provided that I pass my defense and the celebrating afterward doesn’t interfere with my blogging capacity.

Parasitism of a different color

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ResearchBlogging.orgThe common cuckoo is such a lazy parent that brood parasitism—laying its eggs in the nests of other birds—is built into its biology.

No bird will willingly adopt cuckoo chicks, which usually out-compete, and sometimes kill, their adoptive siblings. Given any hint that one of the eggs in her nest isn’t hers, a bird will eject the intruder. So cuckoos have evolved eggs that mimic the coloring of their hosts’ eggs—dividing the species into “host races” that specialize on a single host species, and lay eggs that mimic that host’s.

Cuckoo eggs (indicated by arrows) in the nests of three different host species. Illustration via The Knowledge Project.

As you can see from this illustration, the match is often extremely good—the cuckoo egg is really only obvious when the hosts’ eggs are visibly smaller. In fact, a new study by Mary Caswell Stoddard and Martin Stevens shows that this matching is often even better than it looks to the human eye [$a].

Birds see the world differently than humans—where we have three kinds of color-sensitive cells in our eyes, they have four. This allows them to see colors in the ultraviolet range, which is invisible to us. Birds’ eyes also have an additional class of sensory cell that may help them perceive and discriminate among textures. So to study the match between cuckoo and host eggs, Stoddard and Stevens first had to figure out what each egg looked like to a bird.

A reed warbler feeds the cuckoo chick that has taken over its nest. Photo via Wikimedia Commons.

To do this, they developed a mathematical model of each host species’ vision. The model estimated how similar two eggs should look to a bird, given raw data about what colors of light the eggs reflect and the specific colors the bird can detect. Using the model, Stoddard and Stevens could then calculate the “overlap” between the colors and patterning of a host egg and the egg of a cuckoo specializing on that host species.

Stoddard and Stevens then applied the vision model’s measure of similarity to museum specimens of eggs from the cuckoo-parasitized nests of eleven European bird species. They found that cuckoo eggs matched their hosts’ quite well overall, but the match was best for cuckoos specialized on especially vigilant hosts. For each host, the authors looked up studies of egg rejection behaviors to calculate the probability that each species would eject eggs that didn’t look like their own. Species with higher ejection probabilities were parasitized by cuckoo host races whose eggs were better mimics.

That suggests host rejection behavior exerts strong natural selection on cuckoos, which makes sense given that successfully fooling a host is essential to cuckoo reproduction. In light of evidence that cuckoos can also exert selection on their hosts, it looks as though brood parasitism is a truly coevolutionary interaction between cuckoos and their hosts—one that can cause both to evolve greater diversity.

Reference

Stoddard, M., & Stevens, M. (2011). Avian vision and the evolution of egg color mimicry in the common cuckoo. Evolution DOI: 10.1111/j.1558-5646.2011.01262.x

One snout to rule them all: Does migrating help weevils win the arms race of coevolution?

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ResearchBlogging.orgNatural selection and gene flow have a sort of love-hate relationship. Natural selection acts, on average, to make a population better fit to its environment. Gene flow, the movement of individuals and their genes, can counter the optimizing effect of selection if it introduces less-fit individuals from somewhere a different environment. On the other hand, not all new immigrants are necessarily less fit—sometimes they’re better suited to their new environment than the locals.

This gets more complicated, and more interesting, when the environment in question is another living species. Then, the question is not just how movement of one species changes its response to natural selection, but how movement of the other species changes the nature of that natural selection. That’s the focus of the latest study of a Japanese weevil species and its favorite food plant. The two species are locked in a coevolutionary arms race—but who wins the arms race in any given location depends on the gene flow each species is receiving from elsewhere [$a].

Male and female camelia weevils, caught at an indelicate moment. Evidently he doesn’t find her much longer rostrum intimidating. Photo from Toju et al. (2011), figure 1.

These are camelia weevils, Curculio camelliae. As their name suggests, they like to eat camelias, at least when they’re young. Specifically, weevil larvae eat camelia seeds, which are protected by a thick layer called a pericarp. To deal with camelia pericarps, the weevils have evolved prodigious proboscises, or rostrums, which female weevils use to drill through the pericarp so they can lay their eggs inside. Note that the female in the picture above is the one with the rostrum longer than the rest of her body.

Camelias can reduce their risk of losing seeds to weevil larvae by evolving thicker pericarps; weevils can make sure they’re able to feed their young by evolving longer rostrums. Both species are constrained by costs, though—the cost of producing more pericarp tissue, or carrying around a Pinocchio-grade snout. These costs vary somewhat with climate—camelias grow thinner pericarps in cooler conditions [$a]. This means the arms race won’t proceed equally far in all camelia populations, and introduces the possibility that the way in which weevils and camelias (well, camelia seeds and pollen) move across the landscape may very well determine which species has the upper hand.

A female weevil drills into a camelia fruit. Photo from Toju et al. (2011), figure 1.

The new study sets out to see whether gene flow among populations of the two species determines how far the arms race proceeds in each population. Rather than directly track weevils and camelia seeds, the authors use genetic markers for each species—the more migrants move between two weevil (or camelia) populations, the more similar those two populations’ genetics will be. The populations in question were seven sites on a small island at the south end of the Japanese archipelago, and presumably relatively free from the influence of immigration from the larger islands.

It looks like the movement of weevils, but not camelias, affects how the arms race proceeds. As the genetic difference between weevils at two different sites increased, the difference in how far the arms race had proceeded—that is, how long the local rostrums were, and how thick the local pericarps—increased too. That suggests weevils may be prevented from evolving rostrums of the optimum length for their local camelias by the arrival of less-than-optimal migrants. On the other hand, there was no statistically significant relationship between the genetic similarity of camelia populations and their place in the arms race.

This is where the relationship between selection and gene flow gets complicated, though. Even given the relationship between weevil gene flow and how far the arms race seems to have proceeded, the genetic differences between weevil populations were consistent with very low actual rates of migration. A female weevil arriving in a population of camelias with pericarps too long for her rostrum isn’t going to contribute many offspring to the next generation of weevils at that site. So it’s not impossible that what we’re seeing is selection constricting gene flow rather than gene flow slowing down selection.

Alternatively, weevils from a population with super-long rostrums should be able to lay eggs in any population of camelias they meet. In fact, an analysis that uses the genetic data to estimate rates of immigration and emigration suggests that one of the weevil populations with the longest snouts contributes more migrants to the other sites than it receives from each of them. In arms-race coevolution, size is all that matters—and so the weevils with the longest snouts may be winners no matter where they go.

Reference

Toju, H. (2008). Fine-scale local adaptation of weevil mouthpart length and camellia pericarp thickness: Altitudinal gradient of a putative arms race. Evolution, 62 (5), 1086-102 DOI: 10.1111/j.1558-5646.2008.00341.x

Toju, H., Ueno, S., Taniguchi, F., & Sota, T. (2011). Metapopulation structure of a seed-predator weevil and its host plant in arms race coevolution. Evolution DOI: 10.1111/j.1558-5646.2011.01243.x

One of these mutualists is not like the other

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ResearchBlogging.orgOver the last few months I’ve been writing a lot about how different species interactions have different evolutionary effects. The studies I’ve looked at so far focus on effects over just a few generations—barely time to take notice, in evolutionary time. The February issue of The American Naturalist remedies this short-term perspective with a paper showing that over millions of years, two different kinds of mutualists had very different effects on the history of one group of orchids [$a].

The new study examines the evolutionary history of coryciinae orchids, a group of South African orchids that rely on two major groups of mutualists. The first, and perhaps most obvious, are pollinating bees, which coryciinae orchids attract not with nectar but with oils. Like most other orichids, this group of flowers interacts with its pollinators in very specific ways, to the point that different coryciinae species can share a single pollinator by placing pollen on different parts of the pollinator’s body, as seen in the image below.

Double duty: This bee is carrying pollen from one orchid species on its forelegs, and pollen from another orchid species on its abdomen. Photo from Waterman et al (2011), figure 1.

The second important group of mutualists are mycorrhizae, a class of fungus found in soil, which colonize plants’ roots. Mycorrhizae aid their hosts in taking up minerals, particularly phosphorus, in exchange for sugar supplied by the host. In certain kinds of soil, having the right mycorrhizae is the difference between life and death for a plant.

Although both pollinators and mycorrhizae are vital to an orchid’s success, they should contribute to forming new orchid species in very different ways. Evolving a new pollinator relationship can directly create reproductive isolation for a flowering plant, independent of other ecological considerations. On the other hand, mycorrhizae are closely linked to basic ecology, because the mycorrhizae in a plant’s roots determine what kinds of soils it can use—wet or dry, acidic or alkaline. If new orchid species usually form by adapting to new habitats, they probably acquire new mycorrhizae while doing so.

If changing a trait—in this case, a mutualistic relationship—is related to forming new species, then closely related orchid species will be more likely to differ in that trait. This turns out to be the case for pollinators—the more closely related two orchid species are, the more likely they are to use different pollinators, or different parts of the same pollinator. However, the reverse is true for mycorrhizae. The more closely related two orchids are to each other, the more likely they are to have the same mutualistic fungi in their roots. This finding that pollination matters most to species formation is right in keeping with Verne Grant’s classic study noting that animal pollinated plants tend to differ more in their floral structures—the parts that interact with pollinators—than in other traits.

The authors followed up on these results with field experiments on a few selected species, and found that co-occurring orchids could often successfully pollinate each other, if the pollen was deliberately placed. In these cases, differences in specialized pollination interactions are most of what maintains the orchid species as separate genetic entities. On the other hand, closely related orchids that grow in adjacent habitats did just fine when transplanted into each others’ soil—and mycorrhizae.

Biologists studying the effects of pollination on plant species formation have recently become more aware that correlation does not necessarily imply causation. New pollinator interactions certainly might form new species—but it is also possible that new orchid species created by other forces must rapidly evolve new pollinator interactions to compete with existing species.

References

Armbruster, W., & Muchhala, N. (2008). Associations between floral specialization and species diversity: cause, effect, or correlation? Evolutionary Ecology, 23 (1), 159-79 DOI: 10.1007/s10682-008-9259-z

Waterman, R., Bidartondo, M., Stofberg, J., Combs, J., Gebauer, G., Savolainen, V., Barraclough, T., & Pauw, A. (2011). The effects of above- and belowground mutualisms on orchid speciation and coexistence. The American Naturalist, 177 (2) DOI: 10.1086/657955

Mutualist matchmaking made simple

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This post was chosen as an Editor's Selection for ResearchBlogging.orgBack in September, I wrote about a new economic model of mutualism that proposed mutualists could keep their partner species from cheating—exploiting the benefits of a mutualistic relationship without returning the favor—without explicitly punishing them, so long as failure to play nice led to a reduction in mutualistic benefit [$a]. Now the same research group has published an elaboration of the economic approach to mutualism in the January issue of The American Naturalist, which suggests that mutualists can recruit better partners by manipulating the cost of entering into partnership [$a].

The bobtail squid, whose mutualism with luminescent bacteria is an example for the new model. Photo by megpi.

As a concrete example for their model, the authors refer to the mutualism between bobtail squid and a species of bioluminescent bacteria, which colonize the squid’s light organ and makes it glow. Short of some kind of complicated squid-bacterium signaling system, how does a squid ensure that its light organ is only colonized by bacterial strains that will pay it back and generate light?

They charge a cover.

Continue reading →

Under the mistletoe, coevolution is about s and m

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This post was chosen as an Editor's Selection for ResearchBlogging.orgPlants and plant products, from sprigs of holly to pine boughs, have been traditional winter holiday decorations since before Christmas became Christmas. Nowadays, if we don’t resort to plastic imitations, we deck our halls with garlands from a nursery and a tree from a farm. But seasonal decorations have natural histories apart from mantelpieces and door frames—ecological roles and, yes, coevolutionary interactions with other species.

Mistletoe. Photo by Ken-ichi.

One good example is mistletoe, whose white berries contrast nicely with holly’s red ones, and whose traditional association with kissing is probably responsible for more holiday party awkwardness than anything short of rum-spiked eggnog. Mistletoes are parasites, rooting in the branches of trees and shrubs to make a living at the expense of those hosts.

This sort of intimate interaction might be expected to result in coevolutionary natural selection between mistletoe and its hosts, potentially creating very specific pairings in which individual mistletoe species are only able to infect one or a few host plants with particular immune responses and defense chemistry. Yet mistletoe is dispersed by birds, which like to eat mistletoe berries, or can carry mistletoe seeds in their feathers—so seeds from a single plant might end up on a wide range of hosts. This means the specificity of mistletoe’s host associations is determined in a tug-of-war between selection from individual hosts and gene flow created by wide-ranging seed dispersal.

In population genetics models, we usually use s to represent selection, and m to represent gene flow, or migration. If s from an individual host species or the local climate is stronger than m, it creates local adaptation to those conditions. But even relatively small m from populations experiencing different conditions can wipe out that local adaptation. So in the case of mistletoe, does s win out, or does m?

One approach to answer this question would be to experimentally infect a range of host plants with a particular mistletoe, and compare their success. But with long-lived host plants, this method would be slow and expensive. Conveniently, local adaptation of mistletoe to individual host species should mean that mistletoe collected from different hosts is more genetically differentiated than mistletoe samples from the same host. And that’s quite a bit easier to test.

A 2002 study [PDF] of one North American mistletoe species found exactly this pattern. Coauthors Cheryl Jerome and Bruce Ford sampled dwarf mistletoe, Arceuthobium americanum from several host trees—Jack pine, ponderosa pine, Jeffrey pine, and two subspecies of lodgepole pine—growing across North America. They found that almost a third of the genetic variation they found in A. americanum was distributed among hosts—that is, it could differentiate dwarf mistletoes collected on one host from dwarf mistletoes collected from another.

A lodgepole pine branch supporting dwarf mistletoe in the Uinta Mountains, Utah. Photo by Fool-On-The-Hill.

Within these “host races,” geographic distance did have an isolating effect, but the effect was not as strong as that attributable to host differences. When Jerome and Ford examined the population genetics of the three principal A. americanum host trees—Jack pine and the two lodgepole pine subspecies—they found less differentiation than in mistletoe from the same populations [$a]. That suggests that, although coevolution with the trees strongly shapes mistletoe’s genetics, mistletoe infection is only one of many selective pressures acting on the host trees.

Although this approach is frequently used to test for coevolution, it isn’t entirely conclusive. The observed pattern of genetic differentiation in dwarf mistletoe on different host species could also arise if the A. americanum host races have climactic requirements that closely mirror the distribution of their respective hosts, or if birds carrying mistletoe seeds tend not to move the seeds between host species. Other indirect approaches exist to test these alternatives, but (so far as I can find) they haven’t been applied to dwarf mistletoe.

References

Jerome, C., & Ford, B. (2002). The discovery of three genetic races of the dwarf mistletoe Arceuthobium americanum (Viscaceae) provides insight into the evolution of parasitic angiosperms. Molecular Ecology, 11 (3), 387-405 DOI: 10.1046/j.0962-1083.2002.01463.x

Jerome, C., & Ford, B. (2002). Comparative population structure and genetic diversity of Arceuthobium americanum (Viscaceae) and its Pinus host species: insight into host-parasite evolution in parasitic angiosperms. Molecular Ecology, 11 (3), 407-20 DOI: 10.1046/j.0962-1083.2002.01462.x

Coevolutionary constraints may divide Joshua trees

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Scientists love it when the real world validates our more theoretical predictions. It helps, of course, if those predictions are rooted in the real world to begin with. This is more or less what happened in my own research, with results reported in two just-published scientific papers. In the first, which I discussed last week, my coauthor and I showed that some kinds of species interactions can reduce the diversity of the interacting species [PDF]. Today, I’m turning to the second, in which my coauthors and I found exactly this predicted pattern in one such species interaction, the pollination mutualism between Joshua tree and yucca moths.

The new paper, published this month in the Journal of Evolutionary Biology, examines the phenotypic variation of two forms of Joshua tree and the two different moth species that pollinate it. The data show that although the Joshua trees pollinated by different moths are very different from each other, those pollinated by the same moth species are extremely similar [PDF].

Two forms of Joshua tree pollinated by different moth species, seen here side by side, don’t vary much among themselves. (Flickr: jby)

This is a nice confirmation of the theory paper because it strongly suggests that coevolution between mutualists like Joshua tree and its pollinators works the way the theoretical model assumes it does, with natural selection favoring individuals who best match their partners in the other species.

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