Tag Archives: Research Blogging

Gardening ants grow their own treetop nests

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This post was chosen as an Editor's Selection for ResearchBlogging.orgIf you were to combine ants’ dispersal of seeds and plant protection interactions, and maybe squint a little, you might see something like epiphitic ant gardens. Ant gardens form when tree-nesting ants collect the seeds of some epiphytes—plants evolved to grow in the branches of trees—and the collected seeds sprout. The nests provide congenial conditions for the plants, since gardening ants frequently use dung as a building material. The roots running through the nest help stabilize its structure and suck out moisture to control interior conditions.

Ants cultivate “gardens” of epiphytes like Anthurium gracile to provide nesting space. Photo by gjofili.

This adds up to a mutually beneficial relationship between ant and epiphyte [$a]. A number of tropical epiphytes grow almost exclusively in ant gardens, and the inclusion of plants in the structure of their nests apparently helps gardening ant species to establish nests wherever food is most abundant.

Association with ant gardens has evolved independently in a number of epiphytic species, from arums like Anthurium gracile (pictured to the right) to orchids and philodendrons. When distantly-related species begin to perform the same ecological role, they often evolve convergent traits that facilitate the common role. Almost all ant-dispersed plants attach fatty bodies called elaisomes to their seeds to reward the ants that pick them up. Almost all ant-protected plants grow domatia in which the ants can nest, and nectaries to reward them with sugary sap. But plants that grow in ant gardens don’t seem to have a common trait that prompts ants to collect their seeds. Can it be that every ant-garden plant species has a unique way to be an ant-garden plant?

That’s what studies of ant-garden plants, including a new one just published in PLoS ONE, suggest. Plants associated with ant-gardens don’t have elaisomes on their seeds. Many produce fleshy fruit, but ants will collect their seeds even if no shred of fruit flesh clings to them. In some cases, ants will even collect seeds from the dung of fruit-eating birds and mammals.

This leaves the possibility that ant-garden plants produce some ant-attracting chemical in their seeds. In the new paper, Elsa Youngsteadt and her coauthors set out to identify chemical compounds that might be the common attractant used by nine different ant-garden plants from seven different plant families. Youngsteadt et al. isolated seven different compounds found in the seeds of ant-garden plants but not in closely related species that do not grow in ant gardens. (The absence of the seven compounds from the non-ant-garden relatives is established, rather amusingly, with a blank data table.)

The authors then painted crude extractions of all soluble organic compounds from two ant-garden plants onto seeds from species that gardening ants do not cultivate, and found that the ants were indeed more likely to collect them. (As a control, the ants were also offered seeds coated in the pure solvents used to extract attractive compounds. They didn’t like those.) However, analysis of the extracts failed to find a compound or set of compounds present in all three species.

It’s possible that Youngstead et al. simply failed to isolate the compound or compounds that all three ant-garden plants use to prompt ants to collect their seeds. But it’s not that far-fetched to think that these distantly-related plants might each use different attractive compounds to interact with ants in the same way. Natural selection may often arrive at different solutions when shaping different species for the same ecological role. It might also be that ant-garden relationships were established not by plants evolving a way to prompt ants to pick up their seeds, but by ants evolving to recognize seeds of plants that work well in gardens.

References

Davidson, D. (1988). Ecological studies of neotropical ant gardens. Ecology, 69 (4), 1138-52 DOI: 10.2307/1941268

Youngsteadt, E., Guerra Bustios, P., & Schal, C. (2010). Divergent chemical cues elicit seed collecting by ants in an obligate multi-species mutualism in lowland Amazonia. PLoS ONE, 5 (12) DOI: 10.1371/journal.pone.0015822

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.

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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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Not all species interactions are (co)evolved equal

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Biologists have long thought that coevolutionary interactions between species help to generate greater biological diversity. This idea goes all the way back to The Origin of Species, in which Darwin proposed that natural selection generated by competition for resources helped cause species to diverge over time:

Natural selection, also, leads to divergence of character; for more living beings can be supported on the same area the more they diverge in structure, habits, and constitution, of which we see proof by looking at the inhabitants of any small spot or at naturalised productions.
—Darwin (1859), page 128.

In the twentieth century, this idea was extended into suggestions that coevolution between plants and herbivores or flowers and pollinators helped to generate the tremendous diversity of flowering plants we see today. In general, biologists have found that strong coevolutionary interactions are indeed associated with greater diversity.

Yet although there is a well-established association between coevolution and evolutionary diversification, correlation isn’t causation. Furthermore, every species may coevolve with many others, and diversification that seems to be driven by one type of interaction might actually be better explained by another. It has even been suggested that coevolution rarely causes speciation at all.




Species interact in a lot of different ways, as antagonists, competitors, and mutualists. Do all these interactions shape diversity the same way? (Flickr: jby)

One step toward determining how often coevolution promotes diversification would be to identify what kinds of coevolutionary interaction are more likely to generate diversity. This is precisely the goal of a paper I’ve just published with Scott Nuismer in this month’s issue of The American Naturalist. In it, we present a single mathematical model that compares a wide range of species interactions to see how they shape diversification, and that model shows that coevolution doesn’t always promote diversity [PDF].

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Nibbled to distraction: Gerbils infested with fleas don’t watch for foxes

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ResearchBlogging.orgIn natural communities, each species is embedded in a web of interactions with other species—predators, prey, competitors, mutualists, and parasites. The effects of all these other species combine in complex, unpredictable ways. I recently discussed a study of protozoans living inside pitcher plants that found predators and competitors can cancel out each others’ evolutionary effects. Now another study finds that parasites and predators can interact to make desert-living gerbils adopt less effective foraging strategies [$a].

Allenby’s gerbil is a small desert rodent native to Israel’s Negev Desert. They make a living foraging for seeds, which might seem simple enough—but for small desert mammals, it’s a constant balancing act. Foraging requires continuously judging how profitable it is to continue gathering seeds in one spot compared to looking for another, maybe better, spot; and all the while watching out for predators.

The red fox—a major threat if you’re a tiny rodent, but hard to watch for when you’re scratching fleas all the time. Photo by HyperViper.

For small mammals, parasites like fleas can impose a real physiological cost—but they might also cause irritation that interfere with effective foraging. This idea led a group of Israeli reserachers to experimentally infest captive gerbils with fleas, and release them into an enclosure with a red fox.

It’s okay—the fox was muzzled! The research group was interested in how effectively the gerbils foraged in standardized patches of resources (trays of seed mixed with sand) in the presence of predators, and how being flea-ridden changed that foraging behavior. As metrics of foraging efficiency, they recorded how rapidly the gerbils gave up foraging in a single tray before moving on to another, which approximates how many seeds they left behind.

With no fleas, gerbils spent slightly—but not significantly—less time foraging in a single tray when a fox was in the enclosure with them. But gerbils infested with fleas moved on to a new tray substantially faster in the presence of a fox, leaving behind more seeds in the process. The study’s authors suggest that this is because the irritation caused by fleas distracted the gerbils too much to keep watch for a predator and forage at the same time—so flea-ridden gerbils made up for being less watchful by moving between patches of resources more rapidly.

So for gerbils, the presence of a second, different kind of antagonist amplifies the effects of a nearby predator. Fleas and foxes aren’t just a double whammy—the effects of both together are worse than the sum of each individually.

Reference

Raveh, A., Kotler, B., Abramsky, Z., & Krasnov, B. (2010). Driven to distraction: detecting the hidden costs of flea parasitism through foraging behaviour in gerbils. Ecology Letters DOI: 10.1111/j.1461-0248.2010.01549.x

No, I will not run the Seattle Marathon barefoot

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ResearchBlogging.orgI’m spending a significant chunk of my Thanksgiving break in Seattle, for the purpose of running what will be my second marathon this weekend. Running, like cooking, is helping to keep me sane in the midst of teaching labs, finishing my dissertation research, writing said research up for publication, and trying to sort out what happens after my committee decides I’ve earned a handful of extra letters after my name.

Me at about mile 17 in last year’s Portland Marathon. I’m not quite dead yet.

My first marathon was last year’s Portland Marathon. Prior to 2009, I’d never run a race longer than five miles, but then that spring I let friends talk me into a half-marathon, and after running more than 13 miles, 26.2 suddenly didn’t seem quite so insane. Even so, training up for Portland was more than enough to make me realize that running what was (for me) a 3 hour-45 minute course is not really the same thing as running eight or nine 5k’s in a row.

Feed me!

I can make it through even a half-marathon on a good breakfast and carefully-judged pre-race hydration, but to go much longer I need more food (and water) mid-run. The long-term exercise involved in a long race is fueled by a combination of fat reserves and glycogen stored in the liver and muscle tissue. Glycogen is the more efficient fuel, so as exercise intensity increases, muscles draw on it more heavily.

If his muscles runs out of glycogen, a runner “hits the wall,” and may be forced to stop running altogether. I’ve done this a few times on long training runs, and it’s not pleasant—I’d end up all but walking the last couple painful miles. How long I can go before I hit the wall depends on my glycogen reserves, which in turn depend on the muscle mass in my legs—but it also depends on how fast I’m running, since glycogen use increases with effort. A computational study of the interactions between exercise intensity and glycogen consumption suggests that my first marathon time, 3:45, was close to the upper limit of glycogen consumption for a “trained endurance athlete”—and I probably don’t really qualify as “trained,” in the sense the study uses. So to survive a marathon, I have to take on supplementary energy mid-race, for which I will carry tubes of disgusting sugar syrup.

Supplementary sugar. Nasty but necessary. Photo by size8jeans

Shoes matter. Who knew?

Before I started training for Portland, I didn’t pay much attention to the state of my running shoes—I bought new ones when the holes in the uppers got too obvious. That’s okay when the longest run I do is about eight miles—once my weekly schedule started including longer distances, I noticed more post-run pain when my shoes’ insoles deteriorated. I began investing in gel insole inserts and actually paying attention to how much mileage my shoes had accumulated.

The funny thing about shoes, though, is that familiarity is almost as important as adequate support. Last year I bought new shoes about a month out from the marathon—and ran some truly miserable long runs in them. Lesson learned. It turns out that a new pair of shoes takes some breaking in, especially if you switch brands, as I had. I ended up running the marathon in the shoes I’d considered shot (with new insole inserts), and felt better at the end that I had on a fifteen-mile run in the new ones. I now stick to one brand of shoes, with the same inserts if possible, and I don’t wear new shoes on a long run until I’ve worn them on a number of short ones.

Barefoot running, without the bare feet. Photo by Steven Erdmanczyk

Part of the reason that my running comfort is so sensitive to the quality of my shoes may be that human feet aren’t evolved to run in running shoes. Running on two legs sets humans apart from our closest evolutionary relatives, and we’ve probably been doing it for millions of years—but highly padded running shoes are a very recent invention. This is the central argument in favor of a recent fad for barefoot running [PDF]—that, once you build up some necessary calluses, running without the artificial support and padding of a running shoe is less stressful. A biomechanical comparison of barefoot and shod runners provided some of the first data to support this hypothesis earlier this year. Essentially, barefoot runners tend to land each step toes- or mid-foot-first [PDF], which absorbs the force of a foot-strike more effectively than the heel-first tread of shod runners.

I did see a few of my fellow marathon runners wearing nothing but “barefoot” running shoes like the ones pictured here, which provide protection against rough pavement but no artificial padding. I’m not going to be doing that any time soon. But maybe I’ll try to add some barefoot workouts into my training routine, if I survive Seattle and decide to run a third marathon.

I intend no endorsement of any products pictured or linked to in this post. Thanks to Conor O’Brien, who pointed me to the PLoS Computational Biology article cited above.

References

Jungers, W. (2010). Biomechanics: Barefoot running strikes back. Nature, 463 (7280), 433-4 DOI: 10.1038/463433a

Lieberman, D., Venkadesan, M., Werbel, W., Daoud, A., D’Andrea, S., Davis, I., Mang’Eni, R., & Pitsiladis, Y. (2010). Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature, 463 (7280), 531-5 DOI: 10.1038/nature08723

Rapoport, B. (2010). Metabolic factors limiting performance in marathon runners. PLoS Computational Biology, 6 (10) DOI: 10.1371/journal.pcbi.1000960

Is female orgasm adaptive? Let’s ask the clitoris.

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Correction, 22 December 2010: Vincent Lynch, author of the second paper discussed in this post, notes in the comments that he didn’t actually conclude that female orgasm was an adaptation. I’ve corrected the post accordingly.

ResearchBlogging.orgWhether or not a trait is an adaptation, shaped by natural selection for a specific function, can be a surprisingly contentious question in evolutionary biology. When the trait in question belongs to human beings, though, “contentious” reaches a whole new level—because when evolutionary biologists consider humans, their conclusions get personal.

Erotic sculpture on temple wall, Khajuraho, India. Photo by Abhishek Singh aka Bailoo.

Among the myriad traits and behaviors of Homo sapiens evolutionary biologists might choose to study, few can be as personal as the female orgasm. The adaptive function of male orgasm is about as clear-cut as possible—it’s a mechanistic necessity for uniting a sperm with an egg. But while female orgasm is enjoyable (or so I am told; this is as lousy a point as any to admit that my expertise in this phenomenon is purely academic), it isn’t necessary for fertilization. No man can be a father without having had at least one orgasm, but a woman could conceivably give birth to a huge family without having any.

To explain the existence of female orgasm in an evolutionary context, then, biologists have two options: (1) discover a way in which female orgasm shapes reproductive success indirectly, or (2) conclude that female orgasm isn’t an adaptation. Possibilities advanced for the first option range from the benefits of closer bonding with a mate—sex is, after all, about more than mere reproduction—to suppositions that the contractions associated with orgasm help draw semen into a woman’s reproductive tract.

The argument in support of non-adaptive female orgasm takes a developmental perspective: that female orgasm is really male orgasm, as experienced in a female developmental context. That is, women have orgasms for the same reason men have nipples—because the anatomies of both sexes are constrained by their origins in the same underlying developmental program. If this is the case, natural selection would work to optimize male orgasm, without necessarily affecting female orgasm—and that suggests a way to test whether female orgasm is an adaptation.

Natural selection removes less-fit versions of traits from a population—making that trait less variable within the population under selection. Traits that don’t affect survival or reproductive success, on the other hand, are free to accumulate variation via mutation. So non-adaptive traits can be identified by comparing their variation to traits with known adaptive functions.

Who cares what natural selection thinks, anyway? Photo by JorgeMiente.es.

Psychologist Kim Wallen and philosopher of science Elisabeth Lloyd (who had advanced the hypothesis that female orgasm is non-adaptive in a 2005 book) made just such a comparison in a 2008 study. Variation in female orgasm would be challenging to measure, so they used the clitoris as an anatomic proxy. This let them use the penis—which shares a developmental origin with the clitoris and is presumably under natural selection associated with male sexual function—as an adaptive standard for comparison. In comparison to (flaccid) penis length, Wallen and Lloyd found that clitoris length was indeed more variable [$a]. As a second control, the authors also compared variation in clitoris and penis length to variation in the length of women’s vaginas, understanding that this trait, unlike the clitoris, is important for female reproductive success. Vaginal length turned out to be about as variable as penis length, and much less so than clitoris length.

There are several objections to be made to Wallen and Lloyd’s analysis, and many were made in a response [$a] by evolutionary biologist Vincent Lynch. Lynch objected to the use of length as the focal measure for the size of the clitoris, and showed that clitoral volume was about as variable as penile volume. (I would add that the study of social insects Wallen and Lloyd cite as a precedent for their analysis isn’t actually focused on variation, but on the symmetry of traits under consideration, which is not quite the same thing.) More critically, though, Lynch points out that there isn’t any known relationship between clitoral size and ability to achieve orgasm—so the data don’t have the bearing on the question that Wallen and Lloyd assigned in the first place. Lynch concluded that female orgasm is an adaptation after all—a more conservative interpretation of his result is that we can’t answer the question by measuring clitorises.

Understanding the evolution of human sexual behaviors can help us to figure out how best to navigate the tricky business of a sexual relationship with another person—an approach most recently exemplified in the book Sex at Dawn. But we also tend to view evidence that natural selection favors a particular trait or behavior as a kind of approval, or as evidence of what is “natural.” That’s silly. Whether or not they help to make more babies, orgasms are fun, and they’re a wonderful part of our most intimate expression of affection and love. In some respects, that’s all we need to know.

References

Crespi, B., & Vanderkist, B. (1997). Fluctuating asymmetry in vestigial and functional traits of a haplodiploid insect. Heredity, 79 (6), 624-30 DOI: 10.1038/hdy.1997.208

Lynch, V. (2008). Clitoral and penile size variability are not significantly different: lack of evidence for the byproduct theory of the female orgasm. Evolution & Development, 10 (4), 396-7 DOI: 10.1111/j.1525-142X.2008.00248.x

Wallen K, & Lloyd EA (2008). Clitoral variability compared with penile variability supports nonadaptation of female orgasm. Evolution & development, 10 (1), 1-2 DOI: 10.1111/j.1525-142X.2007.00207.x

In the depths of a pitcher plant, competitors and predators cancel each other out

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ResearchBlogging.orgSpecies interactions are probably pretty important, in the evolution of life. There are all sorts of studies showing that the fitness and evolutionary history of individual species depends upon interactions with pollinators, symbiotes, food plants, herbivores, parasites, predators, and competitors. Most of these studies focus in on a single interaction—but what living thing interacts with only one other organism? Coevolution, when it happens, happens in a community context.

Adding even a second interaction into the scientific picture can be difficult, but it may also dramatically change the evolutionary outcome, as seen in a new study of evolution in the protozoan communities living in purple pitcher plants. Individually, competitors and predators are significant agents of natural selection—but together, they seem to counterbalance each other [$a].

The purple pitcher plant, Sarracenia purpurea. Photo by petrichor.

Carnivorous pitcher plants grow funnel-shaped leaves that collect water to form a pitfall trap for hapless insects, which provide a source of nitrogen in swampy, nutrient-poor habitats. One species’ pitfall is another’s ideal habitat, however, and pitchers also play host to diverse micro-communities [PDF] of protozoans, bacteria, and even mosquito larvae. By recreating—and experimentally manipulating—these communities in the laboratory, the new study’s author, Casey terHorst, was able to disentangle the individual and combined effects of two different kinds of species interaction within pitcher plant pitfalls.

TerHorst focused on a protozoan species in the genus Colpoda, a widespread single-celled critter found in moist soil and standing water. In pitcher plants, Colpoda makes a living feeding on bacteria that break down insects trapped by the pitfall—and they themselves are prey for the larvae of the mosquito Wyeomyia smithii.

An example of genus Colpoda, the group of ciliates studied (but probably not the same species). Photo by PROYECTO AGUA** /** WATER PROJECT.

To determine the individual and combined effects of competition and predation on Colpoda, terHorst allowed experimental populations of the protozoan to evolve for 20 days (about 60-120 Colpoda generations) with either (1) no competitors or predators, (2) competition from another bacteria-eating protozoan, (3) predation by mosquito larvae, or (4) competition and predation. At the end of the experimental period, he sampled each evolved Colpoda population and measured a number of traits, including the size of Colpoda cells and their speed. Larger Colpoda cells are thought to be better competitors but more vulnerable to predators; faster ones should be better able to evade predation.

Individually, predators and competitors had significant effects on Colpoda evolution. In the presence of mosquito larvae, Colpoda evolved smaller, faster cells than it did alone. Unexpectedly, competitors also caused Colpoda to evolve smaller cells, though not faster ones. TerHorst suggests that this is because competition also favored more rapid reproduction by Colpoda, which meant that individual cells grew less before dividing.

Most interestingly, though, Colpoda evolving in the presence of both predators and competitors looked quite a lot like Colpoda that evolved alone. This is apparently because the mosquito larvae ate both Colpoda and its competitor—the mosquitoes acted to relieve some competitive pressure on Colpoda at the same time they ate fewer Colpoda because they had two prey species to pursue. In fact, the mosquitoes preferred to eat the competitor species, since it tended to hang out in the open while Colpoda hid among the plastic beads lining the base of the artificial habitat.

Thus the indirect effects of the predator offsetting competition, and of the competitor drawing away predation, canceled out the natural selection each imposed on Colpoda individually. Species interactions in a community context, even a simple one like this, are far from straightforward.

References

Buckley, H., Burns, J., Kneitel, J., Walters, E., Munguia, P., & Miller, E. (2004). Small-scale patterns in community structure of Sarracenia purpurea inquilines. Community Ecology, 5 (2), 181-8 DOI: 10.1556/ComEc.5.2004.2.6

terHorst, C. (2010). Evolution in response to direct and indirect ecological effects in pitcher plant inquiline communities. The American Naturalist, 176 (6), 675-85 DOI: 10.1086/657047

Between two host plants: The middle road doesn’t work for hybrid butterflies

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This post was chosen as an Editor's Selection for ResearchBlogging.orgNew species form when separate populations of related organisms are no longer able to interbreed. Reproductive isolation can arise if two populations evolve different mating behaviors, or lifestyles so different that individuals from different populations don’t even encounter each other—but it need not mean that matings between the two populations never occur. In fact, speciation can arise in the face of quite a lot of interbreeding, so long as the hybrids produced by interbreeding are less fit than “purebred” individuals.

Edith’s checkerspot in Mount Diablo State Park, California. Photo by davidhoffman08.

This is what seems to be occurring in populations of Edith’s checkerspot, a small butterfly native to Western North America. Checkerspot populations in California use a wide variety of different host plants, and a recent study has shown that the offspring of parents from different host plants are maladapted in the wild.

In the Sierra Nevada mountains, logging has created a new kind of habitat for Edith’s checkerspot [PDF]—patches of cleared forest where the butterfly’s locally preferred host plant, Pedicularis semibarbata, is rare or nonexistent, but an alternative host plant, Collinsia torreyi, is plentiful. In the transition between clearings and less-disturbed forest, the two plants may often grow side by side.


A tale of two host plants: Pedicularis semibarbata and Collinsia torreyi. Photos by Wayfinder_73.

Examination of checkerspot populations that have access to only one of the two host plants suggests that each plant is best used in rather different ways. For instance, Pedicularis-using checkerspot females lay lots of eggs on a few plants, while Collinsia-using females lay a few eggs on each of a large number of plants. Once they hatch, larvae from Pedicularis populations feed on leaves closer to the ground than larvae from Collinsia populations, which makes sense since Pedicularis grows lower in general.

If these differences have a genetic basis, then hybrid checkerspots might exhibit intermediate behaviors, which might not work so well on either host plant. To test for this “hybrid inviability,” the new study’s authors crossed checkerspots from populations encountering only one host plant or the other, and then tested the hybrids’ performance in the field—and what they found confirms those predictions.

The Goldilocks principle—intermediate is better–doesn’t apply to hybrid checkerspots. Hybrid caterpillars foraged on leaves at an intermediate height on both host plants, and grew more slowly than purebred caterpillars. Hybrid females laid an intermediate number of eggs on both host plants, and laid them at an intermediate height. This left their offspring in a poor position for foraging after they hatched—and indeed, they grew more slowly than larvae hatched from eggs that were laid at the “traditional” heights on the host plants.

So it looks as though natural selection for better performance on Collinsia has led to the evolution of checkerspots that are at a disadvantage using Pedicularis (and vice versa). This even to the point that hybrids, which feed and oviposit in ways that are only somewhat different from the optimum, pay performance costs.

What’s interesting, though, is that this hasn’t led to greater genetic differentiation of checkerspot populations using different host plants; as assessed using randomly-selected genetic markers, there is an isolation-by-distance effect, but no effect of host plant use. (The authors cite a previous study using about 400 AFLP loci [PDF].) That suggests that only a few genes are responsible for the observed adaptive differences, and that natural hybridization between checkerspot populations using different hosts may be mixing together the rest of the genome.

References

McBride, C., & Singer, M. (2010). Field studies reveal strong postmating isolation between ecologically divergent butterfly populations. PLoS Biology, 8 (10) DOI: 10.1371/journal.pbio.1000529

Singer, M.C., & Wee, B. (2005). Spatial pattern in checkerspot butterfly-host plant association at local, metapopulation and regional scales. Annales Zoologici Fennici, 42, 347-61