As a final taxonomic catch-all for my 2023 nature photography, let’s go with … invertebrates? If I’m not taking a photo of a plant, a bird, or a mammal, it’s most likely an insect visiting a flower. I do love a good plant-pollinator interaction. And while larger animals are a challenge to manage well with my 150mm lens, I can frequently catch some nice close images of butterflies nectaring, like the blue above, or the Clodius parnassian below.
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A new paper from the lab — coauthored with all three of the Yoder Lab’s graduate student alumni — is now online ahead of print in the journal Evolution Letters. In it, we analyze population genetic data from 20 pairs of plants and herbivores, parasites, and mutualists that live intimately on those plants to test for evidence that the associate species’ population genetic structure aligns with that of their host plants. This is an expected result if adaptation to the host plant drives diversification of the associates — and we found that it is indeed a recurring pattern. This is a pretty neat result, and, I think, a nice contribution to a long-established literature on how intimate associations with plants has driven the diversification of groups like butterflies and beetles.
Medicago truncatula, or barrel clover, a member of the legume family that hosts bacteria in its roots. The bacteria transform nitrogen gas from the atmosphere into fertilizer for their host plant, and the host feeds the bacteria with sugar. Experiments with barrel clover and its mutualists have shown that signals between the plant and the bacteria are important in this interaction, and provide an inspiration for the evolutionary models built by Yoder and Tiffin. (Flickr: jby)
I’m very excited to see this in virtual print — it’s a new model of coevolution between mutualists that takes into account signals between the partners as well as the benefits they provide each other (or don’t).
Yoder JB and P Tiffin. 2017. Sanctions, partner recognition, and variation in mutualism. American Naturalist doi: 10.1086/693472.
I’ll try to write about this in more depth at some point, but here’s the lay summary at the American Naturalist website:
Mutually beneficial relationships between species, or mutualisms, are ubiquitous in the living world, with examples ranging from flowering plants that rely on animal pollinators to fish that clean the teeth and scales of other fish. Mutualisms are often imperfect — one partner or the other varies in the quality of the help it provides. Evolutionary theory predicts that this should break up the relationship, but most mutualisms hold together in spite of partners that take the benefits of mutualism without properly paying them back.
This paradox may be explained by the fact that there’s more to mutualism than trading goods or services. This is a key result of mathematical evolutionary models published in the American Naturalist by Jeremy Yoder and Peter Tiffin, biologists at the University of British Columbia and the University of Minnesota. Yoder and Tiffin built a mathematical evolutionary model of mutualists that communicate before trading resources, and compared it to simpler models with only resource-trading or only communication. In the model with communication and resource-trading, host could “sanction” by cutting off resources to prevent poor quality partners from taking over, but evolution of the signals sent by partners and the hosts’ response to those signals maintained variation over time. Neither of the simpler models could do this. With only resource-trading, sanctions eliminated all poor-quality partners, and all variation; with only communication, poor-quality partners took over the mutualism.
Comparing metrics of diversity (x axis) and geographic differentiation (y axis) for thousands of genes in the Medicago truncatula genome (gray points) reveals that some symbiosis genes (red points, crosses, and triangles) are genome-wide outliers — but they are not all the same kind of outlier. Yoder (2016), Figure 1.
My very latest scientific publication is now online at the American Journal of Botany. It’s sort of an odd paper — something of a review, or an opinion piece, discussing how population genomic data can help us understand why mutualisms stay stable [PDF] in spite of the risk of “cheating” by partners, with a “worked example” with data from the Medicago HapMap Project. Here’s some key bits from the abstract:
Different hypothesized models of mutualism stability predict different forms of coevolutionary selection, and emerging high-throughput sequencing methods allow examination of the selective histories of mutualism genes and, thereby, the form of selection acting on those genes. … As an example of the possibilities offered by genomic data, I analyze genes with roles in the symbiosis of Medicago truncatula and nitrogen-fixing rhizobial bacteria, the first classic mutualism in which extensive genomic resources have been developed for both partners. Medicago truncatula symbiosis genes, as a group, differ from the rest of the genome, but they vary in the form of selection indicated by their diversity and differentiation — some show signs of selection expected from roles in sanctioning noncooperative symbionts, while others show evidence of balancing selection expected from coevolution with symbiont signaling factors.
The paper is my contribution to a Special Section on “The Ecology, Genetics, and Coevolution of Intimate Mutualisms”, which I co-edited with Jim Leebens-Mack. You can view the whole Special Section here, and download my paper here [PDF].
My visualization of key data from Verne Grant’s 1949 paper showing that floral traits are more likely to be important in the taxonomic descriptions of plant species when those species are pollinated by animals — which suggests that those plant-pollinator interactions play a role in the formation of new species.
I got word this morning that the Encyclopedia of Evolutionary Biology, a huge compendium of current knowledge on evolution, systematics, and ecology, is now online. That’s exciting in and of itself, but it’s particularly so because it means you can finally see my contribution, the introduction to the topic of coevolution. Here’s the opening paragraph, of which I’m rather fond:
No organism is an island. Every living thing contends with predators, parasites, and competitors, and most also receive benefits from mutualists (Table 1). These interactions with other species exert natural selection—and predators, parasites, competitors, and mutualists may also experience selection in return. The mutual evolutionary change that results from this reciprocal selection is ‘coevolution’ (Janzen 1980; Thompson 2005).
The rest of the Encyclopedia includes contributions from a tremendous array of other authors, and I’m grateful to subject editor Andrew Forbes for the invitation to contribute. You can browse the whole thing on the publisher’s website, and download a manuscript-format PDF of the final text of my chapter here.
Joshua trees at Tikaboo Valley, Nevada (Flickr: jby)
I’m very excited to announce a new project, with a new model for doing science: The Joshua Tree Genome Project, in which I’m working with a bunch of smart, accomplished folks to sequence the genome of my favourite spiky desert plant. A sequenced Joshua tree genome will provide the framework to understand how coevolution with highly specialized pollinators has shaped the history of Joshua trees, and to use the landscape genomics skills I’ve developed with the Medicago HapMap Project and AdapTree to understand how the trees cope with extreme desert climates — and how to ensure they have a future in a climate-changed world.
Perhaps most excitingly (terrifyingly?) we’re going to raise some of the funds to do the genome sequencing by crowdfunding, using the Experiment.com platform. So please keep an eye on the project site, follow our Twitter feed, and Like our Facebook page to make sure you don’t miss your chance to help understand Joshua trees’ evolutionary past and ensure their future.
Cross-posted from Nothing in Biology Makes Sense!
A huge diversity of flowering plants rely on animals to carry pollen from one flower to another, ensuring healthy, more genetically diverse offpsring. These animal-pollinated species are in a somewhat unique position, from an evolutionary perspective: they can become reproductively isolated, and form new species, as a result of evolutionary or ecological change in an entirely different species.
Evolutionary biologists have had good reason to think that pollinators often play a role in the formation of new plant species since at least the middle of the 20th century, when Verne Grant observed that animal-pollinated plant species are more likely to differ in their floral characteristics than plants that move pollen around via wind. More recently, biologists have gone as far as to dissect the genetic basis of traits that determine which pollinator species are attracted to a flower—and thus, which flowers can trade pollen.
However, while it’s very well established that pollinators can maintain isolation between plant populations, we have much less evidence that interactions with pollinators help to create that isolation in the first place. One likely candidate for such pollinator-mediated speciation is Joshua tree, the iconic plant of the Mojave Desert.
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Photo by ngari.norway.Over at Nothing in Biology Makes Sense!, I’ve written about a new study that tries to disentangle conflicting sources of natural selection to determine whether big herbivores like antelope, zebras, and ostriches have evolved to run because they’re always running away from predators.
An antelope’s frame is under more demands than evading cheetahs—it also needs to travel long distances to follow food availability with the shifting rainy season. In fact, the North American fossil record suggests that big herbivores on that continent evolved long legs for distance running millions of years before there were predators able to chase after them. And then again, not all predators run their prey down; lions, for instance, prefer to pounce from ambush.
To find out whether gazelles are running for their lives, or running for dinner, go read the whole thing.◼
Aphis fabae. Photo by robbersdog.Over at Nothing in Biology Makes Sense!, Devin Drown describes an interaction between aphids and a species of wasp who lay their eggs in the aphids so their larvae can eat the aphids alive. A new study tests whether the success of a wasp larva infecting an aphid depends on the specific genetics of the wasp, and of a bacterial symbiont the aphid carries:
The Vorburger group studies a crop pest aphid, Aphis fabae, and its common wasp parasitoid, Lysiphlebus fabarum. The adult parasitoids lay their eggs in unsuspecting aphid hosts. As the parasitoids develop they battle the hosts defenses. Some aphid hosts are also infected with a bacterium symbiont, Hamiltonella defensa, which can provide protection against the parasitoid by releasing bacteriophages that target the parasitoid invader (Vorburger et al 2009; Vorburger and Gouskov 2011). If the wasp parasitoid can evade all the host defenses then eventually it develops inside the still living aphid. Eventually, as the authors describe in grisly detail
“metamorphosis takes place within a cocoon spun inside the host’s dried remains, forming a ‘mummy’ from which the adult wasp emerges” (Rouchet and Vorburger 2012).
To learn how Vorburger et al. evaluated the importance of wasp genetics for successfully mummifying aphids, go read the whole thing.◼
Photo by futureatlas.
Cross-posted from Nothing in Biology Makes Sense!
It is a truth universally acknowledged in evolutionary biology, that one species interacting with another species, must be having some effect on that other species’ evolution.
Actually, that’s not really true. Biologists generally agree that predators, prey, parasites, and competitors can exert natural selection on the other species they encounter, but we’re still not sure how much those interactions matter over millions of years of evolutionary history.
On the one hand, groups of species that are engaged in tight coevolutionary relationships are also very diverse, which could mean that coevolution causes diversity. But it could be that the other way around: diversity could create coevolutionary specificity, if larger groups of closely-related species are forced into narower interactions to avoid competing with each other.
Part of the problem is that it’s hard to study a species evolving over time without interacting with any other species—how can we identify the effect of coevolution if we can’t see what happens in its absence? If only we could force some critters to evolve with and without other critters, and compare the results after many generations …
Oh, wait. That is totally possible. And the results have just been published.
A team of evolutionary microbiologists has performed exactly the experiment I outlined above. The study’s lead author is Diane Lawrence, a Ph.D. student in the lab of Timothy Barraclough, who is listed as senior author.
For the experiment, the team isolated five bacterial species, of very different lineages, from pools of water at the bases of beech trees—ephemeral pockets of habitat for all sorts of microbes that break down woody debris, dead leaves, and other detritus. They cultured the bacteria on tea made from beech leaves, in vials containing either a single species, or all five species, and let them evolve for eight weeks—several dozens of bacterial generations. In a particularly clever twist on standard experimental evolution methods, they also used nuclear magnetic resonance (NMR) to identify the carbon compounds in sterilized tea that had been “used up” by the bacterial cultures, and compared the compounds in fresh beech tea to determine what the bacteria were eating.
The base of a beech tree. Photo by -nanio-.And, maybe not surprisingly, the bacterial species’ evolution with company turned out to be quite a bit from their evolution alone. Left alone, most of the species evolved a faster growth rate. This is a common result in experimental evolution, because the process of transferring evolving bacteria to fresh growth medium—”serial transfers” that were performed fifteen times over the course of the experimetn—can create natural selection that favors fast-growing mutants. But, grown all together in the same tube, species that had evolved faster growth rates in the solo experiment evolved slower growth instead.
To find out what had evolved in the multi-species tubes, the team tested the growth of the bacterial species on beech tea that had been used to grow one of the other species, then sterilized. The original, ancestral strains of bacteria generally had negative effects on each others’ growth—they lived on similar compounds in the beech tea, and so their used tea wasn’t very nourishing for the other species. The same thing occurred with the strains that had evolved alone, only stronger, which makes sense in light of the increased growth rates, which would’ve depleted the growth medium faster.
But the interactions among the strains of the different bacterial species that had evolved together was strikingly different. Many of them actually made the tea more nutritious for other species in the evolved community. That is, some of the bacteria had evolved the capacity to eat the waste products of another species that was evolving with them. Using the NMR method to track changes in the presence of different carbon compounds in the tea before and after use provided confirmation that the co-evolved species were using, and producing, complementary sets of resources.
In short, the evolving community didn’t simply become more diverse—it evolved new kinds of mutually beneficial relationships between species that began as competitors.
Beech leaves—yum? Photo by Colin-47.That evolutionary shift toward mutual benefit had a significant impact on the bacterial community as a whole, too. Lawrence et al. assembled new communities of bacteria extracted from the end-point of the group evolution experiment, and compared their carbon dioxide production, a proxy for overall metabolic activity, to that of a community assembled from bacteria extracted from the end point of the solo-evolution experiments. The community of co-evolved bacteria produced significantly more carbon dioxide, suggesting they were collectively able to make more use out of the growth medium.
So that’s a pretty nifty set of results, I have to say. But I’m also left wondering what it tells us more generally. In both Lawrence et al.‘s paper, and in accompanying commentary by Martin Tucotte, Michael Corrin, and Marc Johnson, there’s a fair bit of emphasis on the unpredictability of the result. Lawrence et al. write, in their Discussion section,
The way in which species adapted to new conditions in the laboratory when in monoculture—the setting assumed for many evolutionary theories and experiments—provided little information on the outcome of evolution in the diverse community.
And, as Corrin et al. note,
These results imply that predictions constructed from single-species experiments might be of limited use given that most species interact with many others in nature.
So … evolution went differently under different conditions? That isn’t exactly a shocking revelation. The fact that this is one of the study’s major conclusions is a symptom of how little experimental work has actually tested the effects of multiple species on evolution. One experiment I’ve discussed here previously, focused on the joint effects of predators and competitors on microbes that live in pitcher plant pitfalls, similarly emphasized the fact that it wasn’t possible to predict the evolutionary effects of predators and competitors together based solely on their individual effects. Work in this line of inquiry is hanging at the point of establishing that complex conditions lead to complex results.
What I’d really like to know—and I think all the authors of both the paper and the commentary would agree with me on this—is how we can begin to make general predictions about community evolution beyond, “it depends what we put in at the start.” It may be that we’ll need a lot more studies like this current one before we can start to identify common processes, and more interesting trends.◼
References
Turcotte, M., Corrin, M., & Johnson, M. (2012). Adaptive evolution in ecological communities. PLoS Biology, 10 (5) DOI: 10.1371/journal.pbio.1001332
Lawrence, D., Fiegna, F., Behrends, V., Bundy, J., Phillimore, A., Bell, T., & Barraclough, T. (2012). Species interactions alter evolutionary responses to a novel environment. PLoS Biology, 10 (5) DOI: 10.1371/journal.pbio.1001330