I’m delighted to announce a new paper published today in the Biology Letters, coauthored with Colin Carlson at Georgetown University and a CSUN undergrad researcher, Gio Gomez. In it, we examine a big collection of floral visitation records and find a pattern that pollination biologists have talked about but never quite directly demonstrated: the symmetry of flowers seems to shape the diversity of animals that visit them, and potentially provide pollination services. Here’s a brief “lay summary” we wrote to accompany the article:
For centuries, botanists have understood that the symmetry of flowers — whether or not they are “zygomorphic”, with a single line of symmetry — shapes how they attract and interact with pollinators. We examined 53,609 records of animal visits to flowers in 159 communities around the world, and found that zygomorphic flowers are visited by fewer potential pollinator species. This may explain broad patterns in the diversity of flowering plants, in which zygomorphic flowers are associated with faster formation of new species. It also suggests that plant species with zygomorphic flowers may be at greater risk of extinction due to pollinator loss.
This is an exciting paper because it’s my first foray into pollination ecology proper, and because of its place in that broader field of research — and also because it’s the first paper I’ve published with a student coauthor since starting on faculty at CSUN. On top of all that, the project has been a really nice bridge between my interests (mutualism) and Colin’s (host-associate community ecology), and it’s kicked off a collaboration that has produced some even more exciting results, coming soon to a preprint server near you.
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.
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.
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.
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.
This week at Nothing in Biology Makes Sense! I’m discussing a new study that purports to demonstrate that three-toed sloths are in a nutritional mutualism with specialized moths, fueled by algae and poop:
Sloths’ coarse, shaggy fur accumulates its own little microcosm of living passengers. (If you move that slowly in a tropical forest canopy, you’re going to get some hop-ons.) Among these are an assortment of algae, and moths in the genus Cryptoses. It’s been known for a long time that Cryptoses moths lay their eggs in sloth dung, and that their larvae eat it.
To find out why it isn’t completely crazy to think that these poop-eating moths might be helpful to sloths, go read the whole thing.◼
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.
This week at Nothing in Biology Makes Sense! I’m discussing a nifty new study that suggests interacting species can sometimes tolerate stressful environments by helping each other out:
This was the perspective of Peter Kropotkin, a Russian prince and political anarchist who studied the wildlife of Siberia while working as an agent of the Czar’s government. In the harsh conditions of the Siberian winter, Kropotkin reported finding not a bitter struggle over scarce resources, but what he called “Mutual Aid” among species, as well as in the human settlements that managed to eke out a living.
Something like what Kropotkin described is documented in a new paper by Elizabeth Pringle and colleagues. Examining a protection mutualism between ants and the tropical Central American tree Cordia alliodora, Pringle et al. found that drier, more stressful environments supported more investment in the mutualism.
To learn how ants can help a tree deal with drier climates—no, it doesn’t involve little tiny bucket brigades—you’ll have to go read the whole thing.◼
Of course, the ants have a vested interest in keeping the trap effective, since they eat some portion of the critters caught by their host. But it seems pretty straightforward to think that this helps the pitcher plant, too. A more definitive test would be to compare the survival and seed production of pitcher plants grown with and without a colony of ants to keep them clean. ◼
Nitrogen is one of the elemental building blocks of life as we know it—it’s a basic component of amino acids, which are in turn the building blocks of proteins, which form the building blocks and moving parts of every living cell. The nitrogen interwoven in our tissues originated as part of the atmosphere we breathe, but the path from atmosphere to living flesh is far less direct than drawing a breath. Atmospheric nitrogen becomes useful to us animals only via an intimate relationship between a plant and bacterial growing in its roots.
The bacteria, called rhizobia, have the rare ability to “fix” free-floating nitrogen into biologically useable form. In return for this nitrogen source, the host plant allows the rhizobia to infect a specialized knob of root tissue, a root nodule, which it supplies with sugar for the benefit of its nitrogen-fixing guests. The plant uses the fixed nitrogen to make proteins for its own use, and anything that eats the plant afterwards benefits.
If all this sounds familiar, it’s because the interaction between plants and rhizobia is the focus of my developing postdoctoral research, and I’ve been writing about it as I’ve done more reading about it. Specifically, I’ve been interested in how plants might be able to make sure their root nodules house helpful bacteria rather than freeloaders, who enjoy the sugar supply inside the nodule without fixing nitrogen in return.
I’ve discussed a couple of different mathematical models that suggest some options. However, models are really just formal ways to follow through the implications of a particular idea, not necessarily descriptions of what actually transpires between a plant and the rhizobia inside its roots. So I thought it might make sense to step back and survey what we presently know about what goes on inside those root nodules.