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.
Over at Nothing in Biology Makes Sense! I’ve got a new post discussing freshly published results from my dissertation research on Joshua trees and their pollinators. I don’t have to tell you why Joshua trees are interesting, do I?
Joshua trees are pollinated by yucca moths, which are unusually focused, as pollinators go. Your average honeybee will blunder around in a flower, scooping up pollen and drinking nectar, and maybe accidentally pollinate the flower in the process. A yucca moth, on the other hand, gathers up a nice, tidy bundle of pollen in specialized mouthparts, carries it to another Joshua tree flower, and deliberately packs it into place. She does that because the fertilized flower provides more than a little nectar for her—she’s laid her eggs inside the fertilized flower, and when they hatch her offspring will eat some of the seeds developing inside it.
That’s pretty cool in its own right. But what’s especially interesting about Joshua trees, from an evolutionary perspective, is that they’re pollinated by two different moth species. And it turns out that the flowers of Joshua trees associated with the different moth species also look pretty different. The most dramatically different feature is in the length of the stylar canal in the pistil, the part of the flower that determines how the moths lay their eggs.
In the latest development, my collaborators and I tested for genetic evidence that Joshua trees pollinated by different moth species are isolated from each other. To learn what we found, go read the whole thing.◼
I don’t know if I could watch a whole feature film like this, but it’s mighty pretty in two minutes-plus of full-screen viewing. And hey, just as I was wondering if there would be Joshua trees, there were Joshua trees.
My postdoctoral research is shaping up more and more to be hardcore bioinformatics; apart from some time spent trying to get a dozen species of peanut plants to grow in the greenhouse as part of a somewhat long-shot project I’m working on with an undergraduate research associate, I mostly spend my workday staring at my laptop, writing code. It’s work I enjoy, but it doesn’t often give me an excuse to interact directly with the study organism, much less get outdoors. So, when Chris Smith dropped the hint that he could use an extra pair of hands for fieldwork in the Nevada desert this spring, I didn’t need a lot of persuasion.
Chris is continuing a program of research he started back when he was a postdoc at the University of Idaho, and which I contributed to as part of my doctoral dissertation work. The central question of that research is, can interactions between two species help to create new biological diversity? And the specific species we’ve been looking at all these years are Joshua trees and the moths that pollinate them.
Joshua trees, the spiky icon of the Mojave desert, are exclusively pollinated by yucca moths, which lay their eggs in Joshua tree flowers, and whose larvae eat developing Joshua tree seeds. It’s a very simple, interdependent interaction—the trees only reproduce with the assistance of the moths, and the moths can’t raise larvae without Joshua tree flowers. So it’s particularly interesting that there are two species of these highly specialized moths, and they are found on Joshua trees that look … different. Some Joshua trees are tall and tree-ish, and some Joshua trees are shorter and bushy. Maybe more importantly for the moths, their flowers look different, too.
Here’s a photo of two of those different-looking Joshua tree types, side by side in Tikaboo Valley, Nevada. Tikaboo Valley has the distinction of being the one spot where we’ve found both of the tree types, and both of the pollinator moth species, living side by side. That makes Tikaboo Valley the perfect (well, only) place to figure out whether there’s an evolutionary consequence to the divergence of Joshua tree and its association with two different pollinators. Do Joshua trees make more fruit, or fruit with more surviving seeds, when they’re pollinated their “native” moths?
So, over several years of work at Tikaboo Valley, we’ve been edging towards answering that question. We’ve found evidence that, given access to both tree types, the two moth species spend more time on their “native” tree type, and have more surviving offspring when they lay eggs in “native” flowers. But to determine whether plant-pollinator matching matters to Joshua trees, we’d really like to find out what happens when each moth species is forced to use each type of tree, and that’s what Chris has been working on for the last several field seasons.
Installing a Joshua tree chastity device. Photo by jby.
The method for the experiment, developed after some false starts, goes like this:
Find Joshua trees with flowers that haven’t opened yet—untouched by pollinating moths;
Make sure said flowers are far enough off the ground to be out of reach of the open-range cattle that graze all over Tikaboo Valley;
Catalog the tree, measuring how tall it grew before it started branching (a good indicator of which type of tree it is), and its total height, and take a nice photo of it with an ID number placed nearby, for handy future reference;
Seal up the not-yet-open bunch of flowers inside fine-mesh netting, to keep moths out—and also, as we’ll see below, to keep moths in;
Cover the netted flowers in chicken wire, to keep out all the desert critters that like to eat Joshua tree flowers, even if said flowers are served with a side of netting;
While the flowers get closer to opening, go collect some yucca moths, which you do by cutting down clusters of open Joshua tree flowers, dumping them into a bag or a cloth butterfly net, and sorting through the flowers looking for fleeing moths, which can be guided into plastic sample vials—these moths don’t usually like to fly; and finally
Open caged flowers, and insert moths.
By introducing moths of each species into flowers on each variety of Joshua tree, we’ll be able to see whether trees with the “wrong” moth species are less likely to make fruit than trees with the “right” moth species; and directly verify that moths introduced into the “wrong” tree type have fewer surviving larvae than moths introduced into the “right” tree type.
But, being desert plants, Joshua trees aren’t prone to making much fruit even under ideal conditions. After a dry winter (like this last one), it can be hard to find any flowering trees at all. So to obtain a respectable sample size takes a lot of folks—this year, I was one of ten people on the field crew camped in the middle of the valley: a cluster of tents grouped around a rented recreational vehicle, which served as a kitchen/gathering area/lab.
Chris’s lab tech, Ramona Flatz, kept the whole show organized, dividing us into teams to scout for trees with flowers, teams to follow up on scouting reports and install experimental net/cage setups, and teams to go collect moths to put in the cages. This planning was, naturally, conducted in a tent containing a table with laminated maps of the valley, and this tent was called, naturally, the “war tent.”
What results we’ll get remain to be seen; this is the second year with a substantial number of experimental trees, and we won’t know whether all that work has borne fruit until Chris returns in a few weeks to see whether any of the experimental trees have, er, borne fruit. As far as I’m concerned, it was wonderful to return to an old familiar field site, in the middle of the desert, and spend a few days hiking around and harassing yucca moths instead of anwering e-mail. But if the experiment works, the results should be mighty interesting.
Below, I’ve embedded a slideshow of all the photos I took over a few days at Tikaboo Valley—including a special moth-themed production number coordinated by Ramona.◼
Godsoe, W., Yoder, J., Smith, C., & Pellmyr, O. (2008). Coevolution and divergence in the Joshua tree/yucca moth pollination mutualism The American Naturalist, 171 (6), 816-823 DOI: 10.1086/587757
Smith, C. I., C. S. Drummond, W. K. W. Godsoe, J. B. Yoder, & O. Pellmyr (2009). Host specificity and reproductive success of yucca moths (Tegeticula spp. Lepidoptera: Prodoxidae) mirror patterns of gene flow between host plant varieties of the Joshua tree (Yucca brevifolia: Agavaceae) Molecular Ecology, 18 (24), 5218-5229 DOI: 10.1111/j.1365-294X.2009.04428.x
Yoder, J., & Nuismer, S. (2010). When does coevolution promote diversification? The American Naturalist, 176 (6), 802-817 DOI: 10.1086/657048
… whatever that means. I’m doing the family travel circuit during the traditional worst time of year to travel in the U.S., and taking as much of a break as I can while doing it. Regular posting will resume with the start of the new year. In the meantime, here’s Pink Martini’s rockin’ multilingual rendition of “Auld Lang Syne.”
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. Photo by 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.
We already have good direct evidence that selection favors yucca moths who closely match the local Joshua tree population. Joshua tree’s pollinators are entirely dependent on the plant as a food source—they don’t eat nectar or pollen like many other pollinators, but Joshua tree seeds. Female moths lay eggs in Joshua tree flowers, then deliberately pollinate them using pollen carried in unique, specialized mouthparts. When the fertilized flowers develop into fruit, the moth eggs hatch, and the emerging larvae eat some of the seeds inside the fruit.
Scaled comparison of pollinator moth body sizes and Joshua tree pistils. To lay eggs in a flower, moths must drill from near the top of the pistil to the positions marked by dotted lines. Illustration from Smith et al.(2009), fig 1.
If the pollinating moths do too much damage to the flower in the course of laying their eggs, the flower dies off—which helps keep the moths from over-exploiting the relationship by laying lots of eggs or delivering too little pollen. This also means that moths with over-long ovipositors, the appendages used to drill into the flower to lay eggs, may do more damage than necessary and risk killing the flowers they pollinate. As it happens, the two types of Joshua tree have differently-shaped flowers, and the two pollinator species differ in their ovipositor lengths—and moths with overlong ovipositors can’t successfully raise larvae on small-flowered Joshua trees.
The new analysis compares moths’ ovipositor lengths and measurements of Joshua tree flowers from across the entire Mojave Desert, where both are found. The two moth species differ significantly, and so do trees from populations pollinated by different moths species—as we’ve previously found. But because the dataset is more detailed than before, we could also look at how variation is distributed within the two types of Joshua tree and the two pollinator species.
The answer almost feels disappointing: within tree types or pollinator species there just isn’t much variation. In fact, the variation we can detect seems to be random—just statistical noise. That may mean our measurement methods are too imprecise to detect fine-scale patterns in Joshua tree and yucca moth populations. But it’s also what we would expect if Joshua tree and its pollinators were under strong selection to match each other. Natural selection against less-well-matched moths and trees should eliminate heritable variation in moth ovipositor length and Joshua tree flower shape from natural populations. This would leave only non-heritable variation due to causes like developmental errors and environmental effects, which are random with respect to the local plant or pollinator population.
This is the pattern predicted by the mathematical model of coevolution I’ve just published with Scott Nuismer: when coevolution favors closer matching, it should act to reduce variation within the interacting species. The connection was striking enough that we decided to discuss the theory result in a press release about the new Joshua tree study. Coevolutionary constraint might seem to reduce the chances for speciation in interactions like those between Joshua tree and its pollinators. However, constraint might also act to reinforce isolation created by other means; we already have good reason to think that it helps prevent the yucca moths from cross-pollinating the two forms of Joshua tree.
Godsoe, W., Yoder, J.B., Smith, C.I, Drummond, C., & Pellmyr, O. (2010). Absence of population-level phenotype matching in an obligate pollination mutualism Journal of Evolutionary Biology, 23 (12), 2739-46 DOI: 10.1111/j.1420-9101.2010.02120.x
Yoder, J.B., & Nuismer, S. (2010). When does coevolution promote diversification? The American Naturalist, 176 (6), 802-817 DOI: 10.1086/657048
It’s the hot new pigment this season. A just-discovered form of chlorophyll allows the algae that produce it to photosynthesize using infrared light. (Wired Science)
One, two, three … many? Studies of monkeys, babies, and chickens suggest that the ability to count small numbers is innate, and separate from the ability to count larger numbers. (The Thoughtful Animal)
Can you hear me now? On the Galapagos islands, marine iguanas listen for the alarm calls of mockingbirds to know if a predator is approaching. (The Thoughtful Animal)
Crocodile tears from the Adaptationist Programme. Crying confers fitness advantages by eliciting empathetic responses. Or something like that. (NPR)
I start another semester as Teaching Assistant for Mammalogy next week, so here’s David Attenborough discussing mammalian dentition, with reference to an ancient omnivore I’d never heard about up to now.
So another field season came to a close Friday, and after a 1,000-mile drive from Nevada, I’m in Salem for the week, tying up loose ends and getting ready for the Evo-WIBO 2010 meeting next weekend. I’ll probably write up something in greater detail—and announce D&T’s first special series of posts—in the next few days, but for now, here’s my final set of field season photos.