Notes from the field: What's Chris doing to that Joshua tree?

Joshua tree, diverged. Photo by jby.

Cross-posted from Nothing in Biology Makes Sense!

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.

Joshua trees, diverged. Photo by jby.

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.

Camp Tikaboo, 2012 edition. Photo by jby.

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."

In the "war tent," making plans. Photo by jby.

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.◼



References

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