Getting serious mileage out of the new camera!
Posting this from the Las Vegas airport wifi, so my photos will have to speak for themselves. My inbox is packed!
Joshua trees are about to bloom. Which means I’m off to the desert until mid-April first to tour Joshua Tree National Park with my parents for a week, then to spend a month or more at a field site in central Nevada, extending studies of co-divergence in Joshua tree and its pollinator moths.
All of which is to say, posting to D&T is about to drop to near-zero for the foreseeable future. I’ll take lots of photos, and put them online when I get to an Internet connection, but really that’s all I can promise. After all, what good is fieldwork if not as an Internet detox?
Photo by jby.
In a paper just released online at Molecuar Ecology ahead of publication, genetic tests on moth larvae provide the latest piece to the puzzle of why there are two kinds of Joshua tree — because the tree’s pollinators need to match its flowers [PDF].
I’ve written extensively about the interaction between Joshua tree and its pollinators. Like all yuccas, Joshua tree is pollinated only by yucca moths. Female yucca moths collect pollen in special mouthparts and deliberately apply it to a yucca flower after laying eggs inside it. When the eggs hatch, the moth larvae eat some of the seeds inside the developing fruit. Yuccas prevent their pollinators from laying too many eggs by selectively killing flowers too badly damaged by egg-laying [$-a].
TOP: The two forms of Joshua tree (western type on left, eastern on right). BOTTOM: Scaled comparison of moth body sizes and 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. Photo by jby, Illustration from Smith et al.(2010), figure 1.
This last element of the interaction may have had significant consequences for Joshua trees’ evolutionary history. Joshua trees are pollinated by two different species of moths, which occur in different parts of the tree’s range: the larger Tegeticula synthetica in the west, and the smaller T. antithetica in the east. Joshua trees pollinated by the two different moth species are themselves different, both in their overall shape, and in the shape of their flowers’ pistils — specifically, the length of the route that a moth must drill to lay her eggs [PDF].
How does this difference in flower shape affect Joshua tree pollination? If a larger moth attempts to lay eggs in a smaller flower, it may be do more damage to the flower than the “native” pollinator would, triggering the tree to kill the flower. On the other hand, smaller T. antithetica might be able to lay eggs in a larger western-type flower without this risk. If this is the case, moths probably can’t pollinate western trees with eastern pollen, but they might be able to do the reverse.
Such one-way pollen transfer between the two Joshua tree types could produce a population genetic pattern called “chloroplast capture.” Joshua tree pollen doesn’t contain the full genetic code of the tree that produces it — it lacks the genes contained in the chloroplast, the cellular structure that conducts photosynthesis, because pollen grains typically don’t have chloroplasts. The DNA in the cellular nuclei of newly-formed seeds is a mixture of nuclear DNA (nucDNA) from a pollen grain and from one of their “maternal” parent’s ovules, but they get all their chloroplasts, and chloroplast DNA (cpDNA), from the ovule. If moths carry pollen from eastern trees to western trees, then the seeds produced would contain western cpDNA, but also some eastern nucDNA.
Asymmetric pollen transfer can lead to eastern-type trees with western-type chloroplasts. Figure 2 from Smith et al.(2010).
This is what we’ve found in Joshua tree populations near the region where the two tree types and their pollinators come into contact. At these sites, trees look like the eastern type (meaning they likely have eastern nucDNA, though we haven’t tested that yet) but have cpDNA that matches nearby populations of western-type trees [PDF].
The genetic pattern is only suggestive of one-way pollen transfer between the two Joshua tree types, though. We haven’t yet tracked the movement of moths directly, or estimated whether they actually are less successful when laying eggs on the wrong tree type. The newly-published study provides exactly these data. My colleague Chris Smith placed glue traps on Joshua tree flowers at the contact zone to estimate how often adult moths of each pollinator species visited each type of tree in the mixed population. Adult moths were more likely to be trapped on their “native” trees, though they did show up on the other type sometimes.
A yucca moth larva emerges from a Joshua tree fruit in the lab. Photo by jby.
Chris and I then collected fresh fruit from trees in the contact zone, and caught yucca moth larvae as they chewed their way out. Chris and another coauthor, Chris Drummond, then identified the species of each larva based on their genetics (the two pollinators look very similar at that stage) — and in our sample, the pattern of specificity was even stronger than that in the adults. The larger moth species, T. synthetica, never emerged from fruits of the small-flowered eastern trees. The vast majority of larvae of the smaller T. antithetica were also found inside their “native” tree’s fruit — but a handful did emerge from large-flowered western trees.
This mechanism could create the genetic pattern we see in Joshua tree populations. Larger T. synthetica doesn’t seem to lay eggs in (or pollinate) small-flowered eastern trees, but smaller T. antithetica can occasionally lay eggs in (and pollinate) large-flowered western trees. This should create asymmetric gene flow, with pollen moving from eastern trees to western trees, but not the reverse. The two Joshua tree types may not yet be reproductively isolated, separate species — but we won’t know for sure without looking at the plants’ nuclear DNA. As it happens, I’m working on that right now.
Godsoe, W.K.W., Yoder, J.B., Smith, C.I., & Pellmyr, O. (2008). Coevolution and divergence in the Joshua tree/yucca moth mutualism The American Naturalist, 171 (6), 816-823 DOI: 10.1086/587757
Marr, D., & Pellmyr, O. (2003). Effect of pollinator-inflicted ovule damage on floral abscission in the yucca-yucca moth mutualism: the role of mechanical and chemical factors Oecologia, 136 (2), 236-243 DOI: 10.1007/s00442-003-1279-3
Smith, C.I., Godsoe, W.K.W., Tank, S., Yoder, J.B., & Pellmyr, O. (2008). Distinguishing coevolution from covicariance in an obligate pollination mutualism: Asynchronous divergence in Joshua tree and its pollinators. Evolution, 62 (10), 2676-87 DOI: 10.1111/j.1558-5646.2008.00500.x
Smith, C.I., Drummond, C., Godsoe, W.K.W., Yoder, J.B., & Pellmyr, O. (2010). 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 DOI: 10.1111/j.1365-294X.2009.04428.x
Over at LiveScience, my collaborator Chris Smith describes the research we’ve done so far on the interaction between Joshua trees and their pollinators:
First, the match between the Joshua tree flowers and the moths’ ovipositors suggested that coevolution might have molded the relationship between the plant and the pollinator. Second, because the plants are completely dependent on the moths for reproduction, the differences in the flowers might have caused Joshua trees to split into two different species.
Yucca brevifolia in Tikaboo Valley, Nevada. Photo by jby.
A new Joshua tree study is just out in the current issue of New Phytologist, presenting an analysis of the environments occupied by the two different types of Joshua tree. The results demonstrate that the two tree types mostly grow in similar climatic conditions [PDF], which suggests that coevolution with its pollinators, not natural selection from differing environments, is responsible for the evolution of the two different tree types.
The Pellmyr Lab has been studying the two types of Joshua tree, which are pollinated by two separate, highly specialized moths, for several years now. Previous papers have shown that the two types of Joshua tree, first described in the 1970s based only on their vegetative features, are most strongly differentiated by the shape of their flowers [$-a]; and that, although the two moths are separate species, the two tree types are not fully genetically differentiated [PDF].
Hey, those Joshua trees look kinda
Photo by jby.
The latest paper is a chapter from the dissertation of Will Godsoe, who just received his doctorate last week. It presents an analysis that sidesteps a fundamental problem with studying long-lived, specialized organisms — they’re hard use in fully controlled experiments. To determine whether the two types of Joshua tree really evolved as a result of coevolution with their pollinators, we’d like to be able to eliminate the alternative hypothesis that the two types evolved in response to different environmental conditions. Except for a small contact zone in central Nevada, each tree type occurs in a different part of the Mojave desert, and the two regions do have some broad-scale differences in when they receive precipitation.
Ideally, to determine whether two plants have different environmental needs, you just perform an experimental transplant, growing each plant in the other’s environment to see whether it fares as well as it does at home. This isn’t really possible with Joshua trees, which are pretty tricky to sprout from seeds (I’ve tried), and which, in any event, take something like twenty years to mature. So Will proposed to use niche modeling methods instead. Niche models are statistical descriptions of environments where an organism is known to live, often used to predict where it could live. To build niche models for each type of Joshua tree, Will assembled location data we’d collected over several field seasons in the Mojave, then spent another field trip driving around the desert some more to fill in the gaps — he wanted locations where Joshua trees were definitely growing and where they definitely weren’t, to fully “inform” the models.
Using the location data, it was possible to determine what kinds of climates each Joshua tree type tended to occupy by cross-referencing with existing climate databases, then fitting statistical models to the results. The models produced for each tree type could then be compared — and, for the most part, they’re similar. That is, if you collected seeds from one tree type, planted them where the other type grows, and waited around for a few decades to check the result, you’d probably find that it grew as well as it did in its home range.
So, if differing climates don’t explain the origin of the two types of Joshua tree, does that leave no other possibility but the pollinating moths? Not exactly — there are lots of environmental variables that weren’t available for Will’s niche models, for instance, or there could be a third, completely unknown factor. But this does make coevolution with the moths a more plausible explanation. In light of some of our very latest results — which should be going to press fairly soon — coevolution is looking like a better and better possibility.
Godsoe, W., Strand, E., Smith, C.I., Yoder, J.B., Esque, T., & Pellmyr, O. (2009). Divergence in an obligate mutualism is not explained by divergent climatic factors New Phytologist, 183 (3), 589-99 DOI: 10.1111/j.1469-8137.2009.02942.x
Godsoe, W., Yoder, J.B., Smith, C.I., & Pellmyr, O. (2008). Coevolution and divergence in the Joshua tree/yucca moth mutualism The American Naturalist, 171 (6), 816-23 DOI: 10.1086/587757
Smith, C.I., Godsoe, W., Tank, S., Yoder, J.B., & Pellmyr, O. (2008). Distinguishing coevolution from covicariance in an obligate pollination mutualism: Asynchronous divergence in Joshua tree and its pollinators. Evolution, 62 (10), 2676-87 DOI: 10.1111/j.1558-5646.2008.00500.x
Some years, they don’t bloom. I’m just back from a week and a half of attempted fieldwork in Nevada, with a hiatus to Southern California for a lecture to a Desert Institute class. Very few Joshua trees were in flower, so the trip was kind of a bust. But it was still good to get out into the desert. The weather was only really cold a couple nights, and almost too warm in Palm Springs. When I drove back into Moscow this afternoon, it was snowing.
My Spring Break this year is a two-week hiatus for fieldwork in central Nevada and Southern California. Photos when I get back.
Photo by jby.
The latest results from the Pellmyr Lab’s ongoing study of Joshua tree and its pollinators are online as part of the new October issue of Evolution. It’s the cover article, no less. The study, whose lead author is Chris Smith (now on the faculty at Willamette University) compares patterns in the population genetics of Joshua trees and the moths that pollinate them, and shows that although the moths have become two separate species, the trees may not have followed suit [PDF].
Photo by Chris Smith.
Female yucca moths carry pollen between Joshua tree flowers in special mouthparts. When she arrives at a new flower, the female moth lays her eggs inside it, then deliberately applies pollen to the flower’s receptive surface. When the fertilized flower develops into a fruit, the moth eggs hatch, and the larvae eat some of the seeds inside the fruit.
Among the yuccas, Joshua trees are unique because they’re pollinated by two species of moths, which are each other’s closest evolutionary relative. One species is found in the eastern part of Joshua tree’s range, the other in the west. Joshua trees from the east and west have differently-shaped flowers [PDF], which is consistent with the hypothesis that coevolution between moths and trees has driven both toward an evolutionary split.
The new study goes deeper to look at genetic relationships between different populations of the moths and the trees, and what it finds isn’t as tidy as the earlier work might suggest: While Joshua trees’ morphology corresponds nicely to the split in the pollinators, the patterns visible in their chloroplast DNA does not. In some populations, trees look “eastern,” but have chloroplast DNA more closely related to “western” populations. This suggests that, although the moths have become separate species, they’re still moving between the two kinds of Joshua tree frequently enough that the trees haven’t quite split. Why do the two tree types look different, then? One possibility is coevolution with the two moth species, which might exert selection the trees in different ways.
There’s still a lot of work to do before we fully understand what’s going on here. Will Godsoe, the other doctoral student in our lab, is doing some intensive niche modeling to see how much environmental differences might be contributing to the patterns we see here. My own dissertation will look at whether the same incongruities turn up in nuclear DNA, which can have a different evolutionary history than that in the chloroplast.
W. Godsoe, J.B. Yoder, C.I. Smith, O. Pellmyr (2008). Coevolution and Divergence in the Joshua Tree/Yucca Moth Mutualism The American Naturalist, 171 (6), 816-23 DOI: 10.1086/587757
C.I. Smith, W.K.W. Godsoe, S. Tank, J.B. Yoder, O. Pellmyr (2008). Distinguishing coevolution from covicariance in an obligate pollination mutualism: asynchronous divergence in Joshua tree and its pollinators. Evolution, 62 (10), 2676-87 DOI: 10.1111/j.1558-5646.2008.00500.x