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What is the deal with Joshua trees and yucca moths?

Chris Smith examines a cluster of Joshua tree flowers. (Photo: Jeremy Yoder)

Chris Smith examines a cluster of Joshua tree flowers. (Photo: Jeremy Yoder)

One of the strangest things of all about Joshua trees may be the way that they are pollinated.

Many plants attract pollinators with rewards, like sugary nectar, or excess pollen that animal pollinators can eat. A wide variety of insects, birds, and even mammals visit flowers in pursuit of such rewards, incidentally carrying pollen from flower to flower in the process.

Joshua trees, like all yuccas, rely on a different strategy for pollination. Joshua trees produce no nectar and comparatively little pollen. And, instead of attracting a variety of different pollinators, yuccas rely exclusively on a few species of drab moths to assist them with reproduction.

A female yucca moth in the process of laying eggs in a Joshua tree flower. (Photo: Chris Smith)

A female yucca moth in the process of laying eggs in a Joshua tree flower. (Photo: Chris Smith)

These insects, known as yucca moths, are grey, white, or sometimes black, are between one-quarter inch to an inch in size, and at first glance seem entirely unremarkable. However, they have one feature that no other species of moths possess. Surrounding the female yucca moth’s moth is a pair of ‘tentacles’ – long, flexible, coiled appendages. The moths use these tentacle to collect balls of pollen from yucca flowers. The moths then fly to another flower, where they use their tentacles to deposit the pollen onto the floral stigma – the receptive surface where the pollen needs to land in order to fertilize the flower.

Different species of yucca moth use different strategies to get the pollen into the right place. Some moths use a bobbing, pecking behavior, like a child’s drinking bird toy, pushing their tentacles into the stigma to pack the pollen into place. Other species unfurl their tentacle while holding a small batch of pollen and then use their legs and feet to stuff the pollen into the stigma. You can see this in the video below, which shows a female yucca moth inside a cut-open flower, pushing pollen toward the stigma.

Watching the moths’ behavior, it’s hard not to come to the conclusion that the moths are pollinating the plants intentionally. Although it is reasonable to wonder whether moths – not known for their smarts – do anything intentionally, it sure looks like they are trying to pollinate the flowers.

Why would a moth go to so much effort just to pollinate a yucca?

The answer is that the moths are getting something out of the deal. Shortly before they pollinate a flower, the female moths use a needle-like organ called an ‘ovipositor’ to inject their eggs into the developing flower. As the flower matures, it will develop into a fruit and produce seeds. Inside the fruit, the moth’s eggs will hatch into caterpillars that will eat some (but usually not all) of the developing seeds. So, by pollinating the flower, the moth ensures that there will be a nutritious food source for her offspring. And because yucca moths are very reliable pollinators, the trees don’t need to offer ‘bribes’ in the form of nectar, and can get away with producing very little pollen since almost none of it will be wasted.

Things aren’t as simple as they seem.

Although on the surface the relationship between the Joshua tree and yucca moths seems to be very harmonious, a closer look suggests that that it is an uneasy alliance. On multiple occasions yucca moths have developed strategies to ‘cheat’ the system – moths that wait until the yucca has been pollinated by a different species of moth, and then come lay their eggs afterwards without having to do the hard work of pollination themselves. Likewise, there is some evidence that the plants will abort flowers that have too many yucca moth caterpillars (this kills both the caterpillars and the developing seeds, but spares the plant the cost of developing a fruit that will produce few or no viable seeds). Finally, there is some evidence that some species of yucca have evolved changes in the shape of the flower to prevent the moths from laying eggs on the developing seeds.

So what about Joshua trees in particular?

The story of yucca moth pollination in Joshua trees has gotten even more interesting recently. A careful study of the moths that pollinate the Joshua tree revealed that the trees are actually pollinated by two similar, but distinct species. One of the two moths is bigger, and is lighter grey in color. The bigger moth also has a longer ovipositor.

This discovery prompted a closer study of Joshua trees, which showed that trees associated with each of the two different species of moth are actually slightly different from one another. Joshua trees growing in the western Mojave desert, which are pollinated by the larger of the two moth species, tend to be taller and more ‘tree-like’ with a longer trunk. They also have longer leaves. On the other hand, Joshua trees occurring in the eastern Mojave, which are pollinated by the smaller moth, tend be shorter and ‘bush-like’ with lots of branches and shorter leaves. Based on these differences, some botanists have argued that there may actually be two species of Joshua tree: Yucca brevifolia, occurring in the western Mojave and Yucca jaegeriana, occurring in the eastern Mojave. (Preliminary work on the Joshua tree genome suggests that the two tree types are indeed genetically different from one another, but still similar enough that genome sequence data from one species will provide a good starting place for studying the other.)

Examples of Yucca brevifolia and Yucca jaegeriana growing side by side. (Photo: Jeremy Yoder)

Examples of Yucca brevifolia and Yucca jaegeriana growing side by side. (Photo: Jeremy Yoder)

Most interestingly of all, the biggest difference between the trees pollinated by each species of moth is in their flowers, and the biggest difference in flowers is the part of the flower where the moths lay their eggs – the style. Trees pollinated by the larger moth have a longer style, and trees pollinated by the smaller moth have a shorter style. What’s more, if moths accidentally visit the wrong type of tree (which they do sometimes in places where the two trees grow together), the moths are less successful in laying eggs.

All of this leads us to think that the two types of Joshua tree have adapted to the different species of moths. Evolutionary changes in the flowers may have occurred as a way to reduce the number of seeds that get eaten by the moths’ caterpillars. The moths, in turn, may have evolved differences in body size as a way to compensate for the changes in the flowers. This process – changes in one of the organisms causing changes in the other, and vice-versa– is known as ‘coevolution’. Understanding how the genetics of the Joshua tree might have enabled coevolution is one of the questions we hope to answer with a genome sequence.


Keep the momentum going!

Thanks to all of the folks who have pledged their support for our crowdfunding campaign! We’ve had a very exciting first week. We’re more than 1/3 of the way to our fundraising goal, and are pulling ahead in the competition for the most donors, with 59 backers this week.

One of the challenges for crowdfunding, however, is to keep the momentum going after the initial excitement wears off. That nest egg has to be nurtured if it is ever going to take flight. So, we need your help in spreading the word about the project. Please help us reach more people by inviting your friends to like our Facebook page and tweet about the project using @JTGenome.

Also, check out our Experiment.com project page for news about the campaign. We’ll be posting updates over the next week. You can also subscribe on our website to receive email announcements each time our blog here is updated, using the form under “Subscribe by E-mail” in the sidebar.

(Chris Smith)

(Chris Smith)


We’re sequencing the genome of Joshua tree, but we need your help

(Photo: Chris Smith)

(Photo: Chris Smith)

The Joshua Tree Genome Project officially launches today, with a crowdfunding campaign to sequence the genome of one of the most iconic plants in the American southwest. People who love science, Joshua trees, and the Mojave Desert can help finance the development of a Joshua tree genome sequence through Experiment.com.

Why sequence the Joshua tree genome? A reference genome would help answer many important questions about the evolutionary history of this iconic desert species, and about how best to ensure that it survives in a world reshaped by human activity. A sequenced genome will let us:

Discover genes adapted to desert environments. The Mojave Desert contains some of the hottest and driest regions of North America. To survive these inhospitable environments, Joshua trees have an array of physiological and morphological adaptations, from a thick, waxy cuticle on their the leaves, to reduced stomate size and specialized water storage cells. Sequencing the Joshua tree genome will help us find the genes that create these traits, and identify variation in those genes that may allow some Joshua trees to better warmer, drier climates.

Understand the evolution of mutualism. Like all yuccas, Joshua trees rely on highly specialized moths, called yucca moths, to move their pollen from plant to plant. Female moths actively collect and distribute pollen after laying their eggs in Joshua tree flowers; and their larvae eat some of the seeds that develop in the pollinated flower. The moths’ exceptionally reliable pollination service compensates for the loss of a few seeds, and Joshua tree flowers exhibit a suit of adaptations that promote active moth pollination while preventing moth larvae from eating too many seeds. Sequencing of a Joshua tree genome would pave the way to identify genes that contribute to these co-evolved adaptations, and help understand how they have changed over time.

Plan for Joshua tree’s future. Ensuring that Joshua trees will persist into the future means preserving not only the plants themselves, but also the genetic variation that will allow them to adapt to changing climates and environments. We will use a landscape genomics approach to measure the total amount of genetic variation in different populations, and estimate genetic differentiation between populations. This information will let us identify populations of Joshua tree with the greatest potential to adapt to future environmental changes, and give these areas the highest priority for conservation.

Reveal processes of genome evolution. Like all members of the Agavoideae, Joshua trees have a bi-modal karyotype, thought to have resulted from an ancient allopolyploidy event — the combination of two whole genomes by hybridization between species. Sequencing the Joshua tree genome will reveal how genome evolution proceeds following polyploidization events, including the extent of genomic rearrangements among chromosomes of different ancestries, and processes that contribute to diploidization.

The Joshua Tree Genome Project is a collaboration of ecologists, evolutionary biologists, and geneticists, with the support of major Mojave Desert conservation organizations. You can help sequence a Joshua tree genome by donating to the Project through our Experiment.com campaign, and by spreading the word on Twitter and on Facebook.


Calling all fans of science, Joshua trees, and the Mojave Desert!

In just two days, the Joshua Tree Genome Project is launching a crowdfunding campaign through Experiment.com to help sequence a Joshua tree genome. If you love Joshua trees and understand how much a reference genome would help us learn about their evolutionary past and plan for their future, we hope you’ll pitch in. But as we get ready to launch, we also hope you’ll tell everyone you know about our campaign.

The Joshua Tree Genome Project is part of a special Experiment.com challenge to projects administered by smaller undergraduate institutions, like Willamette University. The challenge is not only to raise the funds required for our project, but to earn the largest number of individual contributions, out of all participating projects. If we win, Experiment.com will kick in another $2,000 — money we can use for other aspects of the genome assembly beyond the DNA sequencing covered by our baseline budget.

So help us spread the word! Click this link to tweet about the project, and encourage folks to follow @JTGenome. And follow this link to like our page on Facebook, and share it to your timeline.

Thanks in advance — every tweet, like, and click brings a Joshua tree genome sequence closer to reality. We’re grateful for the support of conservation organizations and agencies with interest in Joshua trees and the conservation of the Mojave Desert’s unique landscapes. With their help and with yours, we’re hopeful that we can meet our fundraising goal, and win the Experiment.com challenge.