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Scientists Want to Unlock the Joshua Tree's Secrets

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In Joshua Tree National Park | Photo: Christopher Michel, some rights reserved

It's a charismatic, distinctive symbol of the California deserts -- and it's in trouble. The Joshua tree, the iconic denizen of the Mojave Desert, faces serious threats from climate change, drought, and human development.

And a group of scientists wants to see if the tree's genome holds potential answers to those threats. The Joshua Tree Genome Project aims to complete the first genome of an individual Joshua tree. That first genome will be used as a baseline with which to compare other Joshua trees, so that scientists can get a sense of the true genetic diversity of the picturesque succulent tree.

Among the questions the project's biologists hope the trees' genetic makeup will help to answer are what drives the plants' reliance on yucca moths for pollination, and whether the wild population of trees might hold traits that allow survival in a warmer, drier, world.

Joshua trees, known to botanists as Yucca brevifolia, are found only in the Mojave Desert and its fringes in California, Nevada, Arizona, and the southwest corner of Utah. They are generally thought to be a relict of cooler, wetter times in the desert. They grow only above 2,000 feet and usually higher, and without a significant stretch of cool, moist winters and springs, the trees generally don't make it through the first few years after germination.

That's a problem in a desert that's getting warmer and drier, and some scientists have forecasted a bleak future for the Joshua tree. In September the group Wild Earth Guardians petitioned the U.S. Fish and Wildlife Service to protect the trees as Threatened under the U.S. Endangered Species Act. USFWS was legally obligated to reply to that petition by January, but has not yet indicated whether they'll consider protecting the trees.

One of the things complicating the tree's outlook is that Joshua trees, like other yuccas, depend on small moths called yucca moths to reproduce. Without the moths' help in pollinating the Joshua tree's flowers, no new generations of seedlings will be produced to try and survive increasingly dry and warm Mojave winters. And yet not much is known about the two species of moth that pollinate Joshua trees, or the signals that cause the moths to emerge in spring just as Joshua tree flowers are ready to be pollinated.

The Genome Project is seeking funding through the science-oriented crowdfunding site Experiment, and at this writing, it's close to its relatively modest initial goal. The first round of funding, if the crowdfunding goal is met, will be used to make a rough overview map of the genome of an individual tree now growing in south-central Nevada.

That overview will allow researchers to get a basic sense of the structure of the Joshua tree genome, and will help guide the subsequent, more intensive step of sequencing the plant's entire genome. After that, then scientists can sample trees from different parts of the species' range to measure the depth of the genetic diversity within the species.

Eastern Joshua trees near the mouth of the Grand Canyon | Photo: Chris Clarke

What, in particular, will the researchers be looking for in that genome? As is true of so much in science, they won't know until they look. "We know, for instance, that certain sections of plants' genomes are associated with flowering," says Willamette University associate professor Chris Smith, the project's lead scientist. "So we'll be looking at that section of the Joshua tree's genome to see if there's anything that explains the details of the plant's relationship with the yucca moth."

Smith is working with a group of scientists that reads pretty much like a Who's Who in knowing things about yuccas, including USGS biologist Todd Esque, University of Georgia plant biologist Jim Leebens-Mack, and University of British Columbia evolutionary biologist Jeremy Yoder, who I interviewed some years back.

Once the project has genomes for a number of trees from throughout the species' range, comparing them may offer up detailed information about how the ancestors of today's trees dispersed throughout the desert, which may in turn give clues about how we can maximize the Joshua tree's chances of surviving over the next 500 years or so.

The project may also help determine one of the big questions now being discussed among Joshua tree aficionados: do the contorted desert trees belong to one species, or two? Joshua trees in the western half of the range, near Los Angeles, tend toward considerably larger size and leaf length than trees in the vicinity of Las Vegas, which branch more abundantly than their western cousins. Each group is pollinated by its own species of yucca moth. In 2007 Lee Lenz of the Rancho Santa Ana Botanic Garden suggested that the eastern form be carved out into its own species, Yucca jaegeriana. Though it's possible that studies of the two Joshua trees' flowers may settle the "one species or two" argument before that, examining the genomes of trees from each population will still offer hints as to how the two groups diverged.

There's one aspect of the study that may have immediate conservation implications. Like most other aspects of plant behavior, Joshua trees' drought hardiness is determined in large part by individual trees' genetic makeup. If the study identifies groups of trees that are especially genetically suited to survive in a warming world, those groups could be singled out for protection. "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," said Smith.

Joshua tree with encroaching wind turbines | Photo: Chris Clarke

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