Study reveals genetic insight into desert survival
A female Mojave desert tortoise. Photo by Sandy Keaton Leander/Arizona State University
The deserts of the American Southwest are home to the Mojave and Sonoran desert tortoises, two seemingly similar yet genetically distinct species of turtles. These tortoises, adapted to different desert ecosystems, hold vital clues about evolution, survival and the challenges posed by climate change.
In a new study, titled “Divergence in Regulatory Regions and Gene Duplications May Underlie Chronobiological Adaptation in Desert Tortoises,” published in Molecular Ecology, researchers from Arizona State University and the University of Alabama at Birmingham (UAB) uncovered the genetic blueprints that allow these iconic creatures to thrive in their harsh environments.
"While some animals can live in both the Mojave and Sonoran deserts, tortoises have diverged into two separate species adapted to each desert over millions of years," said Kenro Kusumi, professor in the School of Life Sciences and senior vice provost and dean of The College and Liberal Arts and Sciences at ASU. "We now have a clearer understanding of what makes them unique, based on finding DNA differences that could underlie their adaptations."
Kusumi emphasized the importance of this research in advancing public knowledge and conservation efforts: “This is an example of research with real public value. It informs our responsibility to conserve biodiversity in our state and beyond.”
A changing desert landscape
The Mojave Desert, which experiences long dry periods and rainfall limited to the winter, contrasts with the Sonoran Desert, which receives winter rainfall and summer monsoons. These environmental differences have driven the evolution of distinct behaviors and physiological traits in the two tortoise species.
"Just as Arizonans adjust their lives around the monsoon and winter rains, these tortoises have adapted their behaviors to fit their environment," said Greer Dolby, an assistant professor at UAB and senior author of the study.
The Mojave desert tortoise hibernates throughout the winter, while the Sonoran desert tortoise can emerge during the winter to take advantage of occasional rainfall and bask in the sun. This divergence, researchers found, could be tied to differences in genes that regulate biological rhythms, such as circadian and circannual clocks, allowing each species to align its activity with its environment.
One of the most significant findings involves mucin genes, which are responsible for producing a component of mucus. These genes have expanded dramatically in the Sonoran desert tortoise, giving it a potential advantage in retaining water and resisting respiratory diseases.
"The Mojave desert tortoise has fewer copies of key mucin genes, which might make them more vulnerable to upper respiratory infections," Kusumi said. "This finding provides a genetic clue as to why the Mojave desert tortoises struggle more with these infections compared to their Sonoran counterparts."
Dolby added that these adaptations are likely tied to the challenges of each desert: "Mucins play a key role in hydration and immune defense. For Sonoran desert tortoises, these genes might have evolved to protect them from pathogens and the dry, dusty conditions of their environment."
Conservation in a changing climate
Both researchers stressed the urgency of conservation in light of these findings. The Mojave desert tortoise is already listed as threatened under the U.S. Endangered Species Act, facing challenges from habitat loss, invasive plant species and predation by ravens.
"The Mojave desert tortoises live in more flat, open desert habitats that are more susceptible to human development and predators, compared with Sonoran desert tortoises, " Kusumi explained. "Their populations are declining at a concerning rate."
In contrast, the Sonoran desert tortoise resides in hilly, rugged terrain with the potential for more microclimate options during periods of climate change — still, it is not immune from risk. While the Sonoran desert tortoise is protected by the state of Arizona, it is not federally listed as threatened.
"We have shared genomic data with federal agencies, and they have currently decided not to list the Sonoran desert tortoise as threatened," Kusumi said. "But this doesn’t mean it’s free from concern — it’s just better positioned than the Mojave desert tortoise at the moment and should continue to be monitored."
Dolby added, “So far, management efforts appear to be successful enough, which is a good thing.”
The study also highlights the broader importance of biodiversity.
"Think of maintaining genetic diversity as a form of future-proofing," Kusumi said. "Populations with greater genetic variation are better equipped to survive unpredictable environmental changes. Preserving this diversity is key to improving the chances that these species endure for generations."
For many residents of the Southwest, desert tortoises are emblematic of their unique environment.
"If you drive from Phoenix to Las Vegas, it’s clear when you transition from the Sonoran to the Mojave Desert. The plants are different, and so is the wildlife," Kusumi said. "The tortoises look similar but have evolved separately to adapt to these distinct deserts."
Kusumi concluded with a poignant thought: "Can you imagine an Arizona without its tortoises, saguaros or other iconic species? Losing them would mean losing part of what makes this region so special. Our research is as much about preserving these hallmarks of the desert as it is about understanding them scientifically."
By uncovering the genetic secrets of the Mojave and Sonoran desert tortoises, researchers hope to protect these species from the mounting threats of climate change and habitat loss, ensuring that future generations can continue to marvel at these remarkable survivors.
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