Secrets of the Sierra Gorda: The Yucca queretaroensis problem and the evolution of Darwin’s ‘Most Wonderful Fertilisation System’ :British Cactus and Succulent Society

Funded by a BCSS Research Grant to Professor Christopher Smith, Willamette University, Salem, Oregon, USA.

The genus Yucca contains approximately 50 species of succulents native to Mexico and the United States, and are cultivated as ornamentals throughout the world. The plants are pollinated exclusively by ‘yucca moths’. The moths possess tentacle-like mouthparts that are unique in the animal world. They use their tentacles to collect pollen and deposit it onto the receptive surface of the flower in a seemingly intentional manner. Darwin famously described the relationship between yuccas and yucca moths as, “The most wonderful case of fertilisation ever published.” The evolutionary origin of these plants has bedeviled scientists for decades because of a problematic species – Yucca queretaroensis. The place where this species fits in the evolutionary tree of Yucca and their relatives has been difficult to determine, with different studies turning up wildly different results. Worse, the pollinator of the plant has never been identified, and it is unclear whether the species is even really a yucca. The problem has remained unresolved partly because the plants grow exclusively on the walls of impossibly steep canyons, buried deep within the Sierra Gorda mountains of central Mexico. In addition, the species is rare, at risk of extinction due to harvesting for construction materials in traditional architecture and poaching for sale on the international black market.

With funding from the British Cactus and Succulent Society, I set out to resolve the ‘Queretaroensis Problem’ once and for all by producing new tissue collections of the plant and DNA sequences from hundreds of genes. In the spring of this year (2022) I traveled to the Mexican states of Guanajuato, Hidalgo, and Queretaro, along with four Mexican scientists from the National Autonomous University. Together we collected leaf tissue and fruits from 34 plants, sampled from 6 populations, including two previously undocumented localities. In addition, I obtained samples of putative yucca moth larvae collected by Professor Emma Fabiola Magallan Hernandez from Yucca queretaroensis fruits. At the Institute of Ecology at the National Autonomous University in Mexico City (UNAM) I extracted DNA from the larvae and from the plants. With support from scientists at UNAM I sequenced DNA from the larvae, which –indeed– proved to be yucca moths, the species Tegeticula tambasi. This species also pollinates the co-distributed species Yucca filifera, which may explain anecdotal reports of hybridization between these two species of yucca. DNA sequencing of the plants is planned for the fall of 2022 and the spring of 2023.

Figure 1: Christopher Smith collecting data from Yucca queretaroensis plants in Guanajuato, Mexico.

Figure 2: Ruins of the Convento de Bucareli in the Mexican state of Queretaro. Steep canyons of the Sierra Gorda are visible in the background.

Progress update, Spring 2024:

After a long wait to get export permits and clearance from customs, the Yucca queretaroensis samples finally arrived at the Smith laboratory in May 2023. I owe a great debt of gratitude to Erika Aguirre Plantar at the Autonomous University In Mexico City (UNAM) for facilitating the permitting and export process for this CITES-listed species.

In addition, with help from Dr. Abisaí Josué García Mendoza at the Institute of Biology at UNAM, Dra. Maria Clara-Arteaga at the Center for Research and Graduate Studies (CICESE) in Ensenada, México, Dr. David Althoff at Syracuse University, Greg Starr Nursery in Tucson, Arizona, and Cistus Gardens in Portland, Oregon, I obtained tissue from 25 species of Yucca, two species of Agave, and one species of Dasylirion.

Throughout summer 2023 my laboratory manager, Ramona Flatz, and my summer research intern, Emelia Sherman, worked to extract DNA from 119 different individual plants. Using sequence probes they physically isolated DNA representing 353 genes from each of these individual plants. The genes were then sequenced at the University of Oregon Genomics core.

At the end of that long process, I am now deep in the arduous process of analyzing the ~1.01 terabytes of resulting data. My initial data analyses and construction of phylogenetic trees suggest that we will get a very clear answer, though I am still struggling with issues of paralogy (That is, am I making an ‘apples to apples’ comparison when I look at different species of Yucca?). If you want to know the final answer, you’ll have to wait for the next chapter — publication.