Editorial NexoScience
A desert plant can wear three different faces and still turn out to be a single crowd. That, roughly, is what came out when researchers finally read the genome of Agave aurea, a spiky rosette endemic to the Baja California Peninsula and the only representative of its group there. For decades the plant had been sorted into three named forms. The genetic data hardly tells them apart.
The three names were built on flowers and body size, never on DNA. Early botanical work described Agave aurea, Agave capensis and Agave promontorii as separate species, mostly on the basis of flower traits. A later revision demoted them to varieties or subspecies of one species, because their leaves and rosettes looked nearly identical and the real differences came down to size and to whether a plant sprouts a crowd of clones around its base or stands alone. Nobody had ever checked whether the genes agreed.
Reading the genome turned the three groups into a blur. A team led by Anna Klimova collected 98 plants across the entire range of the complex, from the western slopes of the Sierra La Giganta down to the southern tip of the peninsula, and compared them at more than 10,000 points in the genome, reporting the results in the journal Ecology and Evolution. Every method they tried pointed the same way. The differences between the named varieties are so shallow that the authors conclude Agave aurea is better described as a set of closely related populations than as separate species, subspecies or varieties.
The groups the genome does recognize are drawn by geography, not by taxonomy. The clearest signal in the data sets the plants of the northern Sierra La Giganta apart from those of the southern mountains and the cape, and even that divide is faint. The imbalance shows up in what geneticists call private alleles, genetic variants that appear in one group and in no other. The common, widespread form carried close to 2,800 of them. The tall mountain form restricted to the northern Sierra La Laguna carried none at all.
Bats are the most likely reason the differences never hardened. Agaves cross-pollinate, take years to flower, and are visited by birds, bees and flies during the day and by nectar-feeding bats at night. One of those bats, Leptonycteris yerbabuenae, ranges across the whole territory of this agave and behaves as a single interbreeding population on the peninsula, which makes it something like a nightly courier service for pollen, carrying it far enough to keep distant stands stirred together. Other agaves and yuccas of the region show the same shallow structure, the authors note, exactly what is expected of a plant whose range is continuous and whose habitat has not been badly broken up.
The thinning of diversity at both ends tells a story of expansion. Genetic variety drops and inbreeding rises at the northern and the southern edges of the range, the pattern a species leaves when it spreads outward in both directions from a single refuge. The models point to the northern part of the Cape Region as that refuge, the one area that stayed habitable through the last ice age and the millennia that followed. The authors also argue that the divide between north and south is too weak to be a scar left by the ancient sea that once flooded the Isthmus of La Paz, and that recent glacial cycles, plus the parched lowland plains standing between the Sierra La Giganta and everything to the south, explain it better.
The climate projections single out the population that looked most distinct. Rainfall seasonality is what best explains where the plant grows today, and the outlook for the second half of the century does not favor it. Across models and emissions scenarios, the harshest run erased roughly 42% of the currently suitable habitat while the most optimistic one left a slight gain, with an average expectation of more than a fifth lost by 2070. Among the areas set to become unsuitable is the Sierra La Giganta, home to the very plants whose genetic identity stood out most, and already under pressure from free-roaming livestock and fast development.
The authors are careful about how far the result reaches. The two rarest forms were thinly sampled, only a handful of plants each and from a single site apiece, so their geographic boundaries still need work, and hybridization between forms appears to be common in the south, which blurs the picture further. Genetic similarity is a pattern, not a formal taxonomic verdict, and the maps of the future are projections rather than observations. Even so, the team proposes treating the three genetic groups it did find as separate management units, and prioritizing the northernmost and southernmost populations, the ones carrying the least genetic variety and the most inbreeding. What the complex needs next, the researchers write, is denser sampling of the rare forms and a closer look at where one group ends and the next begins.
The study, ‘Population genomics and distribution modeling revealed the history and suggested a possible future of the endemic Agave aurea (Asparagaceae) complex in the Baja California Peninsula’, by Anna Klimova, Jesus Gutierrez-Rivera, Alfredo Ortega-Rubio and Luis E. Eguiarte, was published in 2024 in the journal Ecology and Evolution and is available in open access. DOI: 10.1002/ece3.70027.
