Human-mediated dispersal routes structure Sahara mustard genomics

Authors: WINKLER, DANIEL - University Of California Irvine; CHAPIN, KENNETH - University Of California (UCLA); AMER, WAFAA - Cairo University; AUGE, MATTHEW - Bbca-Onlus, Italy; BON, MARIE-CLAUDE - European Biological Control Laboratory (EBCL); CRISTOFARO, MASSIMO - Enea Casaccia Research Center; FRANCOIS, OLIVIER - Universite Grenoble Alpes; GARMON, DAVID - Tubb Canyon Desert Conservancy; GAUT, BRANDON - University Of California Irvine; HEAP, JOHN - Department Of Agriculture - Australia; KASHEFI, JAVID - European Biological Control Laboratory (EBCL); MARINI, FRANCESCA - Bbca-Onlus, Italy; SFORZA, RENE - European Biological Control Laboratory (EBCL); Smith, Lincoln; HUXMAN, TRAVIS - University Of California Irvine

Submitted to: Molecular Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2025
Publication Date: 11/18/2025
Citation: Winkler, D.E., Chapin, K.J., Amer, W.M., Auge, M., Bon, M., Cristofaro, M., Francois, O., Garmon, D., Gaut, B.S., Heap, J.W., Kashefi, J., Marini, F., Sforza, R., Smith, L., Huxman, T.E. 2025. Human-mediated dispersal routes structure Sahara mustard genomics. Molecular Ecology. 0:e70176. https://doi.org/10.1111/mec.70176.
DOI: https://doi.org/10.1111/mec.70176

Interpretive Summary: Exotic annual plants pose an increasingly significant ecological challenge in desert environments, as illustrated by the case of Sahara mustard (Brassica tournefortii Gouan), a species belonging to the Brassicaceae family native to North Africa, the Middle East, and the Mediterranean regions of southern Europe. Sahara mustard was first collected in California in 1927, near Palm Springs in the Coachella Valley, a center of date palm cultivation. Currently, Sahara mustard has spread to southern Nevada, southern California, Arizona, New Mexico, and west Texas. This species is particularly concerning due to its drought tolerance, ability to establish dense populations, potential to outcompete native annual plants in desert ecosystems, and its role in increasing fuel loads and fire risks in desert scrub habitats. Previous research has indicated that Sahara mustard was introduced to the United States on multiple occasions and exists as three genetically distinct populations, two of which are only known from a limited number of locations in the southwestern U.S. However, the native origins of the invasive Sahara mustard populations remain unidentified, complicating efforts to develop and evaluate potential control strategies. We reconstructed the dispersal routes of Sahara mustard from its native range and into the U.S. using population genetic analyses. Our findings suggest that Egypt, along with possibly related populations in Morocco and France, is the most probable source of the largest invasive populations in the U.S. Phylogeographic features of the genomic relationships of Sahara mustard populations are strikingly similar to those of date palms, as evidenced in past molecular studies. Our genomic data, combined with historical records, support the primary hypothesis that Sahara mustard was purportedly introduced multiple times as a contaminant of date palm agricultural imports. Potential to control this species appears challenging, though needed, but this study paves the way for searching natural enemies in the original source populations across the native range aiming for a natural and effective management of Sahara mustard in the U.S.

Technical Abstract: Humans have facilitated the spread and evolution of invasive species, a pattern that has accelerated with globalization of trade and societal development. Consequently, the invasiveness of a species may be determined not only by how many propagules and which genotypes are introduced, but also its evolutionary history and how humans have interacted with it. Here, we document the population structure and movement of Sahara mustard (Brassica tournefortii) within its native range and its invaded U.S. and Australian ranges. We evaluated 312 individuals spanning 31 sites. We found that Sahara mustard has likely undergone substantial mixing within its native range, with genotypes from Egypt potentially representing a human-created connection between North African, European, and West Asian genotypes. Our results suggest Egypt—and perhaps closely related populations in Morocco and France—as the likely origin of invasive U.S. populations. Australian accessions appear most closely related from Qatar and may have acted as a bridgehead and potential source of the most-recently introduced U.S. populations. Further, agricultural varieties from Pakistan and India represent a mix of genotypes from the western part of the species native range and the eastern site in Qatar. We also identified evidence of mixing of some accessions of crop wild relatives. Nearly all populations sampled display excess heterozygosity and negative inbreeding coefficients, likely indicative of selection and potential admixture with cultivated Brassica species. Overall, we reconstructed the probable invasion history of Sahara mustard, inferring significant human-mediated movement of the species within and beyond its native distribution.