Human-Mediated Dispersal Routes Structure Sahara Mustard Genomics

Humans have facilitated the spread and evolution of invasive species, a pattern that has accelerated with the globalisation 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 by 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 to those from Qatar and may have acted as a bridgehead and potential source of the most-recently introduced U.S. population. 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 other 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.