Genetic Diversity and Population Structure in Cities Is Not Consistent Among Cosmopolitan Plant Species

Urbanisation has led to increasing homogenization of plant communities across cities. However, it is unclear whether these patterns extend to cosmopolitan plant species at the genetic level. We examined genome-wide genetic patterns in six widespread plant species (three Poaceae and three Asteraceae) across five cities in the USA (Boston, Baltimore, Minneapolis-St. Paul, Phoenix, and Los Angeles) using reduced-representation sequencing. We assessed genetic structure, differentiation, and patterns of isolation by distance (IBD) and environment (IBE) to determine if species were genetically homogeneous or differentiated by city, percentage of impervious surface, or both. Most species exhibited limited population structure overall, with Poa annua (annual bluegrass), Taraxacum officinale (dandelion), and Cynodon dactylon (Bermuda grass) showing no significant genetic differentiation among cities, a pattern consistent with high gene flow mediated by human activity. Notable exceptions included city-level differences in Erigeron canadensis (horseweed) and Lactuca serriola (prickly lettuce), especially in Phoenix. We also observed low genetic diversity in Digitaria sanguinalis (crabgrass) from Phoenix, suggesting recent founder effects or selection via environmental filtering. Erigeron canadensis, the only native species studied, displayed stronger differentiation by city, along with significant isolation by temperature and distance. Among all species, we found no evidence for population structure by impervious surface. Our findings indicate that widespread population genetic structure patterns of cosmopolitan plants are likely to depend more on species attributes (e.g., self-compatibility) and human-mediated dispersal than on urbanisation per se.