Population genomics of selectively neutral genetic structure and herbicide resistance in UK populations of Alopecurus myosuroides

BACKGROUND Alopecurus myosuroides (blackgrass) is a major weed in Europe with known resistance to multiple herbicide modes of action. In the UK, there is evidence that blackgrass has undergone a range expansion. In this paper, genotyping‐by‐sequencing and population‐level herbicide resistance phenotypes are used to explore spatial patterns of selectively neutral genetic variation and resistance. We also perform a preliminary genome‐wide association study (GWAS) and genomic prediction analysis to evaluate the potential of these approaches for investigating nontarget site herbicide resistance. RESULTS Blackgrass was collected from 47 fields across the British Isles and up to eight plants per field population (n = 369) were genotyped by Restriction site‐associated DNA (RAD)‐sequencing. A total of 20 426 polymorphic loci were identified and used for population genetic analyses. Phenotypic assays revealed significant variation in herbicide resistance between populations. Population structure was weak (FST = 0.024–0.048), but spatial patterns were consistent with an ongoing westward and northward range expansion. We detected strong and consistent Wahlund effects (FIS = 0.30). There were no spatial patterns of herbicide resistance or evidence for confounding with population structure. Using a combination of population‐level GWAS and genomic prediction we found that the top 20, 200, and 2000 GWAS loci had higher predictive abilities for fenoxaprop resistance compared to all markers. CONCLUSION There is likely extensive human‐mediated gene flow between field populations of the weed blackgrass at a national scale. The lack of confounding of adaptive and neutral genetic variation can enable future, more extensive GWAS analyses to identify the genetic architecture of evolved herbicide resistance.