Fig. 1

Sampling locations and variation of copper tolerance across Europe. A Distribution of the nine European locations from the 2015 DrosEU collection (black border) and three additional collection locations (white border). Each population is labelled with the first three letters of the collection location (Additional file 1: Table S1). Label fill corresponds with the copper concentration legend. The map shows the spatial variation in copper topsoil concentrations, as obtained from the Land Use/Land Cover Area Frame Survey (LUCAS) topsoil database, whose samples were taken from 2009 onwards. The grey line marks the 45th parallel. B Boxplots of LT₅₀ values by location. Note that the distance between boxplots is not linear. Locations from the 2015 DrosEU collection are outlined in black, while the three additional locations are outlined in grey. The full list of strains used is provided in Additional file 1: Table S1A-B. C Boxplots of LT₅₀ values of strains split into northern (NE) and southern (SE) European locations by the 45th parallel. This point was chosen because the nine original collection sites could be clearly divided by this line (A). Significance assessed with a one-sided Wilcoxon test: p-value < 0.05 (*). D Boxplots of LT₅₀ values of strains classified by degree of urbanization: U, urban; SU, semi-urban; and R, rural. Significance assessed with a linear model: p-value < 0.01 (**)