Nodule sample collection
White clover (Trifolium repens ) root nodules were sampled from three clover field trials under identical management in Denmark (Store Heddinge, DK), France (Rennes, F), and the United Kingdom (Didbrook, UK). At each site, 18 F2 families (1-18) and two commercial varieties (K and C) were grown in duplicate in a similar block setup (Table S1 and Figure S1 ). In addition, 50 samples were collected from Danish clover fields under organic management. The clover varieties grown at each field can be seen in Table S1. A total of 170 samples was collected (Figure 1 , Table S1 , UK=40, F=40, DK=40, DKO1=14, DKO2=8, DKO3=3, DKO4=15, DKO5=5, DKO6=5). Nodules were stored at -20°C until DNA extraction.