Epistasis analysis of SbPRR37 and SbCO QTL in BTx642/Tx7000 RIL population under field conditions. A. Proportion of phenotypic variance (R2) explained by the QTL corresponding to SbCO-2/Sbco-3 in the portion of the population homozygous for Sbprr37-1 (right) or Sbprr37-2 (left). B. Proportion of the phenotypic variance explained by the QTL corresponding to Sbprr37-1/Sbprr37-2 in the portion of the population homozygous for SbCO-2 (left) or Sbco-3 (right). Each R2 value represents the average obtained under field conditions in three years.