The linkage map was constructed using Joinmap 4.1 (Van Ooijen, 2011) with a haploid model (HAP) using Kosambi’s mapping function and default parameters setting (independence LOD score; significance levels from 2.0 to 10.0 LOD). Groups were defined at the level of LOD ⩾ 5, skewed segregation of markers was tested with chi-square (P < 0.05), and the crossover frequency was calculated as the crossover number per individual per chromosome. Due to a lack of recombination, the ranking order of markers within each linkage group deviates in Joinmap from their known position. For this reason we plotted the cumulative genetic distances (cM) of the SNP markers as calculated with Joinmap against their corresponding cumulative physical distance (kb).