The Vf gene for scab resistance in apple is linked to sub-lethal genes

Vf is the most widely used resistance gene in the breeding for scab resistant apple cultivars. Distorted segregation ratios for Vf-resistance have frequently been reported. Here we revealed that sub-lethal genes caused the distorted segregation. The inheritance of Vf was examined in six progenies by testing linked molecular markers. Three progenies showed distorted segregations that could be explained by three sub-lethal genes (sl1, sl2 and sl3), of which sl1, sl2 were closely linked to Vf. The s11 gene was located at about 14 cM from Vf and expressed itself only in the presence of another independently segregating sub-lethal gene sl3. Only the double homozygous recessive genotypes (sl1sl1 sl3sl3) were lethal, which occurred at first as dwarf and poor vigour plants during the first three months after germination. The sl2 gene was also linked to Vf and its lethality was expressed prior to seed germination and also required the homozygous recessive presence of sl3. The map position of sl3 has not yet been identified. The linkage of Vf to sub-lethal genes usually results in a shortage of Vf-resistant progenies. But in some exceptional crosses, it will lead to abundance of resistant seedling.