Two quantitative trait loci, Dw1 and Dw2, are primarily responsible for rootstock-induced dwarfing in apple

The apple dwarfing rootstock‘Malling9’(‘M9’)has been used worldwide both to reduce scion vigour and as a genetic source for breeding new rootstocks.Progeny of‘M9’segregate for rootstock-induced dwarfing of the scion,indicating that this trait is controlled by one or more genetic factors.A quantitative trait locus(QTL)analysis of a rootstock population derived from the cross between‘M9’בRobusta5’(non-dwarfing)and grafted with‘Braeburn’scions identified a major QTL(Dw1)on linkage group(LG)5,which exhibits a significant influence on dwarfing of the scion.A smaller-effect QTL affecting dwarfing(Dw2)was identified on LG11,and four minor-effect QTLs were found on LG6,LG9,LG10 and LG12.Phenotypic analysis indicates that the combination of Dw1 and Dw2 has the strongest influence on rootstock-induced dwarfing,and that Dw1 has a stronger effect than Dw2.Genetic markers linked to Dw1 and Dw2 were screened over 41 rootstock accessions that confer a range of effects on scion growth.The majority of the dwarfing and semi-dwarfing rootstock accessions screened carried marker alleles linked to Dw1 and Dw2.This suggests that most apple dwarfing rootstocks have been derived from the same genetic source.