Marker-assisted selection for powdery mildew resistance in grapes

The efficiency of marker-assisted selection for powdery mildew (Uncinula ne cator (Schw.) Burr) resistance in grapes (Vitis L, sp,) was studied using m olecular markers associated with a major QTL (quantitative trait loci) for this trait. Initially, genetic maps were constructed from a segregating pop ulation of the cross 'Horizon' x Illinois 547-1 (a hybrid between V. rupest ris Scheele and ii cinerea Engelm,), A major QTL from Ill, 547-1, the resis tant parent, explained 41% of the variation, One RAPD (randomly amplified p olymorphic DNA) marker and one AFLP (amplified fragment length polymorphism ) marker, obtained by bulked segregant analysis, showed the highest associa tion with powdery mildew resistance in the mapping population. Segregation of the QTL was followed in different crosses by CAPS (cleaved amplified pol ymorphic sequence) markers developed from these two markers. An allele-spec ific amplified polymorphism that segregates as present/absent was also deve loped from the CS25b locus. Powdery mildew resistance was evaluated visuall y on a 1 to 5 scale in four different seedling populations. Two populations originated from crosses using Ill, 547-1 as the resistant parent. Two othe r populations were from crosses with NY88.0514.03, a resistant seedling fro m the original 'Horizon' x Ill. 547-1 mapping population, Segregation ratio distortions were observed in some crosses, In these cases, the allele asso ciated with the QTL for powdery mildew resistance was less frequent than th e alternate allele, In all crosses, the markers were closely associated wit h resistance. If selection were based on markers, the percentage of suscept ible individuals (classes 4 and 5) would decrease from 24% to 52% to 2% to 18%. Selection efficiency was greatest in crosses where segregation distort ion was most intense.