Genetic mapping of hop (Humulus lupulus L.) applied to the detection of QTLs for alpha-acid content

The map locations and effects of quantitative trait loci (QTLs) were estimated for alpha-acid content in hop (Humulus lupulus L.) using amplified fragment length polymorphism (AFLP) and microsatellite marker (simple sequence repeat (SSR)) genetic linkage maps constructed from a double pseudotestcross. The mapping population consisted of 111 progeny from a cross between the German hop cultivar 'Magnum', which exhibits high levels of alpha-acids, and a wild Slovene male hop, 2/1. The progeny segregated quantitatively for alpha-acid content determined in 2002, 2003, and 2004. The maternal map consisted of 96 markers mapped on 14 linkage groups defining 661.90 cM of total map distance. The paternal map included 70 markers assigned to 12 linkage groups covering 445.90 cM of hop genome. QTL analysis indicated 4 putative QTLs (alpha1, alpha2, alpha3, and alpha4) on linkage groups (LGs) 03, 01, 09, and 03 of the female map, respectively. QTLs explained 11.9%-24.8% of the phenotypic variance. The most promising QTL to be used in marker-assisted selection is alpha2, the peak of which colocated exactly with the AFLP marker. Three chalcone synthase-like genes (chs2, chs3, and chs4) involved in hop bitter acid synthesis mapped together on LG04 of the female map. Saturation of the maps, particularly the putative QTL regions, will be carried out using SSR markers, and the stability of the QTLs will be tested in the coming years.