Mapping QTLs for opaque2 modifiers influencing the tryptophan content in quality protein maize using genomic and candidate gene-based SSRs of lysine and tryptophan metabolic pathway.

KEY MESSAGE: The mapping analysis resulted in identification of five significant QTLs for opaque2 modifiers influencing the tryptophan content in quality protein maize using functional and genomic SSR markers. Quality protein maize (QPM) was developed by selecting genetic modifiers that convert opaque2 mutant containing high lysine and tryptophan. There are several unlinked opaque2 modifier loci (Opm) in QPM whose location, nature and mode of action are not clear. To identify these Opm QTLs, we developed a population of 218 F2:3 individuals from a cross between VQL2 and VQL8, two isogenic QPM inbreds significantly differing in tryptophan content. Based on the data of the F2:3 population, five significant QTLs on chromosomes 5, 7 and 9 with LOD values more than 2.5 were identified and together explained 38.6 % of the total phenotypic variance (R (2)). The Wx1 gene which has influence on the amino acid composition of the maize endosperm was mapped on chromosome 9 near the marker phi022 and also validated by bulk analysis. The QTL near the SSR marker ZmASK3, developed from the aspartate kinase 2 gene of the lysine pathway, mapped on chromosome 5 and had LOD of 2.7 with R (2) of 5.1 %. On chromosome 9, the QTL between the loci umc1430 and bnlg1401 had an LOD of 4.5 with R (2) of 9.1 %, whereas the QTL between the loci bnlg1401 and phi022 had an LOD of 4.2 with R (2) of 8.4 %. The third QTL was observed to be close to the marker umc2207 with an LOD of 4.8 and R (2) of 8.4 %. The identified QTLs will be very useful in the marker-assisted back-cross breeding and transgressive breeding for the development of QPM maize.