A gene for resistance to the maize streak virus in the African CIMMYT maize inbred line CML202

Resistance to maize streak virus (MSV) is an essential trait of improved maize Varieties in sub-Saharan Africa. We mapped quantitative trait loci (QTL) for resistance to MSV in a population of 196 F-2:3 lines derived from a cross between the maize inbred lines CML202 (resistant) from CIMMYT-Zimbabwe and Lo951 (susceptible) from Italy. Field tests were planted at two locations in Zimbabwe, inoculated with viruliferous leaf hoppers (Cicadulina mbila), and scored twice (21 and 83 days after infesting, DAI) on a 1-5 scale. The mean final streak intensity (score 2) of the parent lines was 2.2 (CML202) and 4.8 (Lo951). Genotype x location interaction was large for score I but negligible for score 2. Consequently, the heritability was higher for score 2 (0.93) than for score 1 (0.62). By composite interval mapping across locations, using a linkage map with 110 RFLP loci, four significant (LOD greater than or equal to 3.0) QTL were identified for score 1 on chromosomes (C) 1, 2, 3, and 4, respectively. All four were contributed by CML202. For score 2, only the QTL on C 1 was significant (LOD = 37), explaining 59% of the phenotypic and 64% of the genotypic variance. The QTL's partially dominant gene action was consistent with the nearly intermediate resistance of the F-1 generation (relative heterosis for resistance 12%). The presence of one major QTL is consistent with the bimodal frequency distribution of the mapping population showing a clear 3:1 segregation. This gene seems to be allelic or identical to Msv1, a major resistance gene which was previously identified in the same genomic region in Tzi4, an inbred line from IITA. Inbred CML202 had lower final disease ratings than Tzi4. The greater resistance of CML202 may be due to allelic differences at the mzv1 locus or due to the minor QTL on C 2, 3, and 4 which were not detected in Tzi4.