Partial Resistance to Maize Streak Virus in the MSR Pool 9 x CML312 Population.

Maize streak, caused by maize streak virus (MSV) affects large areas of maize production in sub-Saharan Africa.  The use of resistant germplasm is an effective method of managing yield losses caused by MSV. Inbred lines and populations with high levels of resistance to MSV have been developed, with most carrying the Msv1 locus on chromosome one (bin 1.04-1.05).  A QTL mapping study was conducted to study the resistance in the MSV-resistant inbred line MSR176, derived from the MSR x Pool-9 population.  The resistant parent was crossed to a susceptible inbred CML312 and twice replicated F_2:3 families were evaluated for MSV resistance at CIMMYT-Harare in Zimbabwe.  Two ten-plant plots were artificially inoculated with viruliferous leafhoppers, then rated for disease on two dates using a 1 (no symptoms) to 5 (severe streaking and plant stunting or plant death) disease severity scale. DNA was extracted from 165 F₂ plants and SSR markers were used to genotype 46 loci on ten chromosomes.  Significant associations between mean disease severity scores and loci on chromosomes 1, 2, 3, 7 and 10 were found using single factor ANOVA.  Markers in bins 1.04, 1.05, and 2.02 were significant at the Pr>F <0.001 level. Markers in bins 3.05 (Pr>F=0.02), 7.06 (Pr>F=0.04) and 10.04 (Pr>F=0.01) also had significant association with resistance.  Nine of the S₅ lines derived from the MSR Pool-9 population had high levels of MSV resistance, but lacked the Msv1 locus, indicating that other loci may also provide partial resistance.  Interval mapping and composite interval mapping were used to further investigate the role of other non-Msv1 loci in these resistant lines.