Genetic analysis of tolerance to maize streak virus in maize

Maize streak, incited by maize streak geminivirus (MSV), is a major disease limiting maize (Zea mays L.) production over widespread areas of Africa. To understand the genetic basis of tolerance to MSV, recombinant inbred lines (RILs) derived from the cross of the MSV tolerant inbred Tzi4 with the MSV susceptible inbred Hi34, were evaluated for MSV tolerance. Experiments were conducted using controlled leafhopper (Cicadulina spp.) infestation in one glasshouse experiment at Namulonge, Uganda, and two field experiments at Centro Internacional de Mejoramiento de Maiz y Trigo, Harare, Zimbabwe. Eighty-seven RILs were genotyped at 82 loci by restriction fragment length polymorphism (RFLP) analysis. The association between genotype at RFLP marker loci and MSV tolerance was determined using single-factor analysis of variance (SFAOV), multiple regression, and interval mapping procedures. There was a significant association of MSV tolerance with RFLP markers on the short arm of chromosome 1. By SFAOV, the portion of the phenotypic variance explained by genotype class (R²) for the association between npi262 and the area under disease progress curve (AUDPC) measure of MSV tolerance was as high as 76% in field experiments. Interval mapping analyses (Knapp and Bridges 1990; Nelson 1997) identified the chromosome region bracketed by bnl12.06a and npi262 as explaining the largest proportion of the variation in MSV tolerance. After classification of symptom responses from the final field ratings into resistant and susceptible classes, qualitative analysis of data fit a chi-square test to a 1:1 Mendelian ratio, further indicating presence of a single major gene. Multipoint linkage analysis placed this gene, designated msv1, at a genetic distance of 3 cM distal to npi262. Identification of the tightly linked molecular marker locus npi262 should greatly aid ongoing conversion of susceptible African varieties to maize streak resistance.