Precise mapping of quantitative trait loci for resistance to southern leaf blight, caused by Cochliobolus heterostrophus race O, and flowering time using advanced intercross maize lines

Abstract

The intermated B73 x Mo17 (IBM) population, an advanced intercross recombinant inbred line population derived from a cross between the maize lines B73 (susceptible) and Mo17 (resistant), was evaluated in four environments for resistance to southern leaf blight (SLB) disease caused by Cochliobolus heterostrophus race O. Two environments were artificially inoculated, while two were not inoculated and consequently had substantially lower disease pressure. Four common SLB resistance quantitative trait loci (QTL) were identified in all environments, two in bin 3.04 and one each in bins 1.10 and 8.02/3. There was no significant correlation between disease resistance and days to anthesis. A direct comparison was made between SLB QTL detected in two populations, independently derived from the same parental cross: the IBM advanced intercross population and a conventional recombinant inbred line population. Several QTL for SLB resistance were detected in both populations, with the IBM providing between 5 and, in one case, 50 times greater mapping resolution.

Figure 1.—

The distribution of weighted mean disease scores for resistance to southern leaf blight of maize (average over four environments), caused by Cochliobolus heterostrophus race O, rated on a 1–9 scale, where 1 represented a symptomless plant and 9 represented a dead plant, in the IBM maize RIL population. The rating for each line was calculated as a least-squares mean of the ratings over the four environments rated. The positions of the average scores of the parental types, B73 and Mo17, are indicated.

Figure 2.—

QTL likelihood plots of SLB resistance from analysis of results averaged over two environments in a conventional 158-line RIL population (the Stuber population) derived from a B73 × Mo17 cross (top plot in each case) compared to QTL likelihood plots of SLB resistance from analysis of data averaged over four environments from the IBM population (bottom plot in each case). Analysis in each case was based on composite-interval mapping using similar parameters. The vertical axes in each graph indicate LOD scores, and the horizontal lines indicate the empirically derived LOD threshold for calling a QTL position (P = 0.05). The top and bottom graphs are scaled in each case so that equivalent genetic locations are vertically in line with each other. Dotted lines around the detected QTL connect the same molecular markers mapped in the two populations. Small triangles on the x-axes denote the position of mapped molecular markers in the two populations. One triangle may represent one or more markers in the case of very closely linked markers. (A) chromosome 1; (B) chromosome 2; (C) chromosome 3.

Figure 2.—

QTL likelihood plots of SLB resistance from analysis of results averaged over two environments in a conventional 158-line RIL population (the Stuber population) derived from a B73 × Mo17 cross (top plot in each case) compared to QTL likelihood plots of SLB resistance from analysis of data averaged over four environments from the IBM population (bottom plot in each case). Analysis in each case was based on composite-interval mapping using similar parameters. The vertical axes in each graph indicate LOD scores, and the horizontal lines indicate the empirically derived LOD threshold for calling a QTL position (P = 0.05). The top and bottom graphs are scaled in each case so that equivalent genetic locations are vertically in line with each other. Dotted lines around the detected QTL connect the same molecular markers mapped in the two populations. Small triangles on the x-axes denote the position of mapped molecular markers in the two populations. One triangle may represent one or more markers in the case of very closely linked markers. (A) chromosome 1; (B) chromosome 2; (C) chromosome 3.

Figure 2.—

QTL likelihood plots of SLB resistance from analysis of results averaged over two environments in a conventional 158-line RIL population (the Stuber population) derived from a B73 × Mo17 cross (top plot in each case) compared to QTL likelihood plots of SLB resistance from analysis of data averaged over four environments from the IBM population (bottom plot in each case). Analysis in each case was based on composite-interval mapping using similar parameters. The vertical axes in each graph indicate LOD scores, and the horizontal lines indicate the empirically derived LOD threshold for calling a QTL position (P = 0.05). The top and bottom graphs are scaled in each case so that equivalent genetic locations are vertically in line with each other. Dotted lines around the detected QTL connect the same molecular markers mapped in the two populations. Small triangles on the x-axes denote the position of mapped molecular markers in the two populations. One triangle may represent one or more markers in the case of very closely linked markers. (A) chromosome 1; (B) chromosome 2; (C) chromosome 3.