Finemapping, cloning, verification, and fitness evaluation of a QTL, Rcg1, which confers resistance to Colletotrichum graminicola in maize

Colletotrichum graminicola (Cg) causes anthracnose stalk rot and anthracnose leaf blight of maize and other graminaceous hosts (Bergstrom and Nicholson, 1999). Anthracnose has been found to occur in the United States since 1855 and occurs in the Americas, Europe, Africa, Asia, and Australia. Over 37.5 million acres are infested annually in the United States with average yield losses of 6.6%. Previously, a QTL region on chromosome 4 was reported to carry resistance to Cg in the maize line DE811ASR (Jung et al. 1994). DE811ASR was further backcrossed to DE811 (susceptible maize line) to develop DE811 Rcg1 which was used to develop a BC7 segregating population for mapping. Utilizing 4784 genotyped BC7 individuals, 856 BC7 individuals were selected as recombinants within the region of interest. The selfed progeny of those 856 individuals were phenotyped to obtain family means, and a high-resolution genetic-linkage map was constructed to further resolve the previously described QTL. Initial markers identified two physical contigs within the region of interest. Using public and private physical and genetic integrated maps, the gap between the contigs was closed, and a complete tiling path through the region was constructed. Additional PCR-based fragment length polymorphic markers were created from BAC end sequences, overgoes and ESTs present on the BAC clones. Subsequent analysis of phenotypic data integrated with the BC7 genetic-linkage map resolved the QTL location to an approximate 3 cM region (∼400kb physical distance based on the B73 and Mo17 physical maps) with a peak LOD score of approximately 55. This 400kb region was sequenced using the corresponding BACs from a library made from the resistant parent, and a candidate gene for resistance to Cg was discovered in the region. Four independent Mu insertion events were identified in putative susceptible mutants derived from a targeted Mu-transposon population containing the resistance gene. Sequence data confirmed the presence of the Mu element within the candidate gene causing these plants to be susceptible to Cg. (Abstract shortened by UMI.)...