Fine Mapping of the Orange Wheat Blossom Midge Resistance Gene Sm1

The orange wheat blossom midge (OWBM) (Sitodiplosis mosellana Géhin; Diptera: Cecidomyiidae) is among the most damaging insect pests of wheat in most temperate wheat growing areas of the world. Genetic resistance is an effective strategy for managing wheat midge and Sm1 is the only described gene for OWBM resistance. The mechanism by which Sm1 confers resistance was proposed to be antibiosis-based through the production of ferulic acid and/or p-coumaric acid. Previous research mapped Sm1 to wheat chromosome arm 2BS. In this study, fine mapping of Sm1 was pursued using a large F₂ population (>5000 plants) from the cross between the wheat cv. Thatcher (susceptible: lacks Sm1) and the breeding line 99B60-EJ2D (resistant: carries Sm1). To better delineate the Sm1 interval and saturate the region, single nucleotide polymorphism (SNP) markers were developed and genotyped using the KASP assay. Flanking markers identified 200 fixed F₃ recombinants in the defined interval of Sm1. Four haplotypes were identified based on recombination breaking points of the Sm1 locus. Phenotypic data of eight lines (2 lines from each haplotype) was consistent and placed Sm1 in the defined interval with one and three recombinants to distal and proximal sides, respectively. Two SNP markers co-segregated with Sm1. These results were consistent with the results from four low resolution Sm1 mapping populations. In Brachypodium distachyon, the Sm1 interval corresponds to an inverted orthologous (syntenic) block of 95 kb (10 genes) on chromosome 5. Marker saturation of the Sm1 interval is currently pursued to further narrow the region.