Chromosome location and markers of Sm1:: a gene of wheat that conditions antibiotic resistance to orange wheat blossom midge

The gene Sm1 in common wheat ( Triticum aestivum L.) confers resistance to orange wheat blossom midge (OWBM) (Sitodiplosis mosellana Gehin). As previously described, resistance was inherited as a single gene ( Sm1) that expresses antibiotic properties ( death or slowed development) against larvae of OWBM. No DNA polymorphism associated with resistance was amplified following screening of about 500 Random Amplification of Polymorphic DNA ( RAPD) primers on bulked segregants but linked Amplified Fragment Length Polymorphism ( AFLP) fragments were identified by screening 57 primer combinations. An allele-specific Sequence Characterized Amplified Region ( SCAR) DNA marker linked to midge resistance was elaborated from the sequence of one of these fragments. Primer sequences of this marker ( WM1; 5' to 3', forward and reverse) were CAC CTG GAA TGT TGG ACT G and ACA TCA TCT GTC AAC GCA CTA and fragment size was 232 base pairs. Genetic tests showed that WM1 was located in the subterminal region of the short arm of chromosome 2B. Sm1 was mapped to a short interval ( similar to 2.5 cM) flanked proximally by WM1 and distally by the microsatellite Xgwm210. Among resistant cultivars, 14/15 carried WM1. However among susceptible common wheats, 33/66 also carried WM1. To permit marker use in combination with marked susceptibles, a trans recombinant carrying the resistance gene Sm1 but lacking the SCAR ( WM1) was isolated in a hard red spring wheat background. In other crosses segregating for leaf rust and midge resistance, adverse linkage drag was detected due to repulsion between Lr16 and Sm1. Two cis recombinants between Lr16 and Sm1 were identified. Due to different crossover sites, these two lines show contrasting alleles of the microsatellite Xbarc35 which is also closely linked to Sm1.