Fine mapping of QSbm.ubo-2BS, a major QTL for resistance to Soil-Borne Cereal Mosaic Virus (SBCMV) in durum wheat

QSbm.ubo-2BS, a major QTL controlling the response to Soil-Borne Cereal Mosaic Virus in durum wheat, maps within a 2 cM-wide interval in the distal region of chromosome arm 2BS. The consensus map developed by Maccaferri et al. (2015) allowed to enrich QSbm.ubo-2BS interval with SNPs from the 90K Illumina array common to several linkage maps. Eleven SNPs were successfully converted to KASP markers, which provided fluorescent high-throughput assays spanning the QTL region. Marker-assisted selection was performed on ~3,000 RILs from the Svevo (resistant) x Ciccio (medium-susceptible) population with two KASP markers flanking the QTL interval, KUBO 9 and KUBO 13. MAS identified a total of 521 lines recombinant between the two markers. In particular, 320 and 189 of these lines were characterized for SBCMV response in a field nursery under severe and uniform SBCMV infection in 2016 and 2017, respectively. The lines were scored for symptom severity on a 0 to 5 scale, where 0 = very resistant and 5 = very susceptible. The same lines were genotyped with six KASP markers distributed along the QTL interval (KUBO 1, KUBO 3, KUBO 27, KUBO 29, KUBO 40 and KUBO 41) and with the DArT marker wPt-2106. The fine mapping conducted on Svevo x Ciccio allowed to locate the most probable QSbm.ubo-2BS support interval to 0.2 cM between the markers KUBO 27 and KUBO 41. Single-marker analysis showed that all markers in the interval are strongly associated with the QTL. Markers order was confirmed through genotyping of Meridiano x Claudio RILs with recombinant events in the interested interval. QSbm.ubo-2BS support interval corresponds to 3.2 Mb on the physical map of Svevo genome, a region that harbors 93 genes, including defensins, proteins belonging to NBS-LRR family and NBS-LRR-like resistance proteins. This work provides the foundation for the positional cloning of QSbm.ubo-2BS.