Inheritance of resistance to cereal soilborne mosaic virus in a durum wheat population of lines derived from the cross ‘Meridiano / Claudio’: results of a two-year study

A population consisting of 184 RILs obtained via single-seed descent by Produttori Sementi Bologna S.p.A., Italy, from a cross between the durum wheat cultivars Meridiano (resistant) and Claudio (moderately susceptible) was assayed during 2006–07 and 2007–08 in a field with natural inoculum sources of CSBMV at Cadriano (Bologna). In each season, the RILs, parents, and the cultivar Grazia (a susceptible control inserted at regular intervals) were evaluated for resistance on the basis of symptomatol¬ogy on four dates and ELISA absorbance on two dates. The plants were grown in 2.4-m2, solid-seeded plots distributed according to a randomized block design with two replicates. Symptoms were scored on a 0–4 scale. Virus concentration was estimated by DAS-ELISA, on leaf samples. Grain yield, kernel weight, and test weight also were measured. The data from the first season were reported (Ann Wheat Newlsett 54:76), those obtained in the 2007–08 trial are summarized in Table 1 and Table 2. Disease pressure was severe in both seasons, as testified by the mean symptom scores (3.8 in 2007 and 3.6 in 2008) recorded for Grazia at the time of maximum symptom expression. Analysis of symptom score and ELISA absorbance frequency distributions indicated a complex inheritance of CSBMV-resistance, involving no less than four major genes, various modifiers, and a dif¬ferent timeline of the expression of resistance. Transgressive segrega¬tion, moreover, indicated that both the resistant (Meridiano) and the moderately susceptible parents (Claudio) contributed favorable alleles. Despite the favorable genes contributed by the latter cultivar, however, RILs completely resistant to CSBMV were not recovered. Indeed, leaf samples from all the RILs assayed were found ELISA-positive on at least one of the four collection dates, and all except two RILs gave ELISA-positive results on at least two dates. Moreover, the only RIL that re¬mained symptom-free throughout the two seasons (i.e., on eight scoring dates) proved ELISA-positive on two out of collection dates. Eighteen of the 184 RILs assayed had overall mean symptom scores lower than the resistant parent; 15 of these RILs also showed lower overall mean ELISA absorbances; 9 produced higher grain yields; 10 showed higher test weights; and 11 showed higher kernel weights. Two RILs performed better than cultivar Meridiano in regard to all of the five parameters considered. Further trials are being set up to establish whether any of the above 18 RILs are indeed more resistant to CSBMV than cultivar Meridiano. The RILs also were profiled with molecular markers (158 SSRs), and an association map spanning 2,050 cM was obtained. Preliminary results indicate that at least four QTL accounted for most of the phenotypic variation observed. A major QTL was associated to Xwmc243 (distal telomeric region of chromosome arm 2BS),with the favorable allele contributed by Meridiano. The additional favorable QTL all located in the distal regions of the short and long arms of chromo¬some group 5 (particularly in chromosome 5A), were contributed by both parents. The QTL identified in the distal regions of 5AL (Xwmc524) and 5BL (Xbarc243) could represent the homoeologous copies of the major QTL identified in bread wheat (Sbm1).