A diagnostic molecular marker allowing the study of Th. intermedium-derived resistance to BYDV in bread wheat segregating populations

Barley yellow dwarf (BYD) is the most important viral disease of small cereal grains. True resistance to the disease is not found in wheat (Triticum aestivtium L.), but it has been introgressed from Thinopyrum intermedium (Ti) on chromosome 7DL of recombinant wheat lines designated TC. The objectives of our study were to identify a high through-put scoring tool for the presence of the translocated Th. intermedium fragment and to assess its suitability for evaluating resistance to BYDV in segregating populations. Segregation of the Ti fragment was followed in the F-2 population of an Anza (bread wheat) by TC14/2*Spear (TC14) cross. Resistance to BYDV isolates PAV-Mex and MAV-Mex in F-3, F-4 and F-5 populations was evaluated under field and/or greenhouse conditions by measuring the virus titers of infected plants using ELISA, and the number of infected plants per plot. The SSR marker gwm37 was polymorphic for the translocation. In F-4 lines it was associated with the physical presence of an intact translocation on chromosome 7DL and with low virus titers of BYDVPAV. Reductions in virus titer of 27% and 55% in the F-3 and 18% and 45% in the F-5 populations were observed when the fragment was present in the heterozygous and homozygous states, respectively, confirming a dosage effect of the resistance allele. A lower proportion of infected individuals in the field was associated with the presence of the fragment, indicating a mechanism that may interfere with aphid feeding or virus translocation within infected plants. Despite significant differences between groups with and without the fragment, the OD values of infected lines overlapped, and it was not possible to definitively detect the fragment based solely on ELISA. We conclude that gwm37 is a reliable marker for the Ti translocation that will allow efficient detection of the translocation in breeding populations and greatly assist in selecting BYDV-resistant wheats in the absence of the disease.