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 cere al grains. True resistance to the disease is not found in wheat (Triticum a estivtium L.), but it has been introgressed from Thinopyrum intermedium (Ti ) on chromosome 7DL of recombinant wheat lines designated TC. The objective s of our study were to identify a high through-put scoring tool for the pre sence of the translocated Th. intermedium fragment and to assess its suitab ility for evaluating resistance to BYDV in segregating populations. Segrega tion of the Ti fragment was followed in the F-2 population of an Anza (brea d 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/o r greenhouse conditions by measuring the virus titers of infected plants us ing 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 wit h 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 fragm ent was present in the heterozygous and homozygous states, respectively, co nfirming a dosage effect of the resistance allele. A lower proportion of in fected individuals in the field was associated with the presence of the fra gment, indicating a mechanism that may interfere with aphid feeding or viru s translocation within infected plants. Despite significant differences bet ween groups with and without the fragment, the OD values of infected lines overlapped, and it was not possible to definitively detect the fragment bas ed solely on ELISA. We conclude that gwm37 is a reliable marker for the Ti translocation that will allow efficient detection of the translocation in b reeding populations and greatly assist in selecting BYDV-resistant wheats i n the absence of the disease.