Tagging downy mildew resistance genes in sorghum.
Sorghum downy mildew (Peronosclerospora sorghi) is one of the most destructive diseases of sorghum and maize throughout the world. In defined examples, downy mildew resistance in sorghum is conditioned by one (for example, in line SC414) or two (in line QL3) dominant genes. Other sources of resistance are known but have not been defined. The availability of tightly linked genetic markers with the resistance genes will permit deploying multigenic resistance to this disease. Two techniques have been used in the study reported here for tagging sorghum downy mildew resistance genes; these include the use of RAPDs and RFLPs. Although eight key sorghum lines were initially selected for this study, only three (SC414, QL3 and RTx7078) were eventually considered. Some 450 RAPD primers were screened with DNA extracted from the parental lines. Some of the primers amplified unique bands in either susceptible or resistant lines only. So far, 247 polymorphic RAPD loci (108 between RTx7078 and SC414, and 139 between RTx7078 and QL3) have been analysed for co-segregation with the resistance phenotype in the F2 populations. One of them appears to be linked to the resistance gene in SC414 at 14.9 cM. A total of 72 RFLP probes (16 maize, 29 sorghum and 29 RAPD fragments) have been screened so far with filters having genomic DNA digested with five restriction enzymes (BamHI, EcoRI, EcoRV, HindIII and XbaI). Sixty-three RFLP loci were identified between susceptible and resistant parents. Three of the markers present on the same linkage group were linked to the different resistance genes.