Inheritance of resistance to stem rust in Medea durum wheat and the role of suppressors

The durum wheat (Triticum turgidum L.) cultivar Medea Ap9d has been used as a supplementary differential in the identification of isolates of stem rust (Puccinia graminis f. sp. tritici Eriks. & Henn.), Genes for stem rust resistance from Medea were transferred to a susceptible hexaploid wheat (T. aestivum L.), LMPG, so that they could be used more efficiently in race identification and wheat breeding. Medea was backcrossed five times to LMPG with selection for resistance to stem rust races LBB and MCC. Ten near-isogenic lines (NILS) were produced and tested with isolates of 10 stem rust races. The NILs fell into four types. Three of the four types had resistance to races to which Medea was susceptible. Apparently, Medea possesses suppressors for some of its genes for resistance and the suppressors were lost during the backcrossing to LMPG, This was confirmed when the cross, Medea/Glossy Hugenot, and the backcross, Medea/2 x Glossy Hugenot, segregated for resistance to races RCH and TMH (15B-4), to which both parents are susceptible. The segregations could not be explained on the basis of complementary genes. Medea was also crossed and backcrossed to a susceptible durum, Glossy Hugenot, for a genetic study using races MCC and TMB. The genetic analysis showed that Medea has four genes that condition resistance to races MCC, two of which also condition resistance to rare TMB. None of the four types of NILs are resistant to both. Apparently, neither of the latter genes was transferred to LMPG. The presence of suppressors in Medea suggests that they may confer a selective advantage on plants that carry them.