DOSAGE-DEPENDENT GENETIC SUPPRESSION OF OAT CROWN RUST RESISTANCE GENE PC-62

Combining crown rust resistance (CRR) genes is an objective of many oa t (Avena sativa L.) breeding programs. Genetic suppressors of CRR have been reported, but the genetic nature of such suppressors has not bee n investigated. An excess of susceptible seedlings in segregating prog eny of crosses involving genotypes with Pc-38 and Pc-62 suggested the Pc-38 parent may have a factor that inhibits the expression of Pc-62. Our objectives were to (i) examine a putative genetic interaction betw een oat CRR gene Pc-62 and a dominant suppressor, (ii) utilize chromos ome interchange stocks to define genetically the location of the suppr essor, (iii) evaluate the effect of gene dosage combinations on the re sistance-suppression relationship, and (iv) examine alternative explan ations of distorted segregation (genetic background and differential g amete transmission). The use of duplication-deficiency (Dp-Df) genotyp es placed a putative suppressor on an interchanged chromosome segment that contains Pc-38. Resistance conferred by Pc-62 was suppressed in F -1 trisomic CRR locus combinations (1Pc-62:2Pc-38) in comparison to th e expression of resistance in F-1 seedlings hemizygous for Pc-62. The Pc-38 segment contributed by two independently derived cultivars produ ced equivalent suppression in F-1 combinations with Pc-62, The presenc e of Pc-38 in an interchanged position in the cultivar Dumont allowed production of mono-and diallelic combinations of resistance genes and evaluation of mono-, di-, and trisomic segregation of the CRR Locus. D iallelic trisomic segregation for the CRR locus indicated that an incr easing dosage of Pc-62, relative to the suppressor and Pc-38, decrease d the degree of suppression, Male transmission of Dp-Df gametes was no t reduced relative to the frequency of gametes with a complete chromos ome complement.