INHERITANCE OF AVIRULENCE FACTORS AND RESTRICTION-FRAGMENT-LENGTH-POLYMORPHISM MARKERS IN OUTCROSSES OF THE OOMYCETE PHYTOPHTHORA-SOJAE

F-1 hybrids from crosses of three P. sojae isolates (P6497, P7064, and P7076) were identified by random amplified polymorphic DNA (RAPD) mar kers specific for each parent. The race types of the isolates were sim ilar to races 2, 7, and 19, respectively, The hybrid nature of the pro geny was confirmed by restriction fragment length polymorphisms (RFLPs ). Selected F-1 progeny were selfed to produce F-2 progeny, RAPD and R FLP markers segregated in regular Mendelian fashion among one set of F -2 progeny (from P6497 x P7064), but considerable bias in the transmis sion of markers was observed among F-2 progeny from the second cross ( P6497 x P7076), Specific avirulence in the pathogen against soybean re sistance genes (Rps genes) was confirmed to be dominant in six cases t ested (Rps1a, Rps1b, Rps1c, Rps1d, Rps1k, and Rps3a) and appeared to b e dominant or semidominant in four others (Rps3b, Rps3c, Rps4, and Rps 6). The segregation of avirulence among the F-2 progeny supported the presence of single dominant avirulence genes (Avr1a Avr1b, and Avr3a) in the pathogen, matching three Rps genes (Rps1a, Rps1b, and Rps3a), a nd was consistent with the possible presence of two others (matching R ps1k and Rps1d). Genetic linkage was observed between some pairs of RF LP markers and between an RFLP marker and Avr1b.