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) markers specific for each parent. The race types of the isolates were similar to races 2, 7, and 19, respectively, The hybrid nature of the progeny was confirmed by restriction fragment length polymorphisms (RFLPs). Selected F-1 progeny were selfed to produce F-2 progeny, RAPD and RFLP markers segregated in regular Mendelian fashion among one set of F-2 progeny (from P6497 x P7064), but considerable bias in the transmission of markers was observed among F-2 progeny from the second cross (P6497 x P7076), Specific avirulence in the pathogen against soybean resistance genes (Rps genes) was confirmed to be dominant in six cases tested (Rps1a, Rps1b, Rps1c, Rps1d, Rps1k, and Rps3a) and appeared to be dominant or semidominant in four others (Rps3b, Rps3c, Rps4, and Rps6). 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), and was consistent with the possible presence of two others (matching Rps1k and Rps1d). Genetic linkage was observed between some pairs of RFLP markers and between an RFLP marker and Avr1b.