The genetics of avirulence in the soybean root and stem rot pathogen, Phytophthora sojae, was studied by constructing two crosses between four different races of the pathogen (race 7 and race 16; race 17 and race 25). An F2 population for both crosses was generated and a subset of randomly chosen F2 progeny from each cross was used to examine the genetic basis of avirulence within P. sojae towards eleven different resistance (Rps) genes in soybean. Avirulence was found to be dominant and determined by a single locus for Avr 1b, 1k, 4, and 6 in the 7/16 cross, and for Avr1d, 3a, 3b, 4, 5, and 6 in the 17/25 cross. However, for Avr3c in both crosses and Avr1k in the 17/25 cross, the F2 population segregated in a 9:7 ratio with avirulence dominant which suggested that avirulence towards Rps3c and Rps1k in the 17/25 cross is controlled by two dominant, independently segregating genes. For Avr1c, the F2 population segregated in a 1:3 ratio and avirulence appeared to be controlled by a recessive allele at a single locus in both crosses with virulence the dominant phenotype observed for P. sojae against Rps1c. For Avr1a, in the 17/25 cross, avirulence appeared to be controlled by a dominant allele at a single locus (3:1 ratio), but, in the 7/16 cross, avirulence was controlled by a recessive allele at a single locus (1:3 ratio). Avr4 and 6 cosegregated in both the 7/16 and 17/25 crosses. In the 7/16 cross, Avr1b and 1k were closely linked, as were Avr3a and 5 in the 17/25 cross. Linkage was also observed between Avr3c and Avr4 and 6 at a distance of 16.2 cM. The remaining avirulence genes showed no linkage to each other.
Information from segregating Rapid Amplified Polymorphic DNA (RAPD), Restriction Fragment Length Polymorphic (RFLP), and Amplified Fragment Length Polymorphic (AFLP) markers screened on a total of 212 progeny from the two crosses described in this investigation and two pre-existing crosses (53 F2 individuals from each cross) were used to construct a comprehensive genetic linkage map of P. sojae. Therefore, the linkage map presented herein is based on four different crosses and consists of 386 markers comprising 38 RFLP markers, 233 RAPD markers, 105 AFLP markers, and 10 avirulence genes. Three new avirulence genes as well as 34 new RAPD and 105 AFLP markers were integrated into the pre-existing linkage map of Whisson et al. (1995). The final map is composed of 21 major linkage groups and 7 minor linkage groups covering a total map distance of 1640 cM.
As a first step in examining the genetic basis of host specificity within Phytophthora species, a cross was generated between two homothallic species of Phytophthora, P. sojae and P. vignae, pathogenic on soybean and cowpea, respectively. A total of 1640 single oospore cultures were isolated and RAPD analysis of 800 of these cultures resulted in two interspecific F1 hybrids, F11121 and F11426, being identified. The true hybrid nature of these individuals was confirmed using a more detailed AFLP analysis. A further ten single zoospore cultures were generated for each F1 hybrid and were used to demonstrate the presence of stable F1 hybrids. One single zoospore culture from each F1 hybrid was used in pathogenicity tests. Both F1 hybrids caused symptoms including lesions, wilting, and death on soybean and cowpea susceptible cultivars. However, the level of aggressiveness displayed by the F1 hybrids was, in general, reduced when compared to the aggressiveness of the parental isolates on their respective hosts. The F1 hybrids were reisolated from infected seedlings of both soybean and cowpea and the hybrid identity of the reisolations was confirmed by RAPD and AFLP analysis. These reisolations were used in further pathogenicity tests and they also caused symptoms of a similar level of severity as the original single zoospore cultures. Inoculation of the F1 hybrids onto a differential set of both soybean and cowpea cultivars did not reveal a change in virulence patterns to those exhibited by the parents. Further avenues of research include examining the genetics of host specificity by constructing F2 populations and backcrosses and using the resultant progeny in pathogenicity tests. The contribution of outcrossing between different homothallic species of Phytophthora toward the evolution of new species could also be examined.