Evidence for outcrossing in Phytophthora sojae and linkage of a DNA marker to two avirulence genes.

Two genetically different isolates of the homothallic Oomycete, Phytophthora sojae, were demonstrated to outcross and form hybrid oospores after co-culturing in vitro. Random amplified polymorphic DNA (RAPD) markers revealed ten hybrids among 354 oospores analysed. One F1 hybrid was allowed to self fertilise and produce an F2 population of 247 individuals. Among 53 F2 individuals, selected at random, 18 polymorphic RAPD markers were observed to segregate at near 3:1 Mendelian ratios, consistent with segregation for dominant alleles at single loci. Segregation of virulence against soybean resistance genes Rps1a, 3a, and 5 revealed that the avirulence genes Avrla, 3a and 5 were dominant to virulence. Avirulence against these three resistance genes appeared to be conditioned by one locus for Avr1a and two independent, complementary dominant loci for both Avr3a and Avr5. Segregation of virulence against Rps6 was in the ratio of 1:2:1 (avirulent:mixed reaction:virulent), suggesting a semi-dominant allele at a single locus. Two avirulence genes and one RAPD marker formed one linkage group, in the order Avr3a, OPH4-1, Avr5, each separated by approximately 5 cM. Our results confirm that outcrossing occurred between the parental isolates, and that sexual recombination under field conditions may play an important role in generating and maintaining genetic diversity in populations of P. sojae.