EVIDENCE FOR OUTCROSSING IN PHYTOPHTHORA-SOJAE AND LINKAGE OF A DNA MARKER TO 2 AVIRULENCE GENES

Two genetically different isolates of the homethallic Oomycete, Phytop hthorn sojae, were demonstrated to outcross and form hybrid oospores a fter co-culturing in vitro. Random amplified polymorphic DNA (RAPD) ma rkers revealed ten hybrids among 354 oospores analysed. One F-1 hybrid was allowed to self fertilise and produce an F-2 population of 247 in dividuals. Among 53 F-2 individuals, selected at random, 18 polymorphi c RAPD markers were observed to segregate at near 3:1 Mendelian ratios , consistent with segregation for dominant alleles at single loci. Seg regation of virulence against soybean resistance genes Rys1a, 3a, and 5 revealed that the avirulence genes Avr1a, 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, complement ary dominant loci for both Avr3a and Avr5. Segregation of virulence ag ainst Rps6 was in the ratio of 1:2:1 (avirulent:mixed reaction:virulen t), suggesting a semi-dominant allele at a single locus. Two avirulenc e genes and one RAPD marker formed one linkage group, in the order Avr 3a, OPH4-1, Avr5, each separated by approximately 5 cM. Our results co nfirm that outcrossing occurred between the parental isolates, and tha t sexual recombination under field conditions may play an important ro le in generating and maintaining genetic diversity in populations of P . sojae.