Phytophthora sojae avirulence genes Avr4 and Avr6 are located in a 24kb, recombination-rich region of genomic DNA.

A cross between two different races (race 7xrace 25) of the soybean root and stem rot pathogen Phytophthora sojae was analyzed to characterize the genomic region flanking two cosegregating avirulence genes, Avr4 and Avr6. Both genes cosegregated in the ratio of 82:17 (avirulent:virulent) in an F(2) population, suggestive of a single locus controlling both phenotypes. A chromosome walk was commenced from RAPD marker OPE7.1C, 2.0cM distant from the Avr4/6 locus. Three overlapping cosmids were isolated which included genetic markers that flank the Avr4/6 locus. The chromosome walk spanned a physical distance of 67kb which represented a genetic map distance of 22.3cM, an average recombination frequency of 3.0kb/cM and 11.7-fold greater than the predicted average recombination frequency of 35.3kb/cM for the entire P. sojae genome. Six genes (cDNA clones) expressed from the Avr4/6 genomic region encompassed by the cosmid contig were identified. Single nucleotide polymorphisms and restriction fragment length polymorphisms showed these six genes were closely linked to the Avr4/6 locus. Physical mapping of the cDNA clones within the cosmid contig made it possible to deduce the precise linkage order of the cDNAs. None of the six cDNA clones appear to be candidates for Avr4/6. We conclude that two of these cDNA clones flank a physical region of approximately 24kb and 4.3cM that appears to include the Avr4/6 locus.