Simple Sequence Repeat Markers Linked to the Soybean Rps Genes for Phytophthora Resistance

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Abstract
Simple sequence repeat (SSR) markers with linkages to the Rps1, Rps2, Rps3, Rps4, Rps5, and Rps6 loci that govern soybean [Glycine max (L.) Merr.] resistance to Phytophthora root rot (caused by Phytophthora megasperma Drechs. f. sp. glycinea Kuan and Ervin) are desired. Near‐isogenic lines (NILs) of Clark or Williams, homozygous resistant (RpsRps) at just one of those Rps loci, were mated to a NIL of Harosoy homozygous susceptible (rpsrps) at all six loci. From the 100 to 120 F2:3 progenies per mating, 20 F3 seedlings were evaluated for resistance (R) or susceptibility (S) following inoculation with the race of P. megasperma affected by the segregating Rps allele. About 15 RpsRps and 15 rpsrps F2 individuals were used to construct contrasting DNA bulks. Presumptive linkage (i.e., SSR marker polymorphism between two bulks) was confirmed or refuted by SSR assay of 15 to 40 F2 individuals within each homozygous class. Recombination values were maximum likelihood estimates from the SSR allelic segregation data of both classes, although the rpsrps class was less prone to phenotypic classification error. SSRs on linkage groups (LGs) N, J, F, and G were identified with linkages to Rps1, Rps2, Rps3, and Rps4, respectively. A skewed R:S segregation in the Rps5 population precluded detection of linked SSRs. The Rps6 locus, whose map position was heretofore unknown, was linked with three SSRs in a region of LG‐G that contains Rps4 and Rps5 SSR–Rps linkages of P < 0.05 could only be identified for the Rps1 alleles because of a paucity of SSR markers and/or parental monomorphism in the genomic regions surrounding other Rps loci.