Soybean mosaic virus (SMV), a potyvirus detected worldwide, can cause serious diseases in soybean (Glycine max L. Merr.). Host resistance to SMV conferred by a single dominant gene, Rsvl, was studied as a model to gain insights of plant virus resistance genes, and to facilitate the breeding of resistant cultivars. DNA restriction fragment length polymorphisms (RFLPs) and microsatellites (or simple sequence repeats, SSRs) were used as genetic markers to identify the chromosomal location of Rsvl1 in a cross between PI 96983 (resistant) and a susceptible cultivar. Twenty five RFLP and three SSR loci polymorphic between the parental lines were analyzed in 107 F, individuals. Genotypes of Rsv1 were determined by inoculating F2.3 progeny with SMV-G1. Genetic analysis revealed that one SSR (HSP176L) and two RFLP (pA186 and pK644a) markers are closely linked to Rsv1, with a distance of 0.5, 1.5, and 2.1 cM, respectively. The tight linkages of the three markers to Rsv1 were confirmed by SSR and RFLP analysis of three near isogenic lines (NILs) of Rsv1 derived from PI 96983 or Marshall.

The three Rsv1-linked markers were then used to screen 67 diverse soybean types. These marker loci showed a remarkably high level of polymorphism, indicating a possible association between disease resistance and rapid sequence divergence. At each Rsv1-linked marker locus, one SSR allele or RFLP haplotype is highly correlated with SMV resistance. These resistance markers, especially the SSR allele at HSP176L which can be detected by the polymerase chain reaction (PCR), may be useful for germplasm screening. The grouping of the 67 accessions according to their Rsv1-linked multilocus marker haplotypes agrees with available pedigree information. A set of differential cultivars known to contain Rsv1 clustered into putative Rsv1- carrying groups. Based on molecular marker analysis and previous inheritance studies, 37 of the 45 resistance accessions probably derive their resistance from Rsv1. The remaining eight accessions include Columbia (Rsv3), and the other potentially diverse resistance sources.

A heat shock protein (HSP) multigene family, HSP176L included, was analyzed for its positional proximity to the Rsv1 gene cluster. A technique termed amplified sequence length polymorphism (ASLP) was developed to convert known DNA sequences to PCR-based genetic markers. Among six pairs of HSP primers used, two (HSP175E and 185C) detected ASLPs between the parents, and segregated in the F₂ population with a size of 174. HSP175E was found to be closely-linked (0.7 cM) to HSP176L, both of which are Class I small HSP genes. HSP185C, however, was mapped to a different linkage group, suggesting that it may belong to another family. ADR11, a member of auxin down-regulated (ADR) multigene family, is known to be linked to HSP173B, also a Class I gene but not mappable in this population. ASLP analysis of ADR11 in a set of Rsv1 NILs indicates that it is linked to Rsv1, and ADR11 co-segregates with HSP175E in the F, population. Thus, the Class I small HSP multigene family including HSP176L, 175E, and 173B, and possibly a family of ADR genes, is located near the Rsvi resistance gene cluster.