Five soybean [Glycine max (L.) Merr.] cultivars that have been used to differentiate strain groups of soybean mosaic virus (SMV) were studied to determine the allelic relationships among genes conditioning reaction to SMV. The cultivars PI 96983, ‘Ogden’, ‘York’, ‘Marshall’, and ‘Kwanggyo’ (PI 406710) are each known to have single dominant gene conditioning resistance to SMV. These parents were crossed in all possible combinations with each other and with a susceptible cultivar. The crosses were evaluated in the F2 and with F2‐derived F3 lines in the field and in the greenhouse for reaction to inoculation with the G1 strain of SMV (SMV‐G1). Results confirm that each of the resistant cultivars has a single dominant gene for resistance to SMV‐G1, but that dominance is sometimes incomplete. The lack of segregation for susceptibility in F2 and F3 progenies from the resistant ✕ resistant crosses indicates a high probability that the resistance genes in these cultivars are alleles at a common locus. Gene symbols Rsvy1, Rsvm1 and Rsvk1 are proposed for the resistance alleles in York, Marshall, and Kwanggyo, respectively. The systemic necrosis reaction that sometimes occurs following SMV‐G1 inoculation of segregating populations is highly associated with plants heterozygous for the resistance gene, but may be influenced by environment and genetic background.
Five soybean [Glycine max (L.) Merr.] cultivars, PI96983, ‘Ogden’, ‘York’, ‘Marshall’, and ‘Kwanggyo’, were studied to determine the inheritance of their resistant (R) or necrotic (N) reactions to soybean mosaic virus (SMV) strains G4, G5, G6, G7, and G7A. Each parent was crossed reciprocally with a susceptible (S) cultivar (‘Essex’ or ‘Lee 68’) to determine the number of genes for resistance or necrosis. The R parents were also crossed with each other to test the allelism of the genes conditioning the R or N reaction. All the F2 populations from N × S crosses segregated in a 3:1 ratio with necrosis being dominant to susceptibility. The absence of S segregants in F2 populations of N × N and R × R crosses and the lack of segregation in S × S crosses indicate that the single dominant genes in the five non‐susceptible parents are alleles at a common locus. All R × S crosses produced a combination of R, N, and S plants and gave a good fit to a 3(R + N):1S ratio. Two different SMV‐strain × resistance‐gene interactions were observed to produce the N reaction: (i) alleles which are necrotic to specific strains in the homozygous state are dominant to alleles which are resistant or susceptible to the same strain, (ii) alleles which are resistant in the homozygous state to a strain often exhibit necrosis when they occur in a heterozygote with a susceptible allele.
The gene symbol Rsv2 was previously assigned to the gene in the soybean [Glycine max (L.) Merr.] line OX670 for resistance to soybean mosaic virus (SMV). The Rsv2 gene was reported to be derived from the Raiden soybean (PI 360844) and to be independent of Rsv1. Accumulated data from our genetic experiments were in disagreement with this conclusion. In this study, Raiden and L88-8431, a Williams BC5 isoline with SMV resistance derived from Raiden, were crossed with two SMV-susceptible cultivars to investigate the mode of inheritance of SMV resistance in Raiden. They were also crossed with five resistant cultivars to examine the allelomorphic relationships of the Raiden gene with other reported genes at the Rsv1 locus. F1 plants, F2 populations, and F2-derived F3 (F2:3) lines were tested with SMV strains G1 or G7 in the greenhouse or in the field. The individual plant reactions were classified as resistant (R, symptomless), necrotic (N, systemic necrosis), or susceptible (S, mosaic). The F2 populations from R x S crosses segregated in a ratio of 3 (R + N):1 S and the F2:3 lines from Lee 68 (S) x Raiden (R) exhibited a segregation pattern of 1 (all R):2 segregating:1 (all S). The F2 populations and F2:3 progenies from all R x R crosses did not show any segregation for susceptibility. These results demonstrate that the resistance to SMV in Raiden and L88-8431 is controlled by a single dominant gene and the gene is allelic to Rsv1. The heterozygous plants from R x S and R x N crosses exhibited systemic necrosis when inoculated with SMV G7, indicating a partial dominance nature of the resistance gene. Raiden and L88-8431 are both resistant to SMV G1-G4 and G7, but necrotic to G5, G6, and G7A. Since the resistance gene in Raiden is clearly an allele at the Rsv1 locus and it exhibits a unique reaction to the SMV strain groups, assignment of a new gene symbol, Rsv1-r, to replace Rsv2 would seem appropriate. Further research is ongoing to investigate the possible existence of the Rsv2 locus in OX670 and its relatives.
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