Diversity, idenification, mapping and pyramiding of genes for resistance to soybean mosaic virus [Glycine max (L.) Merr.]

Soybean Mosaic Virus (SMV) is a prevalent viral pathogen transmitted by aphids and via seed, causing significant yield loss and quality reduction. There are seven SMV strain groups (G1 to G7) and three resistance loci( Rsv1, Rsv3, and Rsv4) reported in soybean. New sources of SMV resistance would be valuable for breeding cultivars with durable resistance to multiple strains. The objectives of this research were to analyze genetic diversity of SMV-resistant soybean germplasm at the molecular level; to pyramid three genes from different sources for durable SMV resistance; and to identify and map new genes/alleles for differential reactions to SMV strains. One hundred and fourteen SMV-resistant germplasm genotypes collected worldwide were screened with 98 simple sequence repeat (SSR) markers. The 114 germplasm genotypes were grouped into four clusters with the largest group containing most of Asian genotypes. Rsv1 alleles were found to be most common, while Rsv3 and Rsv4 were rare among all the SMV resistant germplasm tested. PI 96983 (Rsv1) was crossed with Columbia (Rsv3, Rsv4) to pyramid all three SMV resistance genes with the aid of SSR markers. The molecular screening of a F2 plant population derived from PI 96983 x Columbia showed good fit to the expected genotypic ratios at each marker locus; one out of 70 plants was identified to contain three resistance genes in homozygous condition (Rsv1+Rsv3+Rsv4), while 26 additional plants also contained three genes but at the heterozygous condition. F2:3 lines inoculated with G1 and G7 strains confirmed the presence of specific alleles and SMV resistance. A new SMV resistant parent PI 61944 with unknown gene(s) (Rsv-?) was crossed with the susceptible parent Essex (rsv), and resistant lines PI 96983 (Rsv1), L29 (Rsv3), and V94-5152 (Rsv4) to investigate the inheritance and allelism of SMV resistance. F2 plants were screened with selected SSR markers, and the F2:3 lines were screened with SMV G1 and G7. SMV screening and molecular analysis revealed that the PI 61944 carries a new allele at the Rsv3 locus on linkage group (LG) B2. This research has identified genetic diversity among SMV-resistant soybean germplasm, combined three genes for durable resistance in soybean, and discovered a new SMV resistance allele. These germplasm will be useful in soybean breeding programs where SMV resistance is an objective. This research also demonstrated that SSR technology is extremely useful in marker assisted selection (MAS).