Primary trisomics and SSR markers as tools to associate chromosomes with linkage groups in soybean

Primary trisomics provide an excellent cytogenetic tool to associate genes and linkage groups with their respective chromosomes. A complete set of 20 primary trisomics (2x + 1 = 41) has been established in soybean [Glycine max (L.) Merr.]. A linkage map of soybean with 20 consensus linkage groups has recently been defined. Because simple sequence repeat (SSR) markers map to defined single positions in the soybean genome, the association of a SSR locus with a chromosome will provide an unambiguous association of a linkage group to a specific chromosome. The objective of this work was to demonstrate the use of SSR markers to associate linkage groups with chromosomes by means of primary trisomics. One population of F-2 plants was developed from an F-1 hybrid trisomic for chromosome 13 (Triplo 13) and a second F-2 population was obtained from a F-1 hybrid trisomic for chromosome 5 (Triplo 5). Polymorphic SSR markers from different consensus linkage groups were tested on a subset of 20 plants from each population to identify markers that appeared to show segregation that deviated from normal (1:2:1) disomic inheritance. Markers not associated with the specific chromosome segregated in a disomic (1:2:1) fashion. Markers identified in this manner were further examined in the complete population of F-2 plants to identify those that demonstrated trisomic segregation (6:11:1). By this approach, Triplo 13 was associated with linkage group F and Triplo 5 with linkage group A1. This result was verified by the examination of seven SSR loci on linkage group F and eight loci from linkage group A1 with each showing trisomic segregation with the Triplo 13- and Triplo 5-derived F-2 populations, respectively. These results demonstrate the first association of molecular linkage groups with chromosomes in soybean and indicate that SSR markers provide a tool to associate the remaining 18 trisomics with their respective linkage groups.