Primary trisomics in soybean: Origin, identification, breeding behavior, and use in linkage mapping

A primary trisomic (2n = 2x + 1) is an excellent cytogenetic tool for locat ing Mendelian genes on a particular chromosome and for associating: a conve ntional genetic linkage group with a specific chromosome. The objectives of this study were to produce and identify 20 primary trisomics (2n = 2x + 1 = 41) of soybean [Glycine mar (L,) Merr.] and use them for associating link age groups with specific chromosomes. Primary trisomics isolated from the p rogenies of asynaptic and desynaptic mutants, male sterile lines, neutron-i rradiated plants, and tissue culture-induced sterile mutants were backcross ed (BC) with soybean cv. Clark 63. They were identified at pachytene on the basis of the diagnostic landmarks such as association of the extra chromos ome with its normal homologues in a trivalent configuration, distribution o f euchromatin and heterochromatin, total chromosome length, arm ratios, and by examining chromosome pairing at metaphase I in F-1 plants with 2n = 42 chromosomes isolated from crosses among primary trisomics. The F1 plants fr om similar primary trisomics exclusively showed 21 II and those from dissim ilar primary trisomics exhibited microsporocytes with 20 II + 2 I, 1 III 19 II + 1 I, and 2 III + 18 II configurations. Thus, 20 primary trisomics w ere tentatively identified and were designated as triple 1 through triple 2 0. Female transmission of the extra chromosome in 20 primary trisomics rang ed from 27.3% (triplo 20) to 58.5% (triplo 9). On the basis of the modifica tion ratio (17:1) of a gene located on the extra chromosome in primary tris omic analysis, three marker genes Eul (seed urease), Lx1 (lipoxygenase-1), and P2 (puberulent) were located on chromosomes 5, 13, and 20, respectively , The ultimate aim of this project is to develop by means of primary trisom ics a universal cytogenetic map of soybean by associating existing classica l and several molecular maps with the specific chromosomes.