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 locating Mendelian genes on a particular chromosome and for associating: a conventional 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 linkage groups with specific chromosomes. Primary trisomics isolated from the progenies of asynaptic and desynaptic mutants, male sterile lines, neutron-irradiated plants, and tissue culture-induced sterile mutants were backcrossed (BC) with soybean cv. Clark 63. They were identified at pachytene on the basis of the diagnostic landmarks such as association of the extra chromosome with its normal homologues in a trivalent configuration, distribution of 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 from similar primary trisomics exclusively showed 21 II and those from dissimilar 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 were tentatively identified and were designated as triple 1 through triple 20. Female transmission of the extra chromosome in 20 primary trisomics ranged from 27.3% (triplo 20) to 58.5% (triplo 9). On the basis of the modification ratio (17:1) of a gene located on the extra chromosome in primary trisomic 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 trisomics a universal cytogenetic map of soybean by associating existing classical and several molecular maps with the specific chromosomes.