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 max (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 4 ) 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 F 1 plants from similar primary trisomics exclusively showed 21 II and those from dissimilar primary trisomics exhibited microsporocytes with 20 II + 2I, 1 III + 19 II + 1I, and 2 III + 18 II configurations. Thus, 20 primary trisomics were tentatively identified and were designated as triplo 1 through triplo 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.