Identification of five new primary simple trisomies in soybean based on pachytene chromosome analysis

Primary trisomics are ideal cytogenetic tools for associating genes and linkage groups to known chromosomes and testing their independence. In the cultivated soybean, only 8 of the possible 20 primary simple trisomics are known. In this report cytological evidence for the identification of five more new primary simple trisomics, corresponding to chromosomes 6, 8, 12, 16, and 19, is presented for the first time. The precise identification was based on trivalent configuration of chromosomes at the pachynema stage of meiosis, where the chromosomes were identified by their characteristic total length, arm ratio, and distribution of heterochromatin and euchromatin. Cytological observation of chromosome pairing in the 2n = 42 chromosome F1 plants, obtained from eight crosses between known primary trisomics, also supported the identification of primary trisomics in soybean based on pachytene chromosome analysis. Together with the eight primary trisomics identified previously, 13 of the possible 20 primary simple trisomics have been successfully identified, which accounts for about 76% of the total nuclear euchromatin in soybean.