Use of 4x–2x crosses to determine gene–centromere map distances of isozyme loci in Solanum species

With the meiotic abnormality described as the parallel spindles mechanism operating during anaphase II, first division type restitution 2n gametes (FDR) were identified in selected diploid Solanum clones. Generating 4x–2x FDR testcrosses of the general form aaaa × Aa with these diploid parents provided a means of estimating gene–centromere linkages with the application of half-tetrad analysis to the segregating tetraploid progenies. In this study, gene–centromere map distances were obtained for 10 isozyme loci and the yellow tuber flesh locus (Y). Electrophoretic assay of the tetraploid progenies yielded pooled gene–centromere linkage estimates for Got-1 (0.9 map units, mu), Pgm-2 (2.0 mu), Sdh-1 (8.3 mu), Aps-1 (13.5 mu), Adh-1 (15.8 mu), Prx-3 (18.0 mu), Idh-1 (18.4 mu), Pgi-1 (26.0 mu), 6-Pgdh-3 (30.1 mu), Mdh-1 (33.5 mu), and y (16.8 mu). These linkage data were combined with previously reported diploid testcross linkage data to show that these 11 markers are distributed over at least 8 of the 24 chromosome arms in the potato genome. In addition, gene order for the Idh-1/Sdh-1 linkage block (36.8 mu) was predicted to be of the sequence Id1-1 –centromere–Sdh-1, which is similarly found in Lycopersicon. Because of the codominant nature of these enzyme-coding loci, the theoretical expectations for 4x–2x FDR segregation were tested in this crop. Tests for reciprocity of exchange products of pooled data for 6-Pgdh-3, Mdh-1, Pgi-1, Adh-1, and Sdh-1 families yielded good fits to the expected 1:1 ratio. With these defined electrophoretic markers available, more critical genetic analyses can be extended to a number of genetic and breeding problems concerning the potato. Key words: half-tetrad analysis, 4x–2x, isozymes, Solanum, linkages.