Application of the near-isogenic line gene mapping technique to isozyme markers

Near-isogenic lines (NILs) have been proposed as a genetic resource that can be used to identify linkages between molecular and conventional markers. If the donor parent (DP) and the recurrent parent (RP) possess an allelic contrast for the molecular marker, and if the DP allele is still present in the NIL, then presumptive evidence of linkage between that molecular marker and the intro gressed gene can be inferred. The objective of this study was to evaluate this NIL genemapping technique using isozyme markers and a collection of 63 soybean [Glycine max (L.) Merr.] NILs. The electrophoretic isozyme banding patterns of eight enzymes (12 loci) were examined in 35 ‘Clark’ and 28 ‘Harosoy’ RP/NIL/DP trio sets, where each NIL possessed an introgressed conventional gene. Of the 756 combinations of 12 isozyme loci and 63 trio sets evaluated, 256 exhibited RP/DP isozyme allelic differences. Of these 256, seven (four in Harosoy, three in Clark, total of five conventional markers) exhibited an RP/NIL contrast and a corresponding NIL/DP equality for an isozyme allele. The five different presumptive linkages were: Enp-ln, Mpi-dt₂, Aco4-dt₂, Mpi-ln, and Pgi-d₂. The F₂ cosegregation data confirmed the presumed link-ages of Enp with In (9.38 ± 1.55%) and Mpi with dt₂ (16.07 ± 6.43%), but refuted the presumed linkages of Aco4-dt₂ and Mpi-In. The Enp locus is therefore part of Linkage Group 4, which is currently comprised of the v₁, In, and p₂ loci. Neither Mpi nor dt₂ have been assigned to an existing linkage group, and thus represent a new linkage group. The presumed linkage of Pgi with d₂ was not tested, but other data indicated that this presumption was probably erroneous. The confirmation of two of the five presumptive linkages was consistent with the numbers expected based on a previously published theoretical analysis of the NIL gene-mapping technique.