Near‐Isogenic Lines—A Potential Resource in the Integration of Conventional and Molecular Marker Linkage Maps

Abstract

Near‐isogenic lines (NILs) have been developed in a number crop species via the backcross breeding method. These NILs possess remnant donor parent (DP) DNA, some of which is present in the chromosomal segment linked to and surrounding the introgressed marker. If this DP‐derived DNA contains molecular marker loci (i.e., isozyme variants or restriction fragment length polymorphisms) for which a recurrent parent (RP) and DP possessed allelic contrasts, then DP‐derived molecular alleles may still be present in an NIL and could be detected a posteriori by molecular genetic analysis of specific trio sets of RP/NIL/DP lines. Given this proposition, our objective was to provide a theoretical basis for evaluating the results of such analyses. Using equations derived by previous workers, we calculated that about four of 100 randomly chosen DP‐derived molecular markers would be expected to be retained in a BC 5 S 1 NIL constructed in a hypothetical species with 20 chromosomes of equivalent 50 centimorgan (c M ) genetic map lengths. Of these four markers, two or three would be expected to be located on the introgressed marker chromosome. This suggests that a near‐isogenic gene mapping technique would provide a means for subsetting molecular and introgressed markers into presumptive linkage groups. Such a priori information would make traditional methods of integrating both marker types into a single linkage map a much more efficient undertaking. A combined linkage map would be a valuable resource relative to the goal of eventually cloning some of the conventional markers of scientific interest or economic value.