Characterization of a yield quantitative trait locus on chromosome five of maize by fine mapping

In an earlier study for identifying quantitative trait loci (QTLs) in a maize (Zea mays L.) population generated from the cross B73 x Mo17, a major effect on grain yield and yield related traits was detected on chromosome 5. This chromosomal region has also shown significant associations with grain yield in several other studies. These findings have, thus, provided the impetus to further characterize this segment. A set of BC2S1 lines was created, each containing an introgressed segment of Mo17 in a B73 background. A reciprocal set of lines, each with a B73 donor segment in a Mo17 background, also was created. These BC2S1 lines were genotyped by means of 16 restriction fragment length polymorphism (RFLP) and two isozyme markers that mapped to the targeted region on chromosome 5. From field data based on testcrosses of these lines, this one large region on chromosome 5 was dissected into at least two smaller QTLs. Effects at these two QTLs appear to act in a dominant manner, each showing significance in one testcross but not the other. These genetic factors are in repulsion phase linkage and their effects support the dominance theory of heterosis. One other segment in this region on chromosome 5 showed a significant association with yield, but it was not consistently expressed and may be spurious. The largest of these three segments has been mapped to a 27.5-centimorgan (cM) interval near Amp3. If the observed results are indicative of the true complexity associated with QTLs having large effects, marker-aided breeding involving such regions could be difficult, particularly if the marker-aided breeding is based on early generation (backcross, F-2, or F-3) data, where the intricate nature of a region cannot be resolved.