Relationship of molecular genetic distance of seven quality protein maize (Zea mays L.) inbred lines with specific combining ability and grain yield of hybrids
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Quality protein maize (QPM) with the opaque-2 gene has double the amount of the essential amino acids (tryptophan and lysine) as compared to normal endosperm maize but the gene is associated with undesirable agronomic traits. The aim of this study was to generate genetic information for use in developing breeding strategies for QPM in Southern Africa. Specific objectives were: i) to study the combining ability and type of gene action controlling yield in crosses among QPM inbred lines from Mexico and Southern Africa ii) to determine the relationship between combining ability for grain yield of QPM hybrids and molecular genetic distances between the parent inbred lines. Seven QPM inbred lines were crossed in a diallel mating design. The genetic distances between the maize inbred lines was quantified by using 62 simple sequence repeat (SSR) markers. The lines and the derived F1 hybrids were also evaluated for tryptophan and protein content. The F1 hybrids were evaluated in replicated trials at five locations. The quality of QPM inbred lines was variable with hybrids made up of high quality lines exhibiting high quality values. There was significant variation among the hybrids for yield and QPM traits. While both the general combining ability (GCA) and the specific combining ability (SCA) were significant, the GCA effects were found to be more important than SCA for yield in this study. QPM inbred line SC1 from Southern Africa had the highest GCA value and consistently appeared as one of the parents of the best yielding F1 hybrids. Preponderance of GCA effects indicated that additive gene effects were found to be more important in conferring high yield. Cluster analysis of the lines based on the SSR markers revealed five groups that were in conformity with pedigree information. The correlation between genetic similarity (GS) and SCA was not significant, low and negative (-0.035) suggesting low predictive value. As a result, use of SSR as a predictive tool for heterotic grouping in breeding maize hybrids should be confirmed by phenotypic data from the field evaluations.