2000. Empirical evaluation of marker-assisted selection for improving quantitative traits in sweet corn

2000. Empirical evaluation of marker-assisted selection for improving quantitative traits in sweet corn

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INTRODUCTION

This thesis consists of two related experiments concerning the application of marker-assisted selection (MAS) in plant breeding programs. The first was conducted to compare progress over one cycle of selection in sweet com populations using MAS and phenotypic selection (PS). Single trait selection was conducted for seedling emergence, kernel sucrose content, and cooked kernel tenderness. Multiple trait selection was applied to simultaneously improve emergence/tendemess, emergence/hedonic, and emergence/sucrose/tendemess. From each base population (38 sulSel, 48 sulsel, and 117 sh2 F2:3 families), 20% of families were selected to generate Ci composites for single and multiple traits. In each population, 50% of the families were randomly selected to serve as control (Co). Selection progress was evaluated using the following criteria: selection gain (percent increase of Q over Co), divergence between high and low Ci composites, realized heritability, and cost efficiency.

Results indicated that, although using relatively small base population size particularly in sulSel and sulsel, MAS showed higher selection gains compared to PS. In single trait selection, MAS increased seedling emergence by 8.3% and 14.9% while PS provided 1.5% and 3.1% gains compared to C0 performance in the sulSel and sulsel Ci composites, respectively. In the sh2 population, while the MAS selected composites had higher emergence, the difference was not significantly greater than PS. Selection gains were significantly higher with MAS than PS for kernel sucrose concentration and cooked kernel tenderness. MAS and PS showed comparable selection gains among low-direction composite selections. Additional comparisons were conducted on combinations of three traits under selection where selection efficiency was reduced with increasing numbers of traits in both PS and MAS. The application of MAS resulted in greater cost-efficiency with eating quality traits which are difficult and expensive to measure using phenotypic evaluations. MAS also appeared to be appropriate for seedling emergence since the enhanced selection gain compensated for the lower costs of phenotypic evaluation.

In the second study, three chromosomal regions carrying QTL previously found to improve seedling emergence in a shrunken2 population were introgressed using marker-assisted backcrossing into three sweet com elite inbreds. Comparisons of field seedling emergence were made among 50 different BC2F1 families and Fi hybrids (controls) generated from crosses between the original inbreds. Significant effects associated with the beneficial QTL alleles were observed. Across the three populations, BC2F1 genotypes heterozygous for the beneficial QTL alleles linked to umcl39, bnl9.08, and php200689 displayed an average of 48.6%, 35.6%, and 36.2% increase in seedling emergence, respectively, over the Fi controls.

Results generated from this study suggest that incorporating DNA markers in breeding programs can expedite selection progress and therefore increase the economic return. Also, data suggest that beneficial QTL identified in one population can exert similar effects in other genetic backgrounds .