Genetics, Vol. 177, 1815-1825, November 2007, Copyright © 2007
doi:10.1534/genetics.107.077537

The Role of Epistasis in the Manifestation of Heterosis: A Systems-Oriented Approach

A. E. Melchinger*,1, H. F. Utz*, H.-P. Piepho{dagger}, Z.-B. Zeng{ddagger} and C. C. Schön§

* Institute of Plant Breeding, Seed Science and Population Genetics, {dagger} Bioinformatics Unit, Institute of Crop Production and Grassland Research and § State Plant Breeding Institute, University of Hohenheim, 70599 Stuttgart, Germany and {ddagger} Program in Statistical Genetics, Department of Statistics, North Carolina State University, Raleigh, North Carolina 27695

1 Corresponding author: Institute of Plant Breeding, Seed Science, and Population Genetics, University of Hohenheim, Fruwirthstrasse 21, 70599 Stuttgart, Germany.
E-mail: melchinger{at}uni-hohenheim.de

Heterosis is widely used in breeding, but the genetic basis of this biological phenomenon has not been elucidated. We postulate that additive and dominance genetic effects as well as two-locus interactions estimated in classical QTL analyses are not sufficient for quantifying the contributions of QTL to heterosis. A general theoretical framework for determining the contributions of different types of genetic effects to heterosis was developed. Additive x additive epistatic interactions of individual loci with the entire genetic background were identified as a major component of midparent heterosis. On the basis of these findings we defined a new type of heterotic effect denoted as augmented dominance effect di* that comprises the dominance effect at each QTL minus half the sum of additive x additive interactions with all other QTL. We demonstrate that genotypic expectations of QTL effects obtained from analyses with the design III using testcrosses of recombinant inbred lines and composite-interval mapping precisely equal genotypic expectations of midparent heterosis, thus identifying genomic regions relevant for expression of heterosis. The theory for QTL mapping of multiple traits is extended to the simultaneous mapping of newly defined genetic effects to improve the power of QTL detection and distinguish between dominance and overdominance.


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