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Submitted to: The Alfalfa Genome Conference
Publication Type: Proceedings
Publication Acceptance Date: 2/22/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: I reviewed the empirical evidence regarding the importance of epistasis for important agronomic traits in crops. Epistasis occurs when the effect of one gene is dependent on other genes, such as occurs when two genes are required to complete a biochemical pathway. Modern methods of genetic analysis using DNA markers has provided god evidence that epistasis is of general importance and cannot be assumed to be negligible. Plant breeders have generally ignored epistasis when planning breeding procedures, and I show that by ignoring epistasis, we may limit long term gains from selection. Specific recommendations are made for alfalfa breeders on how to measure epistasis and how to exploit epistasis with appropriate breeding methods.

Technical Abstract: Epistasis is the interaction of alleles at different loci. The value of an allele or genotype at one locus depends on the genotype at other epistatica interacting loci, complicating the picture of gene action. A seemingly "favorable" allele at one locus may be an "unfavorable" allele in a differe genetic background. Classical quantitative genetics methods relate observa aphenotypic measures to the aggregate statistical effects of alleles and allelic combinations in specific populations. Genetic components of varian are population-dependent, often poorly estimated, and do not necessarily reflect the relative importance of different modes of gene action. DNA markers have simplified the direct estimation of gene action effects, and recent QTL and population genetics studies have revealed that epistatic gen action is more important for plant yield and fitness than was previously evident. Implications of strong epistasis for plant breeding include: (1) fitness or yield is not a simple function of allele frequencies; resulting rugged adaptive landscapes are filled with local fitness optima that breedi populations can get "stuck on"; (2) epistatic variance can shift to additiv variance under drift or inbreeding; (3) epistatic variance contributes to "temporary" response to selection which can be captured as heterosis using special breeding procedures. Alfalfa breeding methods that emphasize phenotypic recurrent selection in broad-based populations have the least chance of exploiting epistasis for yield gain.