Submitted to: Plant Breeding Reviews
Publication Type: Book / Chapter
Publication Acceptance Date: 10/20/2000
Publication Date: N/A
Interpretive Summary: 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. I reviewed the interaction of genes and gene products at the molecular level and the empirical evidence regarding the importance of epistasis for important agronomic traits in crops. The possible connections and disconnects between molecular-level gene interactions and phenotypically observable gene interactions are highlighted. Modern methods of genetic analysis using DNA markers has provided good 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 argue that by ignoring epistasis, we may limit long term gains from selection.
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 epistatically interacting loci, complicating the picture of gene action. A seemingly "favorable" allele at one locus may be an "unfavorable" allele in a different genetic background. There are well-defined cases of interactions occurring at the molecular level between gene products, but the relationship between molecular interactions and phenotypic interactions is often not clear. Classical quantitative genetics methods relate observable phenotypic measures to the aggregate statistical effects of alleles and allelic combinations in specific populations. Genetic components of variance 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 gene action is more important for plant yield and fitness than was previously evident. Implications of strong epistasis for plant breeding include: (1) epistatic variance can shift to additive variance under drift or inbreeding; (2) epistatic variance contributes to "temporary" response to selection in outcrossing populations which can be captured as a form of heterosis using appropriate breeding procedures, but may be otherwise squandered; and (3) fitness or yield is not a simple function of allele frequencies; resulting rugged adaptive landscapes are filled with local fitness optima on which breeding populations can become stranded. If epistasis is important, then marker-assisted selection schemes should be designed to exploit it.