Dominance and multi-locus interaction.

The century-old debate about the origin and evolution of non-additive effects (dominance and epistasis) and their roles in adaptation and speciation is far from settled. Dominance describes the observed relative difference in fitness or phenotype between heterozygotes and the average of homozygotes at a given polymorphic locus. Emerging empirical evidence supports and illustrates how the observed dominance at a locus can vary as a result of multi-locus interactions. Wade proposed a model that explains variable dominance at a focal locus through the change in the frequency of dominance-modifying alleles at other loci. Fitness landscape models that integrate epistasis and dominance may serve to quantify the consequences of variable dominance for adaptation and speciation. Dominance is usually considered a constant value that describes the relative difference in fitness or phenotype between heterozygotes and the average of homozygotes at a focal polymorphic locus. However, the observed dominance can vary with the genetic background of the focal locus. Here, alleles at other loci modify the observed phenotype through position effects or dominance modifiers that are sometimes associated with pathogen resistance, lineage, sex, or mating type. Theoretical models have illustrated how variable dominance appears in the context of multi-locus interaction (epistasis). Here, we review empirical evidence for variable dominance and how the observed patterns may be captured by proposed epistatic models. We highlight how integrating epistasis and dominance is crucial for comprehensively understanding adaptation and speciation. [ABSTRACT FROM AUTHOR]