Table 3.

Types of epistasis, compared to a multiplicative model*.

effects of mutations on a trait relative to wild-type value W			synergistic epistasis	antagonistic epistasis	meaning
WAB/W	WA/W	WB/W
<1	<1	<1	ε < 0	ε > 0	only magnitude of epistasis is affected
<1	<1	>1	ε < 0	ε > 0	not all mutational paths decrease the trait
<1	>1	>1	ε < 0		multiple valleys or ‘compensatory’ mutations
signs of mutational effects change the mapping of positive/negative epistasis
>1	<1	<1	ε > 0		multiple peaks or ‘compensatory’ mutations
>1	>1	<1	ε > 0	ε < 0	not all mutational paths increase the trait
>1	>1	>1	ε > 0	ε < 0	only magnitude of epistasis is affected

*ε < 0 is negative epistasis, ε > 0 is positive epistasis. For intuitive explanations see text. Synergistic epistasis is also known as ‘aggravating epistasis’ and corresponding mutations can be called ‘enhancers’, ‘synthetic interactions’ or ‘synthetically sick’. Antagonistic epistasis is also termed ‘buffering epistasis’, ‘diminishing returns’ and corresponding mutations can be called ‘partial suppressors’ and ‘alleviating’ (Segre et al. 2005). Epistasis that generates fitness valleys is also known as ‘compensatory epistasis’ (Phillips 2008), ‘multiple peaks’ (Weinreich et al. 2005) or ‘reciprocal sign epistasis’ (Poelwijk et al. 2007).