role of gene interactions - Visser et al 2011 Flashcards
define epistasis (gene interactions)
> 100 yrs ago, Bateson…genes can suppress the phenotypic effects of other genes; most phenotypes results from intricate gene interactions - basically deviations from additive genetic effects on the phenotype;
deviations from expected additive effects on quantitative traits of alleles occurring at the same (dominance) or different loci - Fisher
why do we study epistasis?
systems and developmental biologists use is to understand the genotype-phenotype map;
evolutionary biologists use it to understand evolution
what might different forms of epistasis lead to?
divergence and speciation;
evolutionary benefits to sex;
affect the robustness and evolvability of organisms
possible causes of epistasis?
proximate (mechanistic) causes eg protein function and metabolic networks;
ultimate (evolutionary) factors eg mutation, recombination, the importance of natural selection and genetic drift
what’s thought to be a prerequisite for epistasis?
pleiotropy
how is epistasis thought to evolve?
epistasis may evolve as an adaptive or intrinsic consequence of changes in genetic robustness and evolvability; gene interactions thought to be shaped by natural selection and genetic drift.
Distinguish between unidimensional and multidimensional epistasis
unidimensional epistasis refers to deviations from a linear relationship between mean log fitness and the number of alleles affecting fitness; aka directional or mean epistasis; can be positive or negative depending on whether the fitness of genotypes carrying multiple mutations is higher or lower than expected from independent effects, respectively. Antagonistic epistasis among deleterious mutations, and synergisitc epistasis among beneficial mutations represent positive epistasis, whereas the opposite situations represent negative epistasis.
Multidimensional epistasis refers to the individual interactions among a given set of alleles - it provides a more complete description of the interactions within a fitness landscape involving these alleles incl. variation of epistasis among pairs of alleles, no. of fitness maxima, measures of accessibility of particular genotypes and pathways; evidence comes from studies of pairwise interactions among deleterious mutations at a genome-wide scale; other evidence comes from the study of all possible interactions among a given set of, often beneficial, mutations - gives details for section of a fitness landscape for a given envt.
Distinguish between magnitude and sign epistasis
Magnitude epistasis refers to interactions where the combined effect of two alleles deviates from multiplicative effects, but in a way that does not change the sign of either allele’s fitness effect;
sign epistasis refers to ‘stronger’ interactions, where the sign of an allele’s contribution to fitness changes with genetic background
what causes epistasis?
Epistasis results from the way in which genetic elements interact with each other in their ‘causation’ of a phenotype and, ultimately, fitness.
(a) metabolic models
(b) pleiotropy as a precondition for epistasis
(c) evolvability
Distinguish between intra-gene epistasis and inter-gene epistasis
intra-gene epistasis may result from non-independent effects of mutations on RNA stability and enzyme activity;
inter-gene epistasis may result from protein interactions and the structure of metabolic networks.
