pleiotropy - senecio7の日記

Pavlicev M & Hansen TF 2011 Genotype-phenotype maps maximizing evolvability: modularity revisited. Evol Biol 38:371-389.

modularization of pleiotropy has been suggested to promote evolvability by restricting genetic covariance among unrelated characters and reducing constraints due to correlated response
modularity may also reduce total genetic variation of characters
maximal evolvability occurs by maximizing genetic variance and minimizing genetic covariance
this does not necessarily require modularity, only patterns of pleiotropy that cancel on average

the advantage of modular genotype-phenotype maps is twofold
first, if pleiotropic effects of most genes are restricted to few traits, the risk of deleterious effects of a mutation is reduced (modularity)
second, if the traits affected by a gene are developmentally or functionally related, a mutation may preserve functionality of this integrated module during evolutionary change (integration)
Hansen (2003) pointed out that modularity in the form of reduced pleiotropy might also hamper evolvability by reducing the potential for genetic variance per trait
below a certain level of pleiotropy, the variational independence gained does not compensate for the genetic variability lost due to smaller genetic basis
while nonzero genetic correlation is usually a sign of pleiotropy, the lack of genetic correlation does not require a lack of pleiotropy
the absence of pleiotropy is not the only way of maintaining high evolvability
if there are no constraints on the type of pleiotropy that can occur, evolvability is generally maximized by maps that tend to generate zero genetic correlation either through modularity, or through patterns of pleiotropy that cancel out

equal variances across underlying loci, the reduction of pleiotropy is not an essential feature of genotype-phenotype maps that maximize evolvability, as measured by the amount of independent variance available to selection per character
in the case of the character model, modular genotype-phenotype maps even result in lower evolvability than the fully pleiotropic maps

a low degree of pleiotropy (i.e., modularity) is not a universal characteristic of the genotype-phenotype maps that maximize evolvability
if there is no constraining selection and all genes have equal variation, the highest evolvability is achieved when all genes affect all characters
the most evolvable maps are to be found among those that can generate a diagonal G-matrix in which all genetic covariances are zero
modular genotype-phenotype maps can achieve this, but so can patterns of variable pleiotropy that cancels out on average (hidden pleiotropy)
this maximizes the amount of genetic variation along any specific direction in morphospace
if there is constraining selection, however, the picture is more complex

pleiotropy has consequences that are not apparent in the multivariate single-generation response to selection
one such consequence is on the stability of genetic correlation over time
the ultimate genetic covariance matrix, determining the response to selection, is determined by the structure of genotype-phenotype map and by the allele frequencies
non-zero covariances could easily evolve from an initially hidden-pleiotropic structure
allele-frequency changes in presence of hidden pleiotropy may therefore cause instability of the G-matrix in the long term
another potential effect of pleiotropy that escapes the model used here involves epistasis
the B matrix represents a linear genotype-phenotype map
epistasis is a common finding in empirical studies across species
modeling the effects of epistasis on evolvability has been restricted to the univariate case
when epistasis affects pleiotropic genes, it can change variances and covariances of traits in a variety of ways, depending on exactly how gene substitutions modify the effects of each other

the effect of pleiotropy on evolution has also been addressed using approaches based on Fisher's (1958) geometric model
they predominantly focus on the evolvability of a single trait (rather than including the constraining trait)
the geometric approach assumes that the rate of evolution depends on the new arising mutational variation
quantitative genetic models such as the present model, assume that the rate of evolution primarily depends on selection on existing variation
the conclusions of the models are not directly comparable
technically, both situations can be seen as simplifications, representing opposing extremes on a continuum combining arising and extant variation