An Individual-Based Diploid Model Predicts Limited Conditions Under Which Stochastic Gene Expression Becomes Advantageous.

Tomotaka Matsumoto,Katsuhiko Mineta,Hitoshi Araki,Naoki Osada

doi:10.3389/fgene.2015.00336

Abstract

Recent studies suggest the existence of a stochasticity in gene expression (SGE) in many organisms, and its non-negligible effect on their phenotype and fitness. To date, however, how SGE affects the key parameters of population genetics are not well understood. SGE can increase the phenotypic variation and act as a load for individuals, if they are at the adaptive optimum in a stable environment. On the other hand, part of the phenotypic variation caused by SGE might become advantageous if individuals at the adaptive optimum become genetically less-adaptive, for example due to an environmental change. Furthermore, SGE of unimportant genes might have little or no fitness consequences. Thus, SGE can be advantageous, disadvantageous, or selectively neutral depending on its context. In addition, there might be a genetic basis that regulates magnitude of SGE, which is often referred to as “modifier genes,” but little is known about the conditions under which such an SGE-modifier gene evolves. In the present study, we conducted individual-based computer simulations to examine these conditions in a diploid model. In the simulations, we considered a single locus that determines organismal fitness for simplicity, and that SGE on the locus creates fitness variation in a stochastic manner. We also considered another locus that modifies the magnitude of SGE. Our results suggested that SGE was always deleterious in stable environments and increased the fixation probability of deleterious mutations in this model. Even under frequently changing environmental conditions, only very strong natural selection made SGE adaptive. These results suggest that the evolution of SGE-modifier genes requires strict balance among the strength of natural selection, magnitude of SGE, and frequency of environmental changes. However, the degree of dominance affected the condition under which SGE becomes advantageous, indicating a better opportunity for the evolution of SGE in different genetic models.

Highlights

The level of gene expression is one of the important factors which affects an individual’s fitness
We tested a two-locus model in which one locus is under viability selection and the other one regulates the magnitude of stochasticity in gene expression (SGE)
Our results suggest that SGE acting on the gene under viability selection causes deleterious effects under the stable environment in our model

Summary

INTRODUCTION

The level of gene expression is one of the important factors which affects an individual’s fitness. Recent studies suggested that for many organisms, a stochasticity in gene expression (SGE) varies phenotypes even across individuals in isogenic populations (Shahrezaei and Swain, 2008a). We considered two loci: first we constructed a simple population genetic model assuming that individual phenotype was basically determined by one locus. SGE may cause more advantageous effects when an individual’s basal genotype is highly disadvantageous To address this issue, we assumed viability selection with 0–1 fitness boundary in the present study. The fitness of each individual was determined by the genotype and the effect of SGE (see Viability Selection section) In this model, we assumed free recombination between loci, and we did not consider new mutation. We assume that each mating pair can make only one offspring per mating event

RESULTS

DISCUSSION

CONCLUSIONS