Phenotypic Diversity Using Bimodal and Unimodal Expression of Stress Response Proteins

Abstract

Populations of cells need to express proteins to survive the sudden appearance of stressors. However, these mechanisms may be taxing. Populations can introduce diversity, allowing individual cells to stochastically switch between fast-growing and stress-tolerant states. One way to achieve this is to use genetic networks coupled with noise to generate bimodal distributions with two distinct subpopulations, each adapted to a stress condition. Another survival strategy is to rely on random fluctuations in gene expression to produce continuous, unimodal distributions of the stress response protein. To quantify the environmental conditions where bimodal versus unimodal expression is beneficial, we used a differential evolution algorithm to evolve optimal distributions of stress response proteins given environments with sudden fluctuations between low and high stress. We found that bimodality evolved for a large range of environmental conditions. However, we asked whether these findings were an artifact of considering two well-defined stress environments (low and high stress). As noise in the environment increases, or when there is an intermediate environment (medium stress), the benefits of bimodality decrease. Our results indicate that under realistic conditions, a continuum of resistance phenotypes generated through a unimodal distribution is sufficient to ensure survival without a high cost to the population.

Exploration-exploitation tradeoffs dictate the optimal distributions of phenotypes for populations subject to fitness fluctuations

“…On the other hand, under the more complex scenario in which the coupling strength varies randomly (rand-x case), the exploration-exploitation tradeoff leads to a non-zero optimal search rate and unimodal phenotypic distributions are generically preferred. This picture is in complete agreement with the results obtained in [29], where the theoretical benefit of a bimodal distribution of stress response proteins was found to be highest in two-state environments, while more variable and structured environments allow for the selection of unimodal distributions. In addition, we have found that adding a small amount of diffusion to a purely exploitative strategy always leads to an increase of fitness in randx environments, while it is always detrimental in const-x environments.…”

“…Such models capture the physical and mathematical complexity of these systems starting from minimal assumptions about the environment and/or the space of feasible phenotypes. In more structured cases, the spectrum of viable behaviors appears to be even richer [29].…”

The Growth Promotion of Peppers (Capsicum annuum L.) by Trichoderma guizhouense NJAU4742-Based Biological Organic Fertilizer: Possible Role of Increasing Nutrient Availabilities

“…This result might have been due to the application of the amino acids which were used by microbes as a food source. The slaughterhouse wastes’ hydrolysates contained abundant free amino acids which provided rich nutrients for NJAU4742, while the adversity from the low pH stress also resulted in an obvious promotion of sporulation [ 31 ]. Jian et al [ 32 ] noted that the evolutions of fungal colonies, spore formation and metabolic activity were significantly affected by nutrition, oxygen, pH and temperature during SSF.…”

Genome heterogeneity drives the evolution of species

“…Since mating is not taken into consideration, we will not look for speciation according to Mayr's biology speciation concept, where different species are separated if mating does not produce fertile offsprings [5]. Instead, we speak of speciation in terms of differences in phenotype distributions [44], where well-separated peaks can be interpreted as different species while the width of each peak accounts for the intraspecies differentiation.…”