Abstract
Robustness and plasticity are essential features that allow biological systems to cope with complex and variable environments. In a constant environment, robustness, i.e., insensitivity of phenotypes, is expected to increase, whereas plasticity, i.e., the changeability of phenotypes, tends to diminish. Under a variable environment, existence of plasticity will be relevant. The robustness and plasticity, on the other hand, are related to phenotypic variances. As phenotypic variances decrease with the increase in robustness to perturbations, they are expected to decrease through the evolution. However, in nature, phenotypic fluctuation is preserved to a certain degree. One possible cause for this is environmental variation, where one of the most important "environmental" factors will be inter-species interactions. As a first step toward investigating phenotypic fluctuation in response to an inter-species interaction, we present the study of a simple two-species system that comprises hosts and parasites. Hosts are expected to evolve to achieve a phenotype that optimizes fitness. Then, the robustness of the corresponding phenotype will be increased by reducing phenotypic fluctuations. Conversely, plasticity tends to evolve to avoid certain phenotypes that are attacked by parasites. By using a dynamic model of gene expression for the host, we investigate the evolution of the genotype-phenotype map and of phenotypic variances. If the host-parasite interaction is weak, the fittest phenotype of the host evolves to reduce phenotypic variances. In contrast, if there exists a sufficient degree of interaction, the phenotypic variances of hosts increase to escape parasite attacks. For the latter case, we found two strategies: if the noise in the stochastic gene expression is below a certain threshold, the phenotypic variance increases via genetic diversification, whereas above this threshold, it is increased mediated by noise-induced phenotypic fluctuation. We examine how the increase in the phenotypic variances caused by parasite interactions influences the growth rate of a single host, and observed a trade-off between the two. Our results help elucidate the roles played by noise and genetic mutations in the evolution of phenotypic fluctuation and robustness in response to host-parasite interactions.
Highlights
Robustness and plasticity are two important properties of biological systems
As a possible origin for the remnant phenotypic variance, we investigated the role of host–parasite interactions such as those between bacteria and phages
Through numerical simulations of the evolution of the host genotype–phenotype mapping, we found that hosts increase phenotypic variation by increasing phenotypic fluctuations if the interaction is sufficiently strong
Summary
Robustness and plasticity are two important properties of biological systems. To maintain function and high fitness against internal noise, environmental variation, and genetic changes, the fitted state must be robust to such disturbance, whereas the phenotype needs to possess plasticity to adapt to environmental variation. The evolution of robustness and plasticity has been investigated extensively both theoretically and experimentally [1,2,3,4,5] In this context, phenotypes are shaped by dynamic processes involving several genetically determined variables, as well as external and internal noise. The dynamical process varies by the genetic changes, so that there exists phenotypic fluctuations by the genetic variation [11,12,13]. The sum of both sources of phenotypic variation defines the genotype–phenotype mapping [14,15,16]. The evolution of genotype– phenotype mapping, is essential to understand the evolution of robustness and plasticity, and has been explored extensively both in numerical [4, 17] and laboratory experiments [18, 19]
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