Evolution of Phenotypic Plasticity under Host-Parasite Interactions
Authors: Nishiura Naoto , Kaneko Kunihiko 
Published: January 2021

Robustness and plasticity are essential features that allow biological systems to cope with complex and variable environments. Through the evolution of a given environment, the former, the insensitivity of phenotypes, is expected to increase, whereas the latter, the changeability of phenotypes, tends to diminish. However, in nature, plasticity is preserved to a certain degree. One possible cause for this is environmental variation, with one of the most important “environmental ” factors being inter-species interactions. As a first step toward investigating phenotypic plasticity in response to an ecological interaction, we present the study of a simple two-species system consisting of hosts and parasites. Hosts are expected to evolve to achieve a phenotype that optimizes fitness and increases the robustness of the corresponding phenotype by reducing phenotypic fluctuations. Conversely, plasticity evolves in order to avoid certain phenotypes being attacked by parasites. By simulating evolution using the host gene-expression dynamics model, we analyze the evolution of genotype-phenotype mapping. If the interaction is weak, the fittest phenotype of the host evolves to reduce phenotypic variances. In contrast, if a sufficient degree of interaction occurs, 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 the threshold, it is increased due to noise-induced phenotypic plasticity. We examine how the increase in the phenotypic variances due to 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 plasticity and robustness in response to host-parasite interactions.

Author summary

Plasticity and phenotypic variability induced by internal or external perturbations are common features of biological systems. However, certain environmental conditions initiate evolution to increase fitness and, in such cases, phenotypic variability is not advantageous, as has been demonstrated by previous laboratory and computer experiments. As a possible origin for such plasticity, we investigated the role of host-parasite interactions, such as those between bacteria and phages. Different parasite types attack hosts of certain phenotypes. Through numerical simulations of the evolution of host genotype-phenotype mapping, we found that, if the interaction is sufficiently strong, hosts increase phenotypic plasticity by increasing phenotypic fluctuations. Depending on the degree of noise in gene expression dynamics, there are two distinct strategies for increasing the phenotypic variances: via stochasticity in gene expression or via genetic variances. The former strategy, which can work over a faster time scale, leads to a decline in fitness, whereas the latter reduces the robustness of the fitted state. Our results provide insights into how phenotypic variances are preserved and how hosts can escape being attacked by parasites whose genes mutate to adapt to changes in parasites. These two host strategies, which depend on internal and external conditions, can be verified experimentally, for example, via the transcriptome analysis of microorganisms.