The role of phenotypic plasticity for rapid evolutionary adaptation: theoretical and experimental approaches using Tribolium castaneum and Bacillus thuringiensis.

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Phenotypic plasticity and host-parasite interactions are thought to be major drivers of fast evolutionary processes. The proposed project aims at elucidating conditions for rapid adaptation by investigating a prime example of phenotypic plasticity, the invertebrate immune memory (i.e. 'priming'). Using a combination of experimental evolution, population genetics, and mathematical modeling, we propose to investigate the following three questions. (1) What is the effect of host phenotypic plasticity on rapid adaptation of a pathogen? Our preliminary theoretical work showed that resistant hosts select for virulent pathogens. This raises the question whether phenotypically plastic hosts (here: with immune priming) might select for more virulent pathogens, too. We will address this topic with a serial passage experiment of the entomopathogen Bacillus thuringiensis in phenotypically plastic vs. non-plastic (i.e. primed vs. non-primed) Tribolium castaneum hosts, and additional mathematical modeling in order to explain the empirical results. (2) What is the effect of host phenotypic plasticity on rapid adaptation of the host itself? It is generally assumed that a high degree of phenotypic plasticity constrains evolutionary adaptation, since it may buffer genetic (i.e. evolutionary) changes. We will address this topic with a series of experiments that focus on host evolution with and without experimentally induced plasticity (here: immune priming). (3) Under which conditions does phenotypic plasticity result in rapid genetic assimilation? It is often stated that fluctuating environments favor phenotypic plasticity and prevent the evolution of genetic assimilation. However, these arguments tend to overlook the difference between the environment that is present when plastic phenotypes are induced, and the environment, in which selection of the phenotypes actually happens. The beauty of 'immune priming' is that it allows disentangling these two environments in evolutionary experiments. We will make full use of this feature and conduct a series of experiments with alternating treatments that mimic different types of environmental fluctuations. The experimental results of research questions (2) and (3) will be described with a population genetics model. This will serve as a starting point to develop a general theoretical framework for the role of phenotypic plasticity on genetic divergence and speciation.