A multidimensional approach to the expression of phenotypic plasticity

Phenotypic plasticity is increasingly recognized as a key element of eco-evolutionary dynamics, but it remains challenging to assess because of its multidimensional nature. Indeed, organisms live in complex environments where numerous factors can impact the phenotypic expression of traits (inter-environment axis), possess multiple traits that can influence each other's expression (inter-trait axis) and differ in their genetic background (inter-genotype axis), which can not only impact the traits' values but also their plasticity. We addressed six questions related to phenotypic plasticity: (a) do different environmental gradients show similar effects on a given trait? (b) Are the effects of two environmental gradients on a trait additive? (c) Do different traits show similar plastic response to a given environmental gradient? (d) Do the (co)variances between traits vary across environmental gradients? (e) Do genotypes differ in their plastic response to a given environmental gradient? (f) Are some genotypes more plastic than others across all traits? We designed a microcosm experiment using the protistTetrahymena thermophilaaimed at encompassing all these aspects of phenotypic plasticity. We exposed 15 distinct genotypes to 25 combinations of temperature and nutrient availability and assessed the plasticity of five phenotypic traits. Our results show strong differences in the plastic response depending on the environmental gradient, not only regarding the shape of the reaction norm of the different traits tested, but also in the overall plasticity of the organisms. We did not find any covariance between traits that was consistent across all environments. Overall, our results suggest independent impacts of the environmental dimension considered on the observed plastic response. These results underline potential difficulties in generalizing findings about plasticity to all environments and all traits. A freePlain Language Summarycan be found within the Supporting Information of this article.