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: (1) do different environmental gradients
show similar effects on a given trait? (2) Are the effects of two
environmental gradients on a trait additive? (3) Do different traits show
similar plastic response to a given environmental gradient? (4) Do the
(co)variances between traits vary across environmental gradients? (5) Do
genotypes differ in their plastic response to a given environmental
gradient? (6) Are some genotypes more plastic than others across all
traits? We designed a microcosm experiment using the protist Tetrahymena
thermophila aimed 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 generalising findings about plasticity to all
environments and all traits.