Evolution of plasticity in gene expression
Diverse transcriptional patterns could accompany evolved differences in behavioral plasticity (Renn & Schumer, 2013), but relevant data characterizing the evolution of gene expression plasticity in the brain has been lacking. In both lineages, we found approximately one third magnitude evolution of plasticity as we did conserved plasticity. As was observed for transcriptomic evolution of gill tissue in stickleback fish (Gibbons, Metzger, Healy, & Schulte, 2017), plasticity evolution showed no consistent pattern, with genes gaining, losing, and switching the direction of expression plasticity between ancestral and derived populations in both lineages. The lack of consistency in these patterns and the dearth of studies that have characterized the evolution of gene expression plasticity make these patterns difficult to interpret at present. Some of the diversity is likely associated with adaptive phenotypic divergence between high- and low-predation populations, in which initially plastic behavioral shifts may become fixed, eliminated, or altered over time. For example, evolution in gene expression plasticity could reflect the gains, losses, and switches in plasticity of different behaviors in these populations (Fischer et al., 2016b). Alternatively, compensatory and homeostatic mechanisms could promote diversity among plastic responses in gene expression without altering higher-level phenotypic traits such as morphology and behavior (Badyaev, 2018; Fischer, Ghalambor, & Hoke, 2016a; Renn & Schumer, 2013). Finally, because fish in low-predation habitats experience relaxed selection on predator-induced plasticity in conjunction with low effective populations sizes, some of the evolution of transcriptional plasticity we report likely arose as a product of genetic drift, rather than selection for altered plastic responses (Lynch, 2007). Documenting the evolution of expression plasticity is an important first step, and additional studies are needed to understand the ubiquity and evolutionary sources of these patterns.