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.