Magnitude and predictability of pH fluctuations shape plastic responses to ocean acidification

Phenotypic plasticity is expected to facilitate the persistence of natural populations as global change progresses. The attributes of fluctuating environments that favor the evolution of plasticity have received extensive theoretical investigation, yet empirical validation of these findings is still in its infancy. Here, we combine high-resolution environmental data with a laboratory-based experiment to explore the influence of habitat pH fluctuation dynamics on the plasticity of gene expression in two populations of the Mediterranean mussel, Mytilus galloprovincialis. We linked differences in the magnitude and predictability of pH fluctuations in two habitats to population-specific gene expression profiles in ambient and stressful pH treatments. The results presented demonstrate population-based differentiation in gene expression plasticity, whereby mussels native to a habitat exhibiting a large magnitude of pH fluctuations with low predictability display reduced phenotypic plasticity between experimentally imposed pH treatments. This work validates recent theoretical findings on evolution in fluctuating environments using an ecologically important marine bivalve, and suggests that populations inhabiting regions exposed to unpredictably fluctuating selection pressures may exhibit reduced plasticity as global change progresses.

Methods
For a full description of the methods, please see the associated manuscript for this Dryad submission.

Breifly, one year of high-frequency pH and temperature monitoring was conducted at a coastal and lagoon habitat in the Northwest Mediterranean Sea (the Bay of Villefranche: 43.682oN, 7.319o E and Thau Lagoon: 43.4158oN, 3.6888oE). Resident mussels at each site were collected and reared in labortory common garden conditions for six weeks, after which a 5 day acclimation to two pH treatments (benign pH - 8.1; stressful pH - 7.75) was conducted. At the end of the acclimation peirod, total RNA was extracted from gill tissue samples,  and gene expression profiling was carried out using 3’ mRNA sequencing. All resulting raw and processed data files are included here.