Is phenotypic plasticity use-it-or-lose-it? Exploring genetic assimilation of salinity-plastic traits across threespine stickleback (<em>Gasterosteus aculeatus</em>) populations

Understanding the response of phenotypically plastic traits to novel environments is critical to predicting evolutionary dynamics. We lab-reared multiple wild-caught threespine stickleback (Gasterosteus aculeatus) populations under a range of salinities and recorded the plasticity of morphological, physiological, and fitness-related traits. Following freshwater colonisation, populations showed a short-term increase or maintenance of salinity tolerance breadth (i.e., consistency of survival across salinities) – while a subsequent loss of salinity tolerance occurred in most, but not all, populations over the longer term. Despite variability amongst physiological and morphological responses to increased salinity across populations, we found that resolution of generalist-specialist trade-offs may drive plasticity loss: less plastic populations grew faster in freshwater. Our findings establish that predicted Simpson-Baldwin dynamics (plasticity gain and then subsequent loss under relaxed sel..., , # Data from: Is phenotypic plasticity use-it-or-lose-it? Exploring genetic assimilation of salinity-plastic traits across threespine stickleback (*Gasterosteus aculeatus*) populations Dataset DOI: [10.5061/dryad.h70rxwdxp](https://doi.org/10.5061/dryad.h70rxwdxp) ## Description of the data and file structure These datasets provide raw and processed data used for analyses of threespine stickleback (*Gasterosteus aculeatus*) population salinity tolerance, using split-clutch experiments raising individuals from embryo to 11 months in 5 salinity treatments at 0ppt, 10ppt, 20ppt, 30ppt, and 40ppt. Clutches were generated from wild-caught individuals from 9 populations in Alaska and Scotland, including a variety of anadromous and freshwater populations of ages between 30 and 16,000 years. Data include metrics of growth and survival to 11 months, as well as morphometric landmark data (and corresponding photographs) of 11-month-old fish, and gill ionocyte characteristics derived from histolo...,