Phenotypic reaction norms are often shaped and constrained by selection and are important for allowing organisms to respond to environmental change. However, selection cannot constrain reaction norms for environmental conditions that populations have not experienced. This may allow cryptic neutral genetic variation for the reaction norm to accumulate such that a release of phenotypic variation occurs when it is exposed to novel conditions. Most genomic diversity behaves as if functionally neutral. Genome-wide diversity metrics may therefore correlate with levels of cryptic genetic variation and, as a result, could exhibit a positive relationship with a release of phenotypic variation in novel environments. To test this hypothesis, we conducted translocations of juvenile brook trout (Salvelinus fontinalis) from 12 populations to novel uninhabited ponds that represented a gradient of environmental conditions. We assessed reaction norms for morphological traits (body size and four morphometric relative warps) across pond environmental gradients and evaluated the effect of genome-wide heterozygosity on phenotypic variability. All traits displayed plastic reaction norms. Overall, we found some evidence that a release of phenotypic variation consistent with cryptic genetic variation can occur in novel environmental conditions. However, the extent to which this release was correlated with average genome-wide diversity was limited to only one of five morphological traits examined. Our results suggest that the link between genomic diversity and the accumulation of cryptic genetic variation in reaction norms may be limited. Similarly, reaction norms were constrained for many of the morphological traits examined. Past conditions may have constrained reaction norms in the putatively novel environments despite significant deviations from contemporary source population habitat. Additionally, as a generalist colonizing species brook trout may exhibit plastic phenotypes across a wide range of environmental conditions.

Environmental data was collected from novel ponds Brook Trout were transplanted to. Relative warp data was collected from photographs taken of surviving trout using the program ImageJ. Relative Warp analysis was conducted using the program TPSdig. Genomic data was derived from Yates et al. 2019, Proc R Soc B, 286:20191989. All other statistical analyses were conducted using the program R using the package lme4.

Individual-level dataset:

Contains relative warp and centroid size data for each individual fish in the dataset (variables RW1-RW24, 'centroid.size). Also contains population of origin, year of translocation, specific pond the individual was translocated to ('pond'), heterozygosity data ('He.genomic'), and relevant environmental variables:

DO = Dissolved Oxygen

Temperature.C = average pond temperature in Celsius

Silt.prop = proportion of substrate that is silt

Depth.cm = average pond depth in cm

Initial.Density = initial density of stocked fish

Pond-level dataset:

Variables as above, except:

Centroid.Size.CoV.unbiased = unbiased estimate of the variance in centroid size of surviving fish from a pond translocation event

RW1-RW4.sd.unbiased = unbiased estimate of the variance in Relative warps of surviving fish from a pond translocation event

Mean.Centroid.Size = mean centroid size of surviving fish from a pond translocation event

Stream.sd.('Centroid', 'RW1'-'RW4') = variance in centroid size/relative warp of fish inhabiting stream of origin of stocked population