Brook Trout (Salvelinus fontinalis) Harbor Similar Phenotypes Despite Isolation and Neutral Genetic Drift

Isolated island populations of wild southern Appalachian Brook Trout (Salvelinus fontinalis) experience a multitude of high elevation, habitat specific stochastic stressors that promote genetic drift and exaggerate neutral genetic differentiation. We were interested in whether populations located in isolated headwater stream fragments have developed uniquely identifiable phenotypes as a result of such isolation and drift. For this investigation, we examined 35 spatially isolated populations across 13 microsatellite loci and discovered considerable neutral genetic differentiation (F’_ST: 0.02-0.94) among all Brook Trout pairwise population comparisons. Interestingly, we observed diminishing patterns of morphometric and meristic (phenotypic) variability as spatial analysis scale increased to the watershed level. While hierarchical ANOVA revealed markedly exaggerated phenotypic variability within and among populations, permutational multivariate analysis of variation revealed no significant differences among pairwise population, basin, or watershed comparisons. Post hoc analyses revealed that habitat specific environmental variables were correlated to phenotypic variation and appear to have influenced plasticity among Brook Trout. In addition, we found no evidence for reduced phenotypic variability in populations, irrespective of sibship, with very small effective population sizes (i.e. <30) and depauperate levels of allelic richness and heterozygosity. However, these data can be used to select candidate populations for genotyping-by-sequencing in order to potentially identify alleles and loci under selection that could aid in elucidating how isolation and extreme habitat conditions have potentially affected the adaptive potential of Brook Trout present at the far southern end of their native range.