Environmental Genomics of Adaptive Phenotypic Plasticity for Temperature in Threespine Stickleback

While the study of reaction norms has provided evidence of how phenotypic variation may be influenced by environmental change, the evolutionary consequences of these changes remain poorly understood. Phenotypic plasticity can occur at different hierarchical levels, from ‘macrophenotypic’ changes in morphology to physiological and transcriptional plasticity.  These levels need not be independent, nor must they be consistent – plasticity for the macrophenotype may be due to canalized molecular traits, while canalized macrophenotypes may be the result of underlying molecular plasticity.  The underlying genomic norm of reaction is therefore necessary to consider when trying to understand how organisms respond to environmental change. Heterogeneous environments are predicted to select for greater phenotypic plasticity than relatively stable environments, but the hierarchical level(s) at which this plasticity exists remains unknown.  The threespine stickleback is an ideal model organism for testing these predictions.  In British Columbia, freshwater populations experience greater thermal variance over the course of the year than ancestral marine populations, and are predicted to have evolved adaptations for coping with this variance.  In this experiment, we reared stickleback from four different freshwater and marine populations at two different temperatures (7º C and 22º C) and measured plasticity for both the macrophenotype (morphology, growth rates) and molecular phenotype (gene expression).  Agilent 8 x 60K single-channel microarrays were used to construct genomic norms of reaction for each population, as well as to identify the number and nature of differentially expressed genes.  Preliminary results suggest that, for growth rates, derived freshwater populations (slope = 0.013) were more plastic than marine populations (slope = 0.006).  The evolutionary significance of the genomic norms of reaction will be discussed.