Interaction of Phenotypic Plasticity and Natural Selection on Gene Expression

Temperature profoundly affects biological systems across all levels of organization. Over generations, species become evolutionarily adapted to specific thermal environments. In addition to evolved adaptive differences, individuals may reversibly modify their phenotype within their lifetimes in response to different thermal environments in a process termed phenotypic plasticity. The interaction between, evolutionary adaptation and phenotypic plasticity is complex and contentious. We utilize Fundulus glycolytic muscle physiology to address this interaction. We conducted a microarray analysis of muscle gene expression using three populations of Fundulus acclimated to three different temperatures. A phylogenetic comparative analysis among populations from different thermal environments demonstrates adaptive variation in mRNA expression for 186 genes. Sixty-seven genes had significant differences in mRNA expression in response to thermal acclimation. Interestingly, evolutionary adaptation and phenotypic plasticity appear to operate primarily orthogonally: few genes (although more than expected by chance alone) exhibit both adaptive variation and phenotypic plasticity. The magnitude and function of the adaptive variation in gene expression is dependent on acclimation temperature (e.g., more genes have adaptive differences at 12° and 28°C than at 20°C), demonstrating the importance of gene-by-environment interactions. Finally, a functional analysis of gene expression provides novel, testable hypotheses regarding adaptation of muscle physiology. A loop design was used for the microarray hybridizations. The loop consisted of 56 individual labeled with Cy3 and Cy5 aRNAs for three populations acclimated to three temperatures (12°C, 20°C 28°C). The loop formed was MA→GA→Fg→ME..., where each arrow represents a separate hybridization (array) with the biological sample at the base of the arrow labeled with Cy3 and the biological sample at the head of the arrow labeled with Cy5. ME are F. heteroclitus individuals from Maine, GA are F. heteroclitus individuals from Georgia, and Fg are F. grandis individuals from Florida. Acclimation temperatures and individuals randomized within this loop. Slides were scanned using a ScanArray Express with 5um resolution at half speed. Images were quantitated using ImaGene software.