Not Too Hot, Not Too Cold: Influences of Temperature Preference on Fish Growth

Most animals are under temperature variations as they move through the variable environment. For ectotherms, the temperature has a strong influence, which in contrast to endotherms, can not hold a constant body temperature via homeostatic mechanisms. While well studied at the species level, the underlying molecular processes are unknown and whether such relationships are affected by growth trajectory. We sequenced and assembled de novo transcriptomes of juvenile Salmo salar exposed developmentally to variable thermal environments and correlated the resulting expression profiles with the epigenomes morphometric variables, and behavioural traits from the same fish. This study examined these issues to determine the extent to which life history (thermal gradient environment versus constant temperature) interact to affect of thermal variance. Growth and metabolic genes were among the most upregulated, suggesting shifts in development and energy production for maintaining performance at variable temperatures. Furthermore, immune- and stress-responsive genes were upregulated, indicating a new complement of genes allowing of fish to better cope with variable temperatures. Other differentially expressed genes were involved with tissue development and temperature variation. Heat-shock protein genes were un-responsive (with exception some of muscle HSP90a), indicating that heat-stress responses are not effected by the thermal variation. However, the thermal constant group displayed a juxtaposed pattern of differentially expressed genes (upregulated-HSP), suggesting that heat-stress responses are triggered under un-variable thermal environments. Our results are the first to reveal the molecular processes that may enable aquatic ectotherms to adjust to a variable thermal environment that drives deeper effects on the fish develop.