Thermal environment and maternal effects shape egg size in a freshwater fish

Offspring size determines offspring survival rates; thus, understanding factors influencing offspring size variability could elucidate variation in population dynamics. Offspring size variation is influenced through multigenerational adaptation to local environments and within-lifetime plastic responses to environmental variability and maternal effects among individuals. Moreover, offspring size variation may represent trade-offs in energy allocation within individuals that influence lifetime reproductive success. However, the mechanisms whereby environmental conditions influence offspring size, e.g., via inducing adaptive and plastic variation in population-scale maternal effects, remain poorly understood. We evaluated intra-specific variation in maternal effects, egg size, and intra-individual egg size variation in six populations of walleye (Sander vitreus) and related among-and within-population patterns to thermal conditions. Egg size was conserved within populations and negatively related to long-term thermal conditions among populations, while maternal effect strengths were positively related to thermal conditions, suggesting that populations inhabiting warmer environments adapted to produce smaller eggs but stronger maternal effects. Within a population, egg size was positively related to colder winters, suggesting cold winters may alter egg size through effects on maternal condition or as an adaptive maternal effect to improve offspring survival. Intra-individual egg size variation varied little among populations or with female size, but declined with increasing summer and decreasing winter temperatures. Our result suggests that environmental conditions could impact not only short-term offspring production but also spur adaptive changes in offspring phenotypes. Thus, it is necessary to account for adaptive responses to predict population dynamics under environmental changes.