Atlantic cod (Gadus morhua) ecotypes, not inversion frequencies, underlie divergence in egg buoyancy

Life-history divergence between Atlantic cod (Gadus morhua) ecotypes in Skagerrak is maintained despite gene flow. Fjord and North Sea ecotypes differ in growth, body size, and reproductive ecology. One environmental heterogeneity in Skagerrak is salinity, as coastal habitats experience substantially lower and more variable salinities than offshore waters. Egg buoyancy is highly sensitive to salinity and represents a critical life-history trait shaping survival and recruitment. Inversions on chromosomes 2, 7, and 12 in cod are linked to salinity adaptation and might be important for egg buoyancy and survival. Here, we conducted a buoyancy test on Atlantic cod eggs, spawned from North Sea and Fjord ecotypes, using Coombs apparatus. Ecotypes and genotypes for the inversions on chromosomes 2, 7, and 12 were determined in all individuals (n = 242). We predicted that inversion genotypes and ecotypes would be associated with egg buoyancy. We found that buoyancy was variable and followed a bimodal distribution sorted by ecotype, with Fjord and North Sea individuals having mean buoyancies at neutral salinity (at 7 °C) of 27 and 31 ppt, respectively. Therefore, individuals with intermediate buoyancies might be selected against. This finding is consistent with our expectations, as Fjord ecotypes experience lower salinities compared to North Sea ecotypes. Interestingly, there was little evidence that the inversions on chromosomes 2, 7, and 12 impacted buoyancy. Overall, we provide experimental evidence of the relative contributions of ecotypic background and inversions on a key life history trait, which helps to explain the maintained divergence among ecotypes.