Chromosomal inversions may assist acute salinity and temperature adaptation in Atlantic cod eggs

Chromosomal inversions have been consistently associated with environmental gradients, suggesting a role for local adaptation. However, the underlying selective mechanisms of inversions remain unresolved in cod. Critically, few experiments have yet tested the fitness consequences of inversions in cod under controlled environmental conditions. Such work is essential to infer causality. Here, we applied acute temperature and low salinity stressors, separately and combined, on Atlantic cod (Gadus morhua) eggs. Inversions on chromosomes 2, 7, and 12 in cod are linked to temperature and salinity in the wild. Cod eggs (N = 1265) were placed into each treatment for 24 hours and the number of eggs that floated initially and finally was counted as a proxy for eggs alive. Genotypes for the inversions on chromosomes 2, 7, and 12 were determined for eggs from the 3°C-25ppt, 7°C-25ppt, 16°C-25ppt, and 16°C-35ppt treatments. We found that sinking increased with temperature and that more eggs sank in 25 ppt than in 35 ppt after 24 hours. We found the inversions on chromosomes 2 and 12 interact with each other and might assist with temperature and salinity adaptation, respectively, in cod eggs, while the inversion on chromosome 7 did not affect floating. These results contrast with previous research that correlated environmental variables with inversions in cod. Our results provide a unique demonstration of experimentally testing genotype-by-environment associations observed in the wild to understand the functions of chromosomal inversions.