Chromosomal inversions assist acute salinity and temperature adaptation in Atlantic cod (Gadus Morhua) eggs

Chromosomal inversions, on chromosomes 2, 7, and 12 in Atlantic cod (Gadus morhua), have been consistently associated with environmental gradients, particularly salinity and temperature, across the species' natural range. While these correlations suggest a role for local adaptation, the underlying selective mechanisms remain unresolved. Critically, few experiments have yet tested the fitness consequences of these inversions under controlled environmental conditions. Such work is essential to establish causality. Here, we conducted an acute stressor experiment with ten treatments of combined temperature and low salinity stress. Cod eggs (N = 1265) were placed into each treatment for 24 hours, and the number of individuals that floated initially and finally was counted. The ability of an egg to float is directly related to its survival, as sunk individuals cannot survive, and individuals lose buoyancy after they die. Inversion genotypes for the inversions on chromosomes 2, 7, and 12 were determined for individuals from the T03-S25, T07-S25, T16-S25, and T16-S35 treatments. We found that sinking increased with temperature and that more individuals sank in 25 ppt than in 35 ppt after 24 hours. Interestingly, some individuals in the 25 ppt treatments recovered (sank initially but floated finally) after 24 hours. We found evidence that the inversions on chromosomes 2 and 12 might assist with temperature and salinity adaptation, respectively, in cod eggs, while the inversion on chromosome 7 did not affect floating. Our results can inform the management and conservation of this endangered species.