Pantophysin (Pan I) locus divergence between inshore v. offshore and northern v. southern populations of Atlantic cod in the north-east Atlantic

More than 6000 cod Gadus morhua, sampled in coastal and offshore waters stretching from the Barents Sea down to the North Sea, were analysed for frequencies of alleles at the scnDNA pantophysin locus (Pan I)[formerly called synaptophysin (Syp I)]. The significant allele frequency difference between the two major stocks of cod in Norway, north-east Arctic cod (NEAC) and Norwegian coastal cod (NCC), was upheld in all years of the investigation (1993 to 2001), and applied both to larval cod and post-juveniles of various ages. On a north-south axis, the appearance of a latitudinal cline of post-juvenile (≥1 year) allele frequencies was exposed. The intermediate allele frequencies in coastal areas of northern Norway, seem to a large extent to be caused by intermingling of the two stocks, although the existence of populations of coastal cod with alternative Pan I frequencies could not be ruled out. The role of selection is yet unresolved. Depth of the sampling location seemed to have an effect on the allele frequencies and their temporal stability, while there was no indication of seasonal variation in the frequencies. Breeding structure was the most likely cause for upholding the extreme divergence in Pan I frequencies between NEAC and NCC.