Number of muscle fibres in adult Atlantic cod varies with temperature during embryonic development and pantophysin (<em>Pan</em>I) genotype.

We investigated the effects of development: temperature and pantophysin (PanI) genotype on muscle fibre recruitment in Atlantic cod Gadus morhua L. reared under controlled laboratory conditions, Fertilized eggs of farmed cod originating from the Norwegian coastal (NC) and northeast Arctic stock (NEA) were incubated at 4, 6 or 8 degrees C until hatching. At hatching, the number of fast myotomal muscle fibres (determined at 0.7 standard length) was generally greater at higher rearing temperature (1.0.4 % greater at 8 than at 4 degrees C in NEA cod) and was greater for NC than for NEA populations (32.7% greater at VC). After first feeding, the temperature groups were reared under identical feeding and environmental conditions until the final number of fast myotomal muscle fibres was reached by adult: fish. Development temperature and PanI genotype affected the final number of myotomal muscle fibres (FFN) in adult cod, In the NEA population, FFN was 11 % higher at 4 degrees C (92 000 +/- 4000 SE, n = 13) than at 8 degrees C (83 000 +/- 3000 SE, n = 20), reversing the trend seen at hatching. Analysis of the NEA population revealed that FFN was 16% higher in PanI(BB) (n = 19) than in PanI(AB) (n = 27) genotypes (Tukey's test; difference of means SE: 12 900 +/- 3400; p = 0.001.). Results indicate association between the PanI locus and the gene(s) regulating the final muscle fibre number, and also demonstrate persistent effects of embryonic environment on adult muscle phenotype.