Phenotypic plasticity of early myogenesis and satellite cell numbers in Atlantic salmon spawning in upland and lowland tributaries of a river system

Early myogenesis was studied in the offspring of Atlantic salmon (Salmo sal ar L.) spawning in a lowland (Sheeoch) and an upland (Baddoch) tributary of the River Dee System, Aberdeenshire, Scotland. Eggs from each population w ere incubated at the simulated natural thermal regimes of each stream, whic h was on average 2.8 degrees C cooler for the Baddoch than for the Sheeoch. Relationships between muscle cellularity variables, the density of myonucl ei and responses to temperature mere investigated using multivariate statis tical techniques. These revealed highly significant temperature effects (P< 0.001) at hatch (H) and first feeding (FF) and significant interactions bet ween population and temperature (P<0.001), indicating that Baddoch and Shee och salmon responded differently to the two temperature regimes. The total cross-sectional area of white muscle (WF.ta) at the adipose fin was relativ ely independent of temperature at hatch and first feeding in the Sheeoch po pulation. In contrast, for alevins of Baddoch origin, WF.ta was 18.9% (H) a nd 30.5% (FF) higher in fish incubated at Baddoch than at Sheeoch temperatu res. At hatch, there were 15.6% more white muscle fibres (WF.no) at the coo ler incubation temperature in fish of Sheeoch origin and 6.0% more in fish of Baddoch origin. However, by first feeding, the difference in WF.no betwe en temperatures had narrowed to 7.2% in the Sheeoch fish and increased to 1 7.4% in the Baddoch population In contrast, at hatch, the density of myonuc lei was 59.8% higher at the warmer incubation temperature in the Sheeoch po pulation and 23.5% higher in the Baddoch population, but differences were l ess evident at first feeding. In Baddoch fish, 22.5% of the total muscle nu clei mere actively dividing at first feeding, as assessed by staining for p roliferating cell nuclear antigen (PCNA), Of the PCNA-positive nuclei, 78% were present in cells that stained for the c-met tyrosine kinase receptor, a marker of satellite cells and their division products. The proportion of c-met-positive cells staining for individual myogenic regulatory factors wa s 72.4% for the myogenic transcription factor MyoD, 76.3% for the myogenic transcription factor Myf-5, 62.1% for myogenin and 48.7% for the myogenic t ranscription factor MSf-6. For the Sheeoch population, there were 26.5% mor e c-met-expressing (P<0.01) and 23.2% more myogenic-regulatory-factor-expre ssing (P<0.05) cells at Sheeoch than at Baddoch temperatures. In contrast, incubation temperature had no significant effects on satellite cell density in the Baddoch population.