Lower environmental temperature delays and prolongs myogenic regulatory factor expression and muscle differentiation in rainbow trout ( Onchrhynchus mykiss) embryos

The effect of different temperatures (4 °C and 12 °C) on myogenic regulatory factors (MyoD and myogenin) and myosin heavy chain (MyHC) expression was investigated in rainbow trout ( Onchrhynchus mykiss) during early development. MyoD is first switched on at stage 14 [about 5 somites are formed (1/2 epiboly)] while myogenin mRNA is expressed at stage 15 [around 15 somites are visible (2/3 epiboly)] at both temperatures. Subsequently (up to at least stage 20), the most caudal somites exhibit less myogenin mRNA at 4 °C compared to 12 °C. At the eyed stage (stage 23 – 24), both myogenin mRNA and protein are present in greater amounts throughout all myotomes at the lower temperature, with mRNA levels in warmer (12 °C) embryos at 83 % for MyoD and 72 % for myogenin of the levels seen in 4 °C embryos. Conversely, however, at this same stage, fast-MyHC mRNA and protein are more abundant in 12 °C than in 4 °C embryos. This indicates relatively advanced muscle differentiation at the warmer temperature. At hatching, myogenin-positive cells are concentrated within the myosepta at both temperatures and they are also sparsely distributed in the myotome at 4 °C, but not at 12 °C. MyoD, myogenin, and MyHC levels provide an indication of differentiation of muscle cells. These findings suggest that myogenic regulatory factor expression is delayed but prolonged by the lowering of temperature.