Temperature and myogenic factor transcript levels during early developmentdetermines muscle growth potential in rainbow trout (Oncorhynchus mykiss) and sea bass (Dicentrarchus labrax)

The influence of changes in environmental temperature on the mRNA levels of myogenic regulatory factors (MRFs), i.e. MyoD and myogenin, as well as myo sin heavy chain (MyHC) were studied during early larval development in rain bow trout and sea bass. Phosphoimager analysis of northern blots indicated that there is an optimum temperature for the RNA transcript levels of MRF a nd MyHC RNA in trout and in sea bass larvae. In the trout strain studied, t he highest concentration for MRF and MyHC transcripts was found at 8 degree sC rather than 4 degreesC or 20 degreesC. In European sea bass, the highest concentrations of MRF and MyHC mRNA were observed at 15-20 degreesC rather than 13 degreesC. Raising sea bass larvae at 15 degreesC was associated wi th higher MyHC gene expression as well as a trend towards an increase in to tal muscle fibre number and higher growth rates after transfer at ambient t emperature. Results suggest that mRNA levels of MRF and MyHC can be used to optimise early development. An experiment in which the temperature was cha nged illustrates the consequence of precise temporal expression of MRF gene s in specifying muscle fibre number at critical stages during early develop ment.