| Summary: | Photoperiod is associated to phenotypic plasticity of somatic growth in several teleost species, however, the molecular mechanisms underlying this phenomenon are currently unknown. The effect of a continuous lighting (LD 24:0), compared with the usual hatchery lighting (HL) regime, on the growth rate and gene expression of myogenic regulatory factors (MRFs: <i>MyoD1</i> paralogs, <i>Myf5</i>, and <i>MyoG</i>) myosin heavy chain (<i>MyHC</i>), and <i>MSTN</i> paralogs in the white muscles of hatchery-reared Atlantic salmon yearlings was evaluated over a 6-month period (May to October). The levels of gene expression were determined using real-time PCR. Continuous lighting was shown to have a positive effect on weight gain. <i>MyHC</i>, <i>MyoD1c</i>, <i>MyoD1b</i>, and <i>MSTN1a/b</i> mRNA expression was influenced by the light regime applied. In all the studied groups, a significant positive correlation was observed between the expression levels of MRFs and <i>MSTN</i> paralogs throughout the experiment. The study demonstrated seasonal patterns regarding the simultaneous expression of several MRFs. <i>MyoD1a</i>, <i>MyoG</i>, and <i>MyHC</i> mRNA expression levels were elevated in the mid-October, but <i>MyoD1b/c</i>, and <i>Myf5</i> mRNA levels decreased by the end of this month. In general, the findings showed that constant lighting affected the regulatory mechanisms of muscle growth processes in salmon.
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