Myosin light chain 2 isoforms in gilthead sea bream (Sparus aurata L.): Molecular growth markers at early stages

• The expression of myosin light chain 2 (MLC2) isoforms was correlated to length.
• Groups of larvae and juveniles exhibiting different growth were studied.
• Individuals of same age, yet different length were compared for MLC2 expression.
• MLC2A/MLC2B expression was significantly and negatively correlated to total length.
• This correlation was independent of age.

The production of high quality juveniles at hatchery stages is important to ensure high growth potential during the on-growing phase. In the present paper, molecular growth markers of muscle development and growth in gilthead sea bream were validated in developmental stages/sizes that exhibit high accelerated growth rate, and rearing practices that are characterized by considerable size disparity. In gilthead sea bream the skeletal myosin light chain 2 (MLC2) is a structural molecule of the muscle that exists in two isoforms, A and B, which are known to mark hyperplasia and hypertrophy, respectively. The expression levels of the two isoforms and the robustness of its correlation with length growth were validated in a) larvae and juveniles of gilthead sea bream originating from the same broodstock and raised under intensive vs semi-intensive conditions, and b) in juvenile gilthead sea bream reared under commercial aquaculture practices and subjected to size grading with sorters of increasing diameter. A replacement of MLC2A by MLC2B was observed at metamorphosis in agreement with previous results. Larvae reared at the semi-intensive system outperformed in length growth and MLC2 expression those reared intensively and an earlier and faster replacement of MLC2A by MLC2B was observed at metamorphosis. Similarly, the ratio of MLC2A/MLC2B expression exhibited a tight, significantly negative correlation to total length of juveniles, independent of age. Overall, comparison of groups of gilthead sea bream of different origin rose under different conditions and of different developmental stages (larvae vs juveniles) provided significant evidence for the robustness of MLC2 expression as a growth marker.