Abstract
The skeletal muscle of the species Arapaima gigas (pirarucu) constitutes the major edible part of the fish and is, therefore, an important protein source for human consumption. Post-natal muscle growth is regulated by the expression of myostatin as well as the Myogenic Regulatory Factors (MRFs) MyoD and myogenin. Once pirarucu reaches large size, we assume that the control of muscle growth by MRFs and myostatin occurs differentially at the initials life stages. In the present work we evaluated the morphological aspects and expression of MRF genes in skeletal muscle of pirarucu during early juvenile stage (Group A, up to 50 g, n=7), and post juvenile stages (Groups B, from 50 to 400 g, n=7; C, from 400 g to 5 kg, n=7, and D, from 5 to 9 kg, n=7). Transverse sections of red and white muscles were obtained to evaluate muscle fiber morphological and morphometric characteristics. MyoD, myogenin and myostatin genes and protein expressions were determined after quantitative real-time polymerase chain reaction and western blotting, respectively. Pirarucu skeletal muscle exhibited similar morphologies at different life stages. It was possible to conclude that both hyperplasia and hypertrophy occur during muscle growth in early and postjuvenile stages. As regard as expression, both mRNA and protein levels were similar among all groups for MyoD and myogenin. Myostatin presented lower mRNA levels and higher protein levels in early-juvenile stage, compared with the other groups. The levels of MRFs and myostatin might be involved in a balance that controls hyperplasia and hypertrophy occurring during post-natal muscle growth. Myostatin does not appear to play a crucial role during pirarucu early-juvenile stages. Since this species represents an interesting model for aquaculture programs due to high growing rates, our data suggest that the best phase to improve muscle growth in pirarucu is at post-juvenile stage and this will turn the employment of pirarucu farming economically better.
Highlights
Pirarucu (Arapaima gigas) is a member of the largest freshwater fish group in the world and is considered one of the most important species of the Amazonian ichthyofauna
The organization of muscle fibers in pirarucu is quite similar to other fish species; red and white muscle fibers showed similar morphological patterns, presenting polygonal or round shapes with peripheral nuclei
Analysis of white muscle fiber morphometry showed many small-diameter fibers surrounding larger fibers. This pattern is observed in most fish species during muscle growth and is characteristic of hypertrophy and hyperplasia [12,17,55,56]
Summary
Pirarucu (Arapaima gigas) is a member of the largest freshwater fish group in the world (the Teleosts) and is considered one of the most important species of the Amazonian ichthyofauna. The pirarucu fastgrowing characteristic allows it to reach up to 3 m length and 250 kg weight, when the 4 years-old pirarucu is called adult [1,2]. This species exhibits some intrinsic characteristics, such as rapid growth rates and rusticity, which enable it to be included in intensive rearing programs with excellent performance results. For this reason, it is important to investigate muscle growth mechanisms in this species. The amount and arrangement of red and white muscle fibers, as well the red/white muscle ratio, can vary according to the fish species and swimming requirements, being an interesting characteristic regarding environmental adaptations [13]
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