| Summary: | Myostatin (Mstn)-A, the main isoform among Mstn splicing variants, functions as a negative regulator, whereas Mstn-B functions as a positive regulator in muscle development. Because broiler chickens are a fast-growing breed raised for meat production and layer chickens are a slow-growing breed raised for egg production, differences in the expression of <i>Mstn</i> isoforms between the two distinct breeds were analyzed in this study. There was no difference in the expression levels of total <i>Mstn</i> (<i>Mstn-A</i> and <i>-B</i> forms) during embryonic development and at D33 between the two breeds. Interestingly, the ratios of <i>Mstn-B</i> to <i>-A</i> were significantly higher in the broiler compared to the layer at most ages. In pectoralis major muscle (PM) tissue, the cross-sectional area (CSA) of muscle fiber was significantly greater in the broiler. The broiler also showed greater bundle CSA and a similar fiber number per bundle compared to the layer at D5 and D33. These data suggest that the greater bundle CSA with myofiber hypertrophy in the broilers is associated with greater muscle growth. The relationship between the expression of <i>Mstn</i> isoforms and growth rate can be used as a potential genetic marker for the selection of higher muscle growth in chickens.
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