Understanding the regulation of skeletal muscle development is important to meet the increasing demand of Indian major carp Labeo rohita. Myogenic regulatory factors (MRFs) along with myocyte specific enhancer factor 2 (MEF2) play the pivotal role in the determination and differentiation of skeletal muscle. The majority of skeletal muscle genes require both MRFs and MEF2 family members to activate their transcription. In this study, the expression pattern of MyoD, myf-5, myogenin, and MEF2A was observed from 6 h after fertilization to 12 months of age using semiquantitative RT-PCR as well as real-time PCR method. MyoD and myf-5 mRNAs were expressed at high level at the early embryonic stages. Myogenin and MEF2A were expressed after MyoD and myf-5 and remained active up to adult stage. Expression of MyoD was lower than that of Myf-5 after the 5th month. Partial sequencing of MyoD, myf-5, and MEF2A was done to draw phylogeny. In phylogenetic study, Labeo MyoD, MEF2A and myf-5 were found to be closely related to those of common carp. The present investigation suggests that the four transcription factors play pivotal role in the regulation of muscle growth of Labeo rohita in an overlapping and interconnected way. 1. Introduction Rohu Labeo rohita (Hamilton 1822) is one of the most important economic carps in India and other South East Asian countries. With increasing demand, more studies are required on growth and differentiation of skeletal muscle to improve the growth rate of this fish. The understanding of the regulation of embryonic and postnatal skeletal muscle growth and development is extremely important in this regard [1]. During vertebrate embryogenesis, skeletal muscle is derived from somites, which is formed by segmentation of the paraxial mesoderm lateral to neural tube [2]. The trunk musculature of fish is originated from the segmental plate mesoderm flanking the notochord and lying underneath the presumptive nerve cord. Studies in zebrafish (Brachydanio rerio) have revealed that the most medial cells in the segmental plate, called adaxial cells, commit to become myoblasts with a slow muscle lineage and the fast muscle fibres are derived from the lateral presomitic mesoderm by fusion of several myoblasts to form multinucleated myotubes [3]. Development and growth of skeletal muscle are complex dynamic processes involving both the recruitment of new muscle fibres (hyperplasia) and growth of existing fibres (hypertrophy) [4]. Fish have an ability to recruit new skeletal muscle fibres throughout the larval life and even during juvenile and adult
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