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Salmonids can accumulate lipids in their fillets, creating marbled features with alternate red (Muscle Fibers, MF) and white stripes (Myosepta, MS). To investigate the regulation of this important quality trait, diets with low and high lipid levels were fed to the fish and found that high lipid diet significantly elevated muscular lipid deposition in MS but not in MF. Then, a whole-transcriptome analysis was performed and results showed that the mRNA expression of ACSL1 and GADD45A was downregulated by the lncRNAs MSTRG.19477.1 and XR_005039693.1, resulting in consistent lipid contents in the MF from both groups. The lncRNAs MSTRG.21618.1, XR_005034756.1, XR_002473790.2, XR_002472790.2, and MSTRG43906.1 increased lipid deposition in MS30 by upregulating the mRNA expression of ELOVL2, DGAT2, LCAT, etc. In conclusion, the present study revealed that selective muscular lipid deposition and several lncRNAs may play key roles in regulating the marbling features of rainbow trout.

Zebrafish myosepta connect two adjacent muscle cells and transmit muscular forces to axial structures during swimming via the myotendinous junction (MTJ). The MTJ establishes transmembrane linkages system consisting of extracellular matrix molecules (ECM) surrounding the basement membrane, cytoskeletal elements anchored to sarcolema, and all intermediate proteins that link ECM to actin filaments. Using a series of zebrafish specimens aged between 24 h post-fertilization and 2 years old, the present paper describes at the transmission electron microscope level the development of extracellular and intracellular elements of the MTJ. The transverse myoseptum development starts during the segmentation period by deposition of sparse and loosely organized collagen fibrils. During the hatching period, a link between actin filaments and sarcolemma is established. The basal lamina underlining sarcolemma is well differentiated. Later, collagen fibrils display an orthogonal orientation and fibroblast-like cells invade the myoseptal stroma. A dense network of collagen fibrils is progressively formed that both anchor myoseptal fibroblasts and sarcolemmal basement membrane. The differentiation of a functional MTJ is achieved when sarcolemma interacts with both cytoskeletal filaments and extracellular components. This solid structural link between contractile apparatus and ECM leads to sarcolemma deformations resulting in the formation of regular invaginations, and allows force transmission during muscle contraction. This paper presents the first ultrastructural atlas of the zebrafish MTJ development, which represents an useful tool to analyse the mechanisms of the myotendinous system formation and their disruption in muscle disorders.

Axial undulations in fishes are powered by a series of three-dimensionally folded myomeres separated by sheets of connective tissue, the myosepta. Myosepta have been hypothesized to function as transmitters of muscular forces to axial structures during swimming, but the difficulty of studying these delicate complex structures has precluded a more complete understanding of myoseptal mechanics. We have developed a new combination of techniques for visualizing the three-dimensional morphology of myosepta, and here we present their collagen-fibre architecture based on examination of 62 species representing all of the major clades of notochordates. In all gnathostome fishes, each myoseptum bears a set of six specifically arranged tendons. Because these tendons are not present outside the gnathostomes (i.e. they are absent from lampreys, hagfishes and lancelets), they represent evolutionary novelties of the gnathostome ancestor. This arrangement has remained unchanged throughout 400 Myr of gnathostome evolution, changing only on the transition to land. The high uniformity of myoseptal architecture in gnathostome fishes indicates functional significance and may be a key to understanding general principles of fish swimming mechanics. In the design of future experiments or biomechanical models, myosepta have to be regarded as tendons that can distribute forces in specific directions.

Myosepta have been subject to comparative and evolutionary studies in aquatic groups of the Craniata, because they are likely to play a role in transmission of muscular forces to axial structures during swimming. Based on gross morphological observations, the V-shaped myosepta of Branchiostoma lanceolatum appear to be simpler than craniate myosepta that lack the dorsal- and ventralmost anterior pointing arm. However, these small and delicate sheets of connective tissue have never been studied in terms of 3D morphology and collagen fibre architecture. We posed the following questions. What are the shape and collagen fibre architecture of the myosepta of Cephalochordata compared to those of Craniata? Do they exhibit the same structures as the corresponding parts of the W-shaped myosepta of Craniata? We adapted methods used for craniate myosepta (clearing, microdissections and polarized light microscopy, DIC microscopy) and additionally used computer-based 3D reconstruction to address these questions in B. lanceolatum. We found four features of complex myoseptal folding that are not present in any craniate group: (1) the medial attachment line is divided into an anterior and posterior line along their traverse on the neural tube, giving rise to a lumen between dorsal nerve cord and medial attachment line, (2 and 3) the myosepta exhibit two vertical anterior lamellae (AVL-1 and AVL-2) and (4) a posterior vertical lamella (PVL) originates from a small anterior depression in the epaxial part. The AVLs and PVL are situated in a paramedian plane near the axis and serve as attachment sites for muscle fibres. Muscle fibres exclusively run from myoseptum to myoseptum and in contrast to the vertebrate condition never attach to the chordal sheath. The myoseptal collagen fibre architecture is different from any of the conditions among Craniata: it is a system of crossing fibres (MLF-1, MLF-2) and longitudinal fibres (LF), that lacks distinct tendons. The MLFs and LFs are hypothesized to be involved in transmission of muscular forces during swimming. Given these findings it is likely that cephalochordate myomeres rather represent a specialized locomotory design than the notochordate ground pattern. Evolutionary transformations of the myoseptal system during early notochordate evolution are discussed in the light of current phylogenies including extinct taxa (for example conodonts, Yunnanozoon, Haikouella).

The caudal fin represents the posteriormost region of the vertebrate axis and is one location where forces are exerted to the surrounding medium. The evolutionary changes of its skeleton have been well analyzed in gnathostomes and revealed transitions from heterocercal to diphycercal and homocercal tails. In contrast, we only know little about the evolutionary transformations of the muscular system of the caudalis and about possible ways of force transmission from anterior myomeres to the caudal fin. The goals of this study are to gain insight into evolutionary transformations of the musculoskeletal system in the four basal actinopterygian groups (Cladistia, Chondrostei, Ginglymodi, and Halecomorphi) and to identify likely pathways of force transmission to the tail. In this context, the connective tissue of the myosepta is considered to be an essential part of the musculoskeletal system. For the first time, this system is analyzed for the whole postanal region. The use of microdissection techniques and polarized light microscopy revealed the collagen fiber architecture and the insertions of all postanal myosepta from cleared and stained specimens. The collagen fiber architecture is similar in all investigated specimens and thus represents the primary actinopterygian condition. All parts of postanal myosepta are dominated by longitudinally arranged myoseptal tendons (lateral and myorhabdoid tendons) that span several vertebral segments. This architecture supports the view that posterior myosepta are well designed to transfer muscular forces that are generated in anterior myomeres. In contrast to the uniform myoseptal architecture, the musculoskeletal system differs between the four basal actinopterygian groups. Among them, chondrosteans have retained the plesiomorphic condition of actinopterygian tails. For the remaining taxa several evolutionary novelties in the musculoskeletal system of the tail are revealed. Most of these have evolved independently in the cladistian and neopterygian stem lineage. In these groups extensions of all epaxial and hypaxial parts of myosepta are present that insert on caudal fin rays. This remarkable contribution of epaxial muscle masses to the caudal fin organization is in contrast to the skeletal organization, that largely derives from hypaxial material only. In contrast to former studies the hypochordal longitudinalis muscle is shown to be a synapomorphy of Halecostomi (Halecomorphi + Teleostei). The morphological framework presented here allows to generate new hypotheses on the function of caudal fins that can be tested experimentally.