Skeletal muscle consists of several tissues, such as muscle fibers and connective and adipose tissues. This review aims to describe the features of these various muscle components and their relationships with the technological, nutritional, and sensory properties of meat/flesh from different livestock and fish species. Thus, the contractile and metabolic types, size and number of muscle fibers, the content, composition and distribution of the connective tissue, and the content and lipid composition of intramuscular fat play a role in the determination of meat/flesh appearance, color, tenderness, juiciness, flavor, and technological value. Interestingly, the biochemical and structural characteristics of muscle fibers, intramuscular connective tissue, and intramuscular fat appear to play independent role, which suggests that the properties of these various muscle components can be independently modulated by genetics or environmental factors to achieve production efficiency and improve meat/flesh quality.

Lipids are the predominant source of energy for fish and are stored in fat depots in different parts of the body regions. This review focuses on visceral, subcutaneous and intramuscular adipose tissues that interfere with carcass and fillet yields and with flesh quality. The morphological, cellular and biochemical characteristics of these tissues are discussed as well as the different mechanisms involved in the regulation of their lipid metabolism. Particular emphasis is given to the modulation of these characteristics and mechanisms by different extrinsic (food composition, water parameters) and intrinsic (selective breeding, life cycle status) factors. This review focuses on recent studies that take into account the present challenges of fin-fish aquaculture, which are principally (1) the replacement of fish oil and meal by vegetable oil and meal due to the need for sustainability and the limited availability of fish to prepare food pellets, and (2) selective breeding programs to improve fish growth and flesh quality. These studies apply various modern technologies to different fish species, including the development of cell culture systems and transcriptomic and proteomic techniques. This review highlights that fish adipose tissues differ in their localization and their morphological characteristics and that they show a large plasticity in their responses to variations of both extrinsic and intrinsic factors. These different responses reinforce the idea of their differential participation in fish lipid homeostasis

Female gamete production relies on coordinated molecular and cellular processes that occur in the ovary throughout oogenesis. In fish, as in other vertebrates, these processes have been extensively studied both in terms of endocrine/paracrine regulation and protein expression and activity. The role of small non-coding RNAs in the regulation of animal reproduction remains however largely unknown and poorly investigated, despite a growing interest for the importance of miRNAs in a wide variety of biological processes. Here, we analyzed the role of miR-202, a miRNA predominantly expressed in male and female gonads in several vertebrate species. We studied its expression in the medaka ovary and generated a mutant line (using CRISPR/Cas9 genome editing) to determine its importance for reproductive success with special interest for egg production. Our results show that miR-202-5p is the most abundant mature form of the miRNA and that it is expressed in granulosa cells and in the unfertilized egg. The knock out (KO) of mir-202 gene resulted in a strong phenotype both in terms of number and quality of eggs produced. Mutant females exhibited either no egg production or produced a dramatically reduced number of eggs that could not be fertilized, ultimately leading to no reproductive success. We quantified the size distribution of the oocytes in the ovary of KO females and performed a large-scale transcriptomic analysis approach to identified dysregulated molecular pathways. Together, cellular and molecular analyses indicate that the lack of miR-202 impairs the early steps of oogenesis/folliculogenesis and decreases the number of large (i.e. vitellogenic) follicles, ultimately leading to dramatically reduced female fecundity. This study sheds new light on the regulatory mechanisms that control the early steps of follicular development, including possible targets of miR-202-5p, and provides the first in vivo functional evidence that a gonad-predominant microRNA may have a major role in female reproduction.

Archimer http://archimer.ifremer.fr Highlights ► Application of ultrasound and 2D imagery to predict processing yields in rainbow trout. ► Estimation of genetic parameters and genetic correlations with yields to predict. ► Comparison of different strategy of selection.

Brown trout (Salmo trutta) (2.7 g initial mean weight) were reared in freshwater for 8 months at water velocities of <0.1 (control group), 1 or 2 body lengths per second (BL s−1) (exercise groups). Growth (body weight, body length and body width), muscle structure (muscle fibre diameter and width of myosepta) and flesh quality parameters (dry matter, muscle pH, collagen content and solubility, instrumental evaluation of texture) were measured at the end of the experiment. The body weight of fish at 1 BL s−1 was 22% higher than the control group. Muscle development was stimulated at 1 and 2 BL s−1, leading to a higher condition factor, greater body height and width, and muscle fibre hypertrophy (55 vs. 59.5 μm fibre diameter in the control and 2 BL s−1 groups respectively). Connective tissue and collagen were only slightly affected by exercise (no difference in collagen solubility, but a greater proportion of γ trimer and fewer α chains in the control compared with the exercised group). Flesh quality was affected, with greater dry matter content and lower post‐mortem pH in the 1 BL s−1 group compared with the control and 2 BL s−1 groups. The mechanical resistance of the raw fillets was slightly but significantly increased by exercise. The exercise‐related changes in muscle structure and texture are discussed.

Genetic parameters of production traits (growth, carcass yield, fillet yield) and bony tissues (head and vertebral axis) were estimated for large all-female rainbow trout Oncorhynchus mykiss reared in freshwater. Genetic parameters were estimated using REML at 16 months of age (1636 g) on 1962 DNA-assigned progenies from a partial factorial mating design with 60 dams and 100 sex-reversed sires. Highlights► Genetic parameters of production traits in rainbow trout ► Negative correlation between production traits and bony tissues ► Selection on fillet yield

Genetic parameters for carcass and fillet percentage were estimated in 760 European sea bass reared under commercial conditions and slaughtered at 573 days post fertilization (395 g mean body weight). Phenotyped fish were the offspring of 45 sires and 20 dams crossed in a factorial mating design. Pedigrees were reconstructed with 90.7% success using 12 microsatellites. The heritability of fillet yield was moderately low (0.21), while it was high for carcass yield (0.57). Both traits were poorly correlated (− 0.01 to 0.28) making space for their combined improvement. We investigated different predictors derived from measurement of surfaces on digital pictures and ultrasound measurements at several points of the body. The accuracy of the phenotypic prediction was rather low for fillet yield (r2 = 0.02-0.18), but higher for carcass yield (r2 = 0.27-0.41). However, genetic correlations of predictors with the traits to predict were reasonably high (up to 0.67 for fillet yield and 0.95 for carcass yield), thus allowing to consider them for performing indirect individual selection instead of sib selection. However, it was difficult to design a predictor that would simultaneously increase fillet yield and carcass yield because of contradicting effects of relative head size, an important component of the predictors which was positively correlated to carcass yield but not to fillet yield. Highlights ► We investigated morphological predictors and heritability of fillet and carcass yields in European sea bass. ► Fillet and carcass yield were heritable, but uncorrelated. ► It was not possible to find unequivocal morphological predictors applicable to both fillet and carcass.

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