Ian A. Johnston

Energy metabolism of carp swimming muscles

SummaryElectromyography has been used to study the recruitment of red, pink and white muscle fibres of the Mirror carp at different swimming speeds. Locomotion below 0.3–0.5 L/S (lengths per second) is achieved primarily by fin movements after which the red myotomal muscle becomes active. Pink muscle fibres are the next type to be recruited at speeds around 1.1–1.5 L/S. White muscle is only used for fast cruising in excess of 2–2.5 L/S and during bursts of acceleration.Studies of the myofibrillar ATPase activities of these muscles have shown a ratio of 1∶2∶4 for the red, pink and white fibres respectively. The myosin low molecular weight components, which are characteristic of the myosin phenotype, have been investigated by SDS polyacrylamide electrophoresis. The light chain patterns of the pink and white muscles were identical and characteristic of the fast myosin phenotype. Red muscle myosin had a light chain pattern characteristic of slow muscles. It would appear that there is a relationship between the speed of contraction of the fibre types and the locomotory speed at which they are recruited.The activities of some enzymes of energy metabolism have also been determined in the three muscle types. Enzymes associated with oxidate metabolism have high, intermediate and low activities in the red, pink and white muscles respectively. Pyruvate kinase and lactate dehydrogenase activities were considerably higher in the pink than in either red or white muscles. It is suggested that the high capacity for anaerobic glycolysis of the pink muscle is associated with its recruitment for sustained effort at swimming speeds above which the fish can no longer meet all its energy requirements by gas exchange at the gills.

Muscle development and growth: potential implications for flesh quality in fish

Growth in fish involves the recruitment and hypertrophy of muscle fibres. Muscle recruitment is particularly important in species that reach a large ultimate body size. The number of muscle fibres recruited to reach a particular girth varies between families and strains and is influenced by environmental factors including diet, exercise, light and temperature regimes. The resulting variation in muscle cellularity and associated changes in connective tissue matrix are thought to be important determinants of texture and other flesh quality characteristics. The state of knowledge about muscle development and growth is reviewed and potential practical applications of the research to flesh quality issues are discussed.

Environment and plasticity of myogenesis in teleost fish

SUMMARY Embryonic development in teleosts is profoundly affected by environmental conditions, particularly temperature and dissolved oxygen concentrations. The environment determines the rate of myogenesis, the composition of sub-cellular organelles, patterns of gene expression, and the number and size distribution of muscle fibres. During the embryonic and larval stages, muscle plasticity to the environment is usually irreversible due to the rapid pace of ontogenetic change. In the early life stages, muscle can affect locomotory performance and behaviour, with potential consequences for larval survival. Postembryonic growth involves myogenic progenitor cells (MPCs) that originate in the embryo. The embryonic temperature regime can have long-term consequences for the growth of skeletal muscle in some species, including the duration and intensity of myotube formation in adult stages. In juvenile and adult fish, abiotic (temperature, day-length, water flow characteristics, hypoxia) and biotic factors (food availability, parasitic infection) have complex effects on the signalling pathways regulating the proliferation and differentiation of MPCs, protein synthesis and degradation, and patterns of gene expression. The phenotypic responses observed to the environment frequently vary during ontogeny and are integrated with endogenous physiological rhythms, particularly sexual maturation. Studies with model teleosts provide opportunities for investigating the underlying genetic mechanisms of muscle plasticity that can subsequently be applied to non-model species of more ecological or commercial interest.

REVIEW Growth and the regulation of myotomal muscle mass in teleost fish

Summary Teleost muscle first arises in early embryonic life and its development is driven by molecules present in the egg yolk and modulated by environmental stimuli including temperature and oxygen. Several populations of myogenic precursor cells reside in the embryonic somite and external cell layer and contribute to muscle fibres in embryo, larval, juvenile and adult stages. Many signalling proteins and transcription factors essential for these events are known. In all cases, myogenesis involves myoblast proliferation, migration, fusion and terminal differentiation. Maturation of the embryonic muscle is associated with motor innervation and the development of a scaffold of connective tissue and complex myotomal architecture needed to generate swimming behaviour. Adult muscle is a heterogeneous tissue composed of several cell types that interact to affect growth patterns. The development of capillary and lymphatic circulations and extramuscular organs – notably the gastrointestinal, endocrine, neuroendocrine and immune systems – serves to increase information exchange between tissues and with the external environment, adding to the complexity of growth regulation. Teleosts often exhibit an indeterminate growth pattern, with body size and muscle mass increasing until mortality or senescence occurs. The dramatic increase in myotomal muscle mass between embryo and adult requires the continuous production of muscle fibres until 40–50% of the maximum body length is reached. Sarcomeric proteins can be mobilised as a source of amino acids for energy metabolism by other tissues and for gonad generation, requiring the dynamic regulation of muscle mass throughout the life cycle. The metabolic and contractile phenotypes of muscle fibres also show significant plasticity with respect to environmental conditions, migration and spawning. Many genes regulating muscle growth are found as multiple copies as a result of paralogue retention following whole-genome duplication events in teleost lineages. The extent to which indeterminate growth, ectothermy and paralogue preservation have resulted in modifications of the genetic pathways regulating muscle growth in teleosts compared to mammals largely remains unknown. This review describes the use of compensatory growth models, transgenesis and tissue culture to explore the mechanisms of muscle growth in teleosts and provides some perspectives on future research directions.

Evolution and adaptive radiation of antarctic fishes

There are few instances where a knowledge of the thermal physiology, habitats and lifestyles of a group of closely related species can be mapped onto a well-supported phylogeny and a detailed climatic history. The unique fish fauna of the Southern Ocean, dominated by a single group of fish whose phylogeny is known from traditional and molecular techniques, provides one such opportunity. Furthermore, these fish are living at an extreme temperature for marine organisms. Physiological and molecular studies are revealing details of the mechanisms of temperature compensation and, combined with knowledge of the thermal history, are throwing new light on the process of evolution in this unique group of fish.

Muscle fibre density in relation to the colour and texture of smoked Atlantic salmon (Salmo salar L.).

Muscle fibre cellularity was quantified during seawater growth in populations of predominantly early (strain X) and late maturing (strain Y) Atlantic salmon (Salmo salar L.). The fibre density (number mm−2 white muscle cross-sectional area) in the fresh fillet was related to pigment concentration, colour as determined with the Roche SalmoFan™, and lipid content. The relationship between fibre density and the textural characteristics of the smoked fillet, as assessed by trained taste panels, was also determined. There was no significant correlation between astaxanthin concentration and muscle fibre density. However, significant positive relationships were obtained between Roche SalmoFan™ score and fibre density, explaining 33% and 44% of the total variation in colour visualisation in strains X and Y, respectively. Significant positive correlations were observed between muscle fibre density and all four measures of texture assessed by the taste panels, “chewiness”, “firmness”, “mouth-feel” and “dryness”. A firm texture was therefore associated with a high muscle fibre density. At harvest, the lipid content of the fillet was significantly higher in strain X (11.2%) than strain Y (7.0%). There was, however, no significant correlation found between sensoric “oiliness” score and the percentage lipid content of the fillet. The results indicate that muscle fibre cellularity is an important factor in several key flesh quality traits. The potential for manipulating muscle cellularity to produce desirable flesh quality characteristics is briefly discussed.

A well-constrained estimate for the timing of the salmonid whole genome duplication reveals major decoupling from species diversification

Whole genome duplication (WGD) is often considered to be mechanistically associated with species diversification. Such ideas have been anecdotally attached to a WGD at the stem of the salmonid fish family, but remain untested. Here, we characterized an extensive set of gene paralogues retained from the salmonid WGD, in species covering the major lineages (subfamilies Salmoninae, Thymallinae and Coregoninae). By combining the data in calibrated relaxed molecular clock analyses, we provide the first well-constrained and direct estimate for the timing of the salmonid WGD. Our results suggest that the event occurred no later in time than 88 Ma and that 40–50 Myr passed subsequently until the subfamilies diverged. We also recovered a Thymallinae–Coregoninae sister relationship with maximal support. Comparative phylogenetic tests demonstrated that salmonid diversification patterns are closely allied in time with the continuous climatic cooling that followed the Eocene–Oligocene transition, with the highest diversification rates coinciding with recent ice ages. Further tests revealed considerably higher speciation rates in lineages that evolved anadromy—the physiological capacity to migrate between fresh and seawater—than in sister groups that retained the ancestral state of freshwater residency. Anadromy, which probably evolved in response to climatic cooling, is an established catalyst of genetic isolation, particularly during environmental perturbations (for example, glaciation cycles). We thus conclude that climate-linked ecophysiological factors, rather than WGD, were the primary drivers of salmonid diversification.

Endurance exercise training in the fast and slow muscles of a teleost fish (Pollachius virens)

Summary1.The recruitment of muscle fibre types has been investigated in the coalfish (Pollachius virens) using electromyography. Red trunk muscles were active at all swimming speeds examined (0.25–3.6 lengths/s). Interestingly, white fibres were recruited at 0.8–2.0 lengths/s providing evidence that this muscle type is also used during sustained activity.2.The effect of endurance exercise training on muscle fibre size and enzymes of energy metabolism has also been investigated. Fish were exercised continuously at 2.1 lengths/s for a period of three weeks in an experimental swimming chamber. This swimming speed represents a significant increase in work load relative to non-exercised fish as evidenced by muscle fibre hypertrophy and an increase in creatine kinase activities in both red (184%) and white (260%) muscles.3.Glycogen storage levels increased to a greater extent in red (+520%) than white (+200%) muscles. Phosphofructokinase activity was eight times higher in the red muscle of exercised fish. In contrast, there was only a small increase in citrate synthetase (+30%) and no change in either hexokinase or cytochrome oxidase activities in the red muscle of trained fish.4.Increased hydroxyacyl CoA dehydrogenase activities in both muscle types indicate an enhanced capacity for fatty acid catabolism with training.5.White muscle phosphofructokinase activities were not significantly different in trained and untrained fish. It is likely that the maximum potential of white muscle for anaerobic glycogenolysis is already sufficient to meet all its energy requirements at this swimming speed.6.The results suggest that the capacity of coalfish red muscle to do aerobic work remains essentially unchanged by endurance exercise training and that any increase in the ability to produce ATP must be met anaerobically.

Biochemical Correlations of Power Development and Metabolic Fuel Preferenda in Fish Hearts

Hearts from 16 species of fishes, including representatives of cyclostomes, elasmobranchs, and teleosts, were examined. As a biochemical index of maximal ATP demand, ATPase activity of crude homogenates of ventricular tissue was measured under conditions which optimize activity of myofibrils and inhibit nonphysiological mitochondrial F₁-ATPase by inclusion of oligomycin in the assay medium. The activities of nine additional enzymes from pathways of energy metabolism and concentration of myoglobin-an intracellular protein involved in storage and diffusion of oxygen-were measured. Of all possible pairwise regressions between these biochemical markers, only the relationships among ATPase, carnitine palmitoyltransferase (CPT), and hexokinase (HK) and between HK and phosphofructokinase (PFK) were significant. The inability of elasmobranch myocardium to catabolize fatty acid is confirmed. ATP yields calculated from HK and CPT and corrected to reflect physiological conditions of the tissue suggest that catabolism of either carbohydrate or fatty acid fuels can meet maximal energetic demands of the tissue. The range of power developments encompassed by hearts of ectothermic fishes, although substantial, has not resulted in selective expansion of capacity to oxidize one particular metabolic fuel.

Temperature during embryonic development has persistent effects on thermal acclimation capacity in zebrafish

Global warming is intensifying interest in the mechanisms enabling ectothermic animals to adjust physiological performance and cope with temperature change. Here we show that embryonic temperature can have dramatic and persistent effects on thermal acclimation capacity at multiple levels of biological organization. Zebrafish embryos were incubated until hatching at control temperature (TE = 27 °C) or near the extremes for normal development (TE = 22 °C or 32 °C) and were then raised to adulthood under common conditions at 27 °C. Short-term temperature challenge affected aerobic exercise performance (Ucrit), but each TE group had reduced thermal sensitivity at its respective TE. In contrast, unexpected differences arose after long-term acclimation to 16 °C, when performance in the cold was ∼20% higher in both 32 °C and 22 °C TE groups compared with 27 °C TE controls. Differences in performance after acclimation to cold or warm (34 °C) temperatures were partially explained by variation in fiber type composition in the swimming muscle. Cold acclimation changed the abundance of 3,452 of 19,712 unique and unambiguously identified transcripts detected in the fast muscle using RNA-Seq. Principal components analysis differentiated the general transcriptional responses to cold of the 27 °C and 32 °C TE groups. Differences in expression were observed for individual genes involved in energy metabolism, angiogenesis, cell stress, muscle contraction and remodeling, and apoptosis. Therefore, thermal acclimation capacity is not fixed and can be modified by temperature during early development. Developmental plasticity may thus help some ectothermic organisms cope with the more variable temperatures that are expected under future climate-change scenarios.