Thermal imprinting of muscle growth in fish

Muscle growth in teleost fish is a plastic process and is strongly influenced by ambient temperature. However, studies to date do not provide a consistent picture of temperature effects on muscle growth dynamics but they are indeed often contradictory. Results of recent research also demonstrate that the thermal history experienced by a teleost during embryonic and larval life is likely to be `imprinted` and to have continued effects upon muscle growth during the later periods of ontogeny. The causes of this phenomenon at the muscle precursor cell level are as yet unknown. The proposed work will investigate temperature-related changes in muscle growth dynamics in relation to changes of muscle precursor cell proliferation and differentiation rates in a salmonid (brown trout) and a cyprinid (pearlfish) fish. Specifically, the aims of the work are (i) to establish the effects of different temperature treatment on dermomyotome-generated muscle precursor cells during embryonic life, (ii) to test whether different temperature treatment modulates the specific phases of teleost muscle growth in different ways, (iii) to test how the different thermal experience in the period until onset of free swimming influences muscle development later in ontogeny, and (iv) to examine myonuclear addition to growing fibres occurring from myoblast fusion. The methods to be used are semithin section histology, morphometry, in situ hybridization and immunolabelling techniques, single fibre analysis and cell culture. It is expected that the results will be of major significance to aspects of both pure and applied developmental biology and zoological research of teleosts and other vertebrates.

Muscle growth in teleost fish is a plastic process and is strongly influenced by ambient temperature. Research by the time of the start of this project had demonstrated that the thermal history experienced by a teleost during embryonic and larval life is likely to be `imprinted`, thus having lasting effects upon muscle growth during the later periods of ontogeny. This means that within an individual species, fish that are large at hatching as a result of their early thermal history, may end up as small adults, and vice versa. Here, we present data explaining that muscle precursor cell (MPC) behaviour is an important underlying reason of this phenomenon. We were able to show that the MPCs of warm-reared pearlfish embryos reduce proliferation in favour of differentiation and thus develop into large hatchlings. However, the limited MPC reserves of these warm-reared fish finally lead to smaller adults. By contrast, cold-reared embryos show enhanced MPC proliferation but reduced differentiation, thus leading to smaller hatchlings but allowing for a larger MPC pool utilisable for enhanced posthatching growth, finally resulting in larger adults. The work also provided relevant new information regarding the general patterns of MPC behaviour in fish. Previous work from our lab had already identified the dermomyotome (DM) as the main source of MPCs. The present work now demonstrates that the teleost DM is heterogeneous in regard to myogenic cell commitment. While cells in the anterior DM appear to be responsible for proliferation only, cells from the posterior DM migrate into the myotome to give rise to new muscle fibres. Results further show that mitotically competent DM cells immigrate into the lateral fast muscle when the DM de-epithelialises by the time of hatching. This establishes a new intramyotomal myogenic stem cell pool taking over the role of the DM to promote muscle fibre formation during post-embryonic life. The results of this project are of major significance to aspects of both pure and applied developmental biology, and teleost ecology. They provide a means to assess reproductive success in the wild in the situation of climate change. The new information on thermal imprinting can be directly utilised for follow-up aquacultural research and commercial fish production. Optimising stocking success is important to meet increasing demands of fish protein in the situation of declining catches worldwide, this all the more as the optimisation can be achieved by variation of rearing temperature only by tapping the natural growth potential of the fish, without hormonal or genetic interventions.