Developmental plasticity of muscle growth in fish ecotypes

Coregonids are abundant and economically important food fish in European inland waters. Several lakes are inhabited by sympatric ecotype pairs that belong to a single species but diverge in life style and ultimate size (small and large forms). Such pairs provide well-suited model systems for study of intraspecific phenotypical diversification and its dependence upon ambient factors. One of the most important among these factors is temperature. Recent research has demonstrated that the thermal experience of a teleost embryo is likely to become `imprinted` and to have continued effects upon the growth of the body and its dominant tissue, the axial muscle, during the animal`s entire later life. However, it is as yet unknown to which extent thermal imprinting determines these growth features in phenotypically diverging coregonid forms, thus perhaps aiding the segregation of small and large ecotypes. The proposed work will use an ecotype pair of Coregonus lavaretus from a post-glacial lake in Austria (Traunsee) to investigate into this issue at the muscle level until yearling stage. Specifically, the work is intended to establish for the two coregonid forms how thermal experience until hatching affects (i) general body growth and (ii) muscle growth as expressed in cellularity change (hyperplasia and hypertrophy) including (iii) the underlying dynamics of muscle precursor cell proliferation and differentiation. Methods to be used include immunolabelling, in situ hybridization, single fibre analysis and digital planimetry of semithin sections. Results are expected to be of major significance to aspects of both pure and applied developmental biology, ecology and fisheries biology (eg. optimisation of aquaculture procedures and fish yields from stocked populations) and from the evolutionary perspective.

Several lakes around the world are inhabited by ecotype pairs of teleost fish that differ in habitat utilisation, spawning behaviour and maximum size (dwarf type and regular type). Such ecotype pairs provide well-suited model systems for study of intraspecific phenotypical diversification. Recent research has demonstrated that the thermal experience of a teleost embryo is likely to become 'imprinted', thus having continued effects on the growth of the body and its dominant tissue mass, the axial muscle. Here, we present data that demonstrate that such thermal imprinting aids the segregation of small and large forms within an ecotype pair of whitefish Coregonus lavaretus. We were able to show that fish of the regular form are much smaller and had less muscle mass when imprinted at thermal conditions typical for the spawning sites of the dwarf form (6C) than when imprinted at the conditions usually experienced at their own spawning sites (2C). Surprisingly, the fish of the dwarf form exhibit a similar response pattern to thermal history (2-fish much larger than 6-fish), indicating that in their case, normal spawning site temperature (6C) is indeed likely to act as a growth limiting factor. In addition, immunolabelling was performed to quantitatively examine numbers of muscle precursor cells including such that are mitotically active (Pax7+/H3P+) or have entered differentiation (Pax7+/MEF2+). Results demonstrate that incubation temperature has an important influence on the proliferation/differentiation balance of such cells in the two ecotypes and this behaviour is able to explain the observed growth effects. Thus, muscle precursor cells of warm-incubated embryos reduce proliferation in favour of differentiation. This reduces the MPC reserves of such warm-reared fish which may then be responsible for the reduced growth until juvenile life. By contrast, cold-reared embryos show enhanced MPC proliferation but reduced differentiation, thus allowing for a larger MPC pool utilisable for enhanced posthatching growth, finally resulting in larger juveniles. This is of major significance to aspects of ecological developmental biology, fisheries biology and from the evolutionary perspective.