Expression profiles of growth-related genes during the very early development of rainbow trout embryos reared at two incubation temperatures

Quantitative RT-PCR was used to determine the profiles of expression of 10 growth- or development-related genes in rainbow trout (Oncorhynchus mykiss) embryos prior to the formation of the somatotropic (ST) axis (pituitary somatotrops and liver); embryos were sampled immediately after fertilization and water-hardening (t0), 1-h post-fertilization, and 1-, 2-, 5-, 7-, 10- and 13-days post-fertilization (dpf); expression profiles were examined in embryos reared at two temperatures (6.0 and 8.5 °C), which had different developmental rates. Accumulation of mRNA encoding for GH1, GH2, IGF-1, IGF-2, two isoforms of GH receptors (GHR1, GHR2), two isoforms of IGF receptors (IGF-RIa, IGF-RIb) and two isoforms of thyroid receptor (TR), TRα and TRβ, was measured. All of these genes were expressed in the t0 samples, but the rates of expression of the different genes varied markedly. For most of the genes examined, the expression rates tended to fall within the first hour after fertilization, and remained at the lower level for between 2 and 7 days, after which there was a significant (P < 0.05) and progressive increase in the number of accumulated copies of mRNA. This increase is probably associated with the commencement of embryonic genome transcription activity (EGTA), and it was generally, although not always, found later in embryos that were reared at 6.0 °C compared with the faster developing embryos reared at 8.5 °C. The study suggests that the EGTA begins between 2- and 5-dpf, with a staged increase in EGTA between 5- and 13-dpf.