Thyroid hormone receptor expression during metamorphosis of Atlantic halibut (Hippoglossus hippoglossus).

Flatfish such as the Atlantic halibut (Hippoglossus hippoglossus) undergo a dramatic metamorphosis that transforms the pelagic, symmetric larva into a benthic, cranially asymmetric juvenile. In common with amphibian metamorphosis, flatfish metamorphosis is under endocrine control with thyroid hormones being particularly important. In this report we confirm that tri-iodothyronine (T(3)) levels peak at metamorphic climax during halibut metamorphosis. Moreover, we have isolated cDNA clones of TRalpha and TRbeta genes and confirmed the presence in halibut of two TRalpha isoforms (representing the products of distinct genes) and two TRbeta isoforms (generated from a single gene by alternative splicing). Real-time PCR was used to assess expression of these genes during metamorphosis. TRbeta shows the most dramatic expression profile, with a peak occurring during metamorphic climax.

Troponin T isoform expression is modulated during Atlantic halibut metamorphosis.

BACKGROUND: Flatfish metamorphosis is a thyroid hormone (TH) driven process which leads to a dramatic change from a symmetrical larva to an asymmetrical juvenile. The effect of THs on muscle and in particular muscle sarcomer protein genes is largely unexplored in fish. The change in Troponin T (TnT), a pivotal protein in the assembly of skeletal muscles sarcomeres and a modulator of calcium driven muscle contraction, during flatfish metamophosis is studied. RESULTS: In the present study five cDNAs for halibut TnT genes were cloned; three were splice variants arising from a single fast TnT (fTnT) gene; a fourth encoded a novel teleost specific fTnT-like cDNA (AfTnT) expressed exclusively in slow muscle and the fifth encoded the teleost specific sTnT2. THs modified the expression of halibut fTnT isoforms which changed from predominantly basic to acidic isoforms during natural and T4 induced metamorphosis. In contrast, expression of red muscle specific genes, AfTnT and sTnT2, did not change during natural metamorphosis or after T4 treatment. Prior to and after metamorphosis no change in the dorso-ventral symmetry or temporal-spatial expression pattern of TnT genes and muscle fibre organization occurred in halibut musculature. CONCLUSION: Muscle organisation in halibut remains symmetrical even after metamorphosis suggesting TH driven changes are associated with molecular adaptations. We hypothesize that species specific differences in TnT gene expression in teleosts underlies different larval muscle developmental programs which better adapts them to the specific ecological constraints.

Analysis of the Sox gene family in the European sea bass (Dicentrarchus labrax).

Sox (SRY-related genes containing a HMG box) genes encode a family of transcription factors that are involved in a variety of developmental processes including sex determination. Twenty Sox genes are present in the genomes of humans and mice, but far less is known about the Sox gene family in other vertebrate types. We have obtained clones representing the HMG boxes of twelve Sox genes from European sea bass (Dicentrarchus labrax), a fish species whose farming is complicated by a heavily skewed sex ratio, with between 70% and 99% of offspring typically being male. The cloned Sox genes are members of the SoxB, SoxC, SoxE and SoxF groups. Sequence analysis shows that some of the clones represent genes duplicated in sea bass with respect to the mammalian Sox gene family.