Abstract
Larval metamorphosis in bivalves is a key event for the larva-to-juvenile transformation. Previously we have identified a thyroid hormone receptor (TR) gene that is crucial for larvae to acquire “competence” for the metamorphic transition in the mussel Mytilus courscus (Mc). The mechanisms of thyroid signaling in bivalves are still largely unknown. In the present study, we molecularly characterized the full-length of two iodothyronine deiodinase genes (McDx and McDy). Phylogenetic analysis revealed that deiodinases of molluscs (McDy, CgDx and CgDy) and vertebrates (D2 and D3) shared a node representing an immediate common ancestor, which resembled vertebrates D1 and might suggest that McDy acquired specialized function from vertebrates D1. Anti-thyroid compounds, methimazole (MMI) and propylthiouracil (PTU), were used to investigate their effects on larval metamorphosis and juvenile development in M. coruscus. Both MMI and PTU significantly reduced larval metamorphosis in response to the metamorphosis inducer epinephrine. MMI led to shell growth retardation in a concentration-dependent manner in juveniles of M. coruscus after 4 weeks of exposure, whereas PTU had no effect on juvenile growth. It is hypothesized that exposure to MMI and PTU reduced the ability of pediveliger larvae for the metamorphic transition to respond to the inducer. The effect of MMI and PTU on larval metamorphosis and development is most likely through a hormonal signal in the mussel M. coruscus, with the implications for exploring the origins and evolution of metamorphosis.
Highlights
Larval metamorphosis in bivalves is a key event for the larva-to-juvenile transformation
The conserved adenines in the apical loop and selenocysteine insertion sequence (SECIS) grade of two deiodinases were predicted as A, which proved that McDx and McDy were selenoproteins
Maximum Likelihood (ML) phylogenetic tree based on the full-length of McDx and McDy amino acids sequences was constructed to investigate the evolutionary relationship (Fig. 3)
Summary
Larval metamorphosis in bivalves is a key event for the larva-to-juvenile transformation. Anti-thyroid compounds, methimazole (MMI) and propylthiouracil (PTU), were used to investigate their effects on larval metamorphosis and juvenile development in M. coruscus. Both MMI and PTU significantly reduced larval metamorphosis in response to the metamorphosis inducer epinephrine. Despite differences in the function of selectively eliminating iodine moieties from two phenyl rings in iodothyronines, the removal of iodide in three enzymes is both dependent on the rare amino acid selenocysteine (Sec) in their core active center[14]. Anti-thyroid compounds such as 6-n-propyl-2-thiouracil (PTU) and methimazole (MMI) have been employed to disrupt the thyroid axis for investigating the effect of THs synthesis and metabolism in larval development and growth of v ertebrates[18,19]. The presence of putative deiodinases and thyroid peroxidase genes in M. coruscus transcriptome leads us to speculate that accumulated iodine in mussels could be used to iodinate their proteins which may be important for growth and development
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