HSP90 proteins are major chaperones that play pivotal roles in controlling multiple regulatory pathways such as hormone signaling,cell differentiation,cell proliferation and apoptosis,morphogenesis and morphological evolution,metamorphosis and stress defense.Using realtime fluorescent quantitative RT-PCR approach,expression profiles of HSP90 genes were examined during larval development and in adult tissues in Japanese flounder,Paralichthys olivaceus.In adult tissues,HSP90α was mainly expressed with higher transcript levels in skeletal muscle,intestine and stomach; but HSP90β exhibited higher mRNA levels in brain,spleen and kidney.During metamorphosis,HSP90α transcript increased distinctly with the highest mRNA levels at the metamorphosis climax; whereas HSP90β mRNA levels did not change.Due to the important role of thyroid hormone(TH)for the flounder metamorphosis,the transcriptional regulation of HSP90 genes by TH was evaluated using exogenous TH and thiourea(TU)treatment.Compared with untreated control,HSP90α transcript increased significantly in larvae sampled 8 and 13 days after TH treatment,and TU-treated larvae exhibited lower HSP90α mRNA levels.In contrast,the transcript levels of HSP90β did not vary after TH and TU treatment.Overall,these results demonstrate that HSP90α transcription is upregulated by thyroid hormone and plays an important role during metamorphosis in Japanese flounder.