Differential Gene Expression of Thyroid Hormone Receptor α and β in Fish Development

Abstract

To understand the mechanisms of flounder metamorphosis, which is controlled by thyroid hormone, gene expression of the thyroid hormone receptors (TRαA, TRαB, TRβ1, and TRβ2) was studied in developing flounder larvae, using quantitative reverse-transcriptase polymerase chain reaction andin situhybridization. TR gene transcripts were found at very low levels in fertilized flounder eggs. Substantial amounts of TR mRNAs were present in premetamorphic larvae, except for TRαB mRNA, which was low throughout larval development. TRαA gene transcripts increased rapidly in metamorphic climax and decreased rapidly postclimax. The expression level of TRβs increased in climax, reached its peak postclimax, and remained high in metamorphosed juveniles.In situhybridization confirmed the decrease in TRα transcripts in most tissues postclimax and further revealed the ubiquitous expression of TR genes and distinct tissue specificity of α and β subtypes in the overall fish body. These results suggest that thyroid hormone exerts effects directly on each tissue during fish metamorphosis and that gene expression of TR subtypes is differentially regulated both temporally and regionally. Thus, the results suggest that the development of each tissue of the flounder by thyroid hormone is further controlled at the receptor level by the differential expression of TRs.