DIFFERENTIAL GENE-EXPRESSION OF THYROID-HORMONE RECEPTOR-ALPHA AND RECEPTOR-BETA IN FISH DEVELOPMENT

To understand the mechanisms of flounder metamorphosis, which is contr olled by thyroid hormone, gene expression of the thyroid hormone recep tors (TR alpha A, TR alpha B, TR beta 1, and TR beta 2) was studied in developing flounder larvae, using quantitative reverse-transcriptase polymerase chain reaction and in situ hybridization. TR gene transcrip ts were found at very low levels in fertilized flounder eggs. Substant ial amounts of TR mRNAs were present in premetamorphic larvae, except for TR alpha B mRNA, which was low throughout larval development. TR a lpha A gene transcripts increased rapidly in metamorphic climax and de creased rapidly postclimax. The expression level of TR beta s increase d in climax, reached its peak postclimax, and remained high in metamor phosed juveniles. In situ hybridization confirmed the decrease in TR a lpha transcripts in most tissues postclimax and further revealed the u biquitous expression of TR genes and distinct tissue specificity of al pha and beta subtypes in the overall fish body. These results suggest that thyroid hormone exerts effects directly on each tissue during fis h metamorphosis and that gene expression of TR subtypes is differentia lly regulated both temporally and regionally. Thus, the results sugges t that the development of each tissue of the flounder by thyroid hormo ne is further controlled at the receptor level by the differential exp ression of TRs. (C) 1998 Academic Press.