TAIL REGRESSION, APOPTOSIS AND THYROID-HORMONE REGULATION OF MYOSIN HEAVY-CHAIN ISOFORMS IN XENOPUS TADPOLES

T-3 effects on myosin heavy chain gene expression were analysed in mus cles undergoing different fates during metamorphosis. Muscle fate was followed by somatic gene transfer of a constitutively expressed lucife rase vector. Persistent expression was found in dorsal muscle which is remodelled during metamorphosis whilst the signal disappeared in apop tosing caudal muscle. RNAse protection assay was used to follow produc tion of myosin heavy chain isoforms: two isoforms identified as embryo nic (E3 and E19) and one adult form (A7). The effects of T-3 treatment were followed over 120 h. During this time frame E3 and A7 expression patterns were found to be similar in both caudal and dorsal muscles. Most notably, at 48 h E3 expression was significantly down-regulated a nd production of A7 significantly upregulated in both caudal and dorsa l muscle. Thus T-3-induced transitions in muscle gene expression are i ndependent of muscle fate during amphibian metamorphosis. (C) 1997 Els evier Science Ireland Ltd.