Temporally restricted expression of transcription factor βFTZ-F1: significance for embryogenesis, molting and metamorphosis in Drosophila melanogaster

ABSTRACT FTZ-F1, a member of the nuclear receptor superfamily, has been implicated in the activation of the segmentation gene fushi tarazu during early embryogenesis of Drosophila melanogaster. We found that an isoform of FTZ-F1, βFTZ-F1, is expressed in the nuclei of almost all tissues slightly before the first and second larval ecdysis and before pupation. Severely affected ftz-f1 mutants display an embryonic lethal phenotype, but can be rescued by ectopic expression of βFTZ-F1 during the period of endogenous βFTZ-F1 expression in the wild type. The resulting larvae are not able to molt, but this activity is rescued again by forced expression of βFTZ-F1, allowing progression to the next larval instar stage. On the other hand, premature expression of βFTZ-F1 in wild-type larvae at mid-first instar or mid-second instar stages causes defects in the molting process. Sensitive periods were found to be around the time of peak ecdysteroid levels and slightly before the start of endogenous βFTZ-F1 expression. A hypomorphic ftz-f1 mutant that arrests in the prepupal stage can also be rescued by ectopic, time-specific expression of βFTZ-F1. Failure of salivary gland histolysis, one of the phenotypes of the ftz-f1 mutant, is rescued by forced expression of the ftz-f1 downstream gene BR-C during the late prepupal period. These results suggest that βFTZ-F1 regulates genes associated with ecdysis and metamorphosis, and that the exact timing of its action in the ecdysone-induced gene cascade is important for proper development.