SUMMARYStudies of the onset of metamorphosis have identified an ecdysone-triggered transcriptional cascade that consists of the sequential expression of the transcription-factor-encoding genes DHR3, FTZ-F1, E74A and E75A. Although the regulatory interactions between these genes have been well characterized by genetic and molecular studies over the past 20 years, their developmental functions have remained more poorly understood. In addition, a transcriptional sequence similar to that observed in prepupae is repeated before each developmental transition in the life cycle, including mid-embryogenesis and the larval molts. Whether the regulatory interactions between DHR3, FTZ-F1, E74A and E75A at these earlier stages are similar to those defined at the onset of metamorphosis, however, is unknown. In this study, we turn to embryonic development to address these two issues. We show that mid-embryonic expression of DHR3 and FTZ-F1 is part of a 20-hydroxyecdysone (20E)-triggered transcriptional cascade similar to that seen in mid-prepupae, directing maximal expression of E74A and E75A during late embryogenesis. In addition, DHR3 and FTZ-F1 exert overlapping developmental functions at the end of embryogenesis. Both genes are required for tracheal air filling, whereas DHR3 is required for ventral nerve cord condensation and FTZ-F1 is required for proper maturation of the cuticular denticles. Rescue experiments support these observations, indicating that DHR3 has essential functions independent from those of FTZ-F1. DHR3 and FTZ-F1 also contribute to overlapping transcriptional responses during embryogenesis. Taken together, these studies define the lethal phenotypes of DHR3 and FTZ-F1 mutants, and provide evidence for functional bifurcation in the 20E-responsive transcriptional cascade.

Animals balance nutrient storage and mobilization to maintain metabolic homeostasis, a process that is disrupted in metabolic diseases like obesity and diabetes. Here, we show that DHR38, the single fly ortholog of the mammalian nuclear receptor 4A family of nuclear receptors, regulates glycogen storage during the larval stages of Drosophila melanogaster. DHR38 is expressed and active in the gut and body wall of larvae, and its expression levels change in response to nutritional status. DHR38 null mutants have normal levels of glucose, trehalose (the major circulating form of sugar), and triacylglycerol but display reduced levels of glycogen in the body wall muscles, which constitute the primary storage site for carbohydrates. Microarray analysis reveals that many metabolic genes are mis-regulated in DHR38 mutants. These include phosphoglucomutase, which is required for glycogen synthesis, and the two genes that encode the digestive enzyme amylase, accounting for the reduced amylase enzyme activity seen in DHR38 mutant larvae. These studies demonstrate that a critical role of nuclear receptor 4A receptors in carbohydrate metabolism has been conserved through evolution and that nutritional regulation of DHR38 expression maintains the proper uptake and storage of glycogen during the growing larval stage of development.

Insertional mutagenesis with a heterologous transposon provides a method to rapidly determine the molecular identity of mutated genes. The Drosophila transposon Mos1 can be mobilized to cause mutations in Caenorhabditis elegans (Bessereau et al. 2001); however, the mutagenic rate was initially too low for use in most forward genetic screens. To increase the effectiveness of Mos1-mediated mutagenesis we examined the conditions influencing Mos1 transposition. First, optimal transposition occurs 24 hr after expression of the transposase and is unlikely to occur in differentiated sperm or oocytes. Second, transposition is limited to germ-cell nuclei that contain donor elements, but the transposase enzyme can diffuse throughout the gonad syncytium. Third, silencing of transposition is caused by changes in the donor array that occur over time. Finally, multiple transposition events occur in individual germ cells. By using screening techniques based on these results, Mos1 mutagenicity was increased to within an order of magnitude of chemical mutagens.

Neuroendocrine signaling pathways play a central role in modulating animal body size in response to environmental signals. Little is known, however, regarding how these neuroendocrine circuits are controlled. An important advance in this area is reported in this issue of Genes & Development by Kaplan and colleagues (pp. 1877–1893), who show that serotonergic neurons regulate the growth of peripheral tissues in Drosophila through the insulin/IGF pathway.