The thisbe (ths) gene encodes a Drosophila fibroblast growth factor (FGF), and mutant females are viable but sterile suggesting a link between FGF signaling and fertility. Ovaries exhibit abnormal morphology including lack of epithelial sheaths, muscle tissues that surround ovarioles. Here we investigated how FGF influences Drosophila ovary morphogenesis and identified several roles. Heartless (Htl) FGF receptor was found expressed within somatic cells at the larval and pupal stages, and phenotypes were uncovered using RNAi. Differentiation of terminal filament cells was affected, but this effect did not alter ovariole number. In addition, proliferation of epithelial sheath progenitors, the apical cells, was decreased in both htl and ths mutants, while ectopic expression of the Ths ligand led to these cells’ over-proliferation suggesting that FGF signaling supports ovarian muscle sheath formation by controlling apical cell number in the developing gonad. Additionally, live imaging of adult ovaries was used to show that htl RNAi mutants, hypomorphic mutants in which epithelial sheaths are present, exhibit abnormal muscle contractions. Collectively, our results demonstrate that proper formation of ovarian muscle tissues is regulated by FGF signaling in the larval and pupal stages through control of apical cell proliferation and is required to support fertility.

SUMMARY Rapid mitotic divisions and a fixed transcription rate limit the maximal length of transcripts in early Drosophila embryos. Previous studies suggested that transcription of long genes is initiated but aborted, as early nuclear divisions have short interphases. Here, we identify long genes that are expressed during short nuclear cycles as truncated transcripts. The RNA binding protein Sex-lethal physically associates with transcripts for these genes and is required to support early termination to specify shorter transcript isoforms in early embryos of both sexes. In addition, one truncated transcript for the gene short-gastrulation encodes a product in embryos that functionally relates to a previously characterized dominant-negative form, which maintains TGF-β signaling in the off-state. In summary, our results reveal a developmental program of short transcripts functioning to help temporally regulate Drosophila embryonic development, keeping cell signaling at early stages to a minimum in order to support its proper initiation at cellularization.

Graded transcription factors are pivotal regulators of embryonic patterning, but whether their role changes over time is unclear. A light-regulated protein degradation system was used to assay temporal dependence of the transcription factor Dorsal in dorsal-ventral axis patterning of Drosophila embryos. Surprisingly, the high-threshold target gene snail only requires Dorsal input early but not late when Dorsal levels peak. Instead, late snail expression can be supported by action of the Twist transcription factor, specifically, through one enhancer, sna.distal. This study demonstrates that continuous input is not required for some Dorsal targets and downstream responses, such as twist, function as molecular ratchets.

Morphogen concentration changes in space as well as over time during development. However, how these dynamics are interpreted by cells to specify fate is not well understood. Here, we focus on two morphogens: the maternal transcription factors Bicoid and Dorsal, which directly regulate target genes to pattern Drosophila embryos. The actions of these factors at enhancers has been thoroughly dissected and provides a rich platform for understanding direct input by morphogens and their changing roles over time. Importantly, Bicoid and Dorsal do not work alone; we also discuss additional inputs that work with morphogens to control spatiotemporal gene expression in embryos.

In Drosophila embryos, genes expressed along the dorsal-ventral axis are responsive to concentration of the Dorsal (Dl) transcription factor, which varies in space; however, levels of this morphogen also build over time. Since expression of high-threshold Dl target genes such as snail (sna) is supported before Dl levels peak, it is unclear what role increasing levels have if any. Here we investigated action of two enhancers that control sna expression in embryos, demonstrating using genome editing that Dl binding sites within one enhancer located promoter proximally, sna.prox, can limit the ability of the other distally-located enhancer, sna.dis, to increase sna levels. In addition, MS2-MCP live imaging was used to study sna transcription rate in wildtype, dl heterozygote, and a background in which a photo-sensitive degron is fused to Dl (dl-BLID). The results demonstrate that, when Dl levels are high, Dl acts through sna.prox to limit the activity of sna.dis and thereby influence sna transcription rate. In contrast, when Dl levels are kept low using dl-BLID, sna.prox positively influences sna transcription rate. Collectively, our data support the view that Dl’s effect on gene expression changes over time, switching from promoting sna expression at low concentration to dampening sna expression at high concentration by regulating enhancer interactions. We propose this differential action of the Dl morphogen is likely supported by occupancy of this factor first to high and then low affinity binding sites over time as Dl levels rise to coordinate action of these two co-acting enhancers.Significance statementA gradient of the maternal transcription factor Dorsal is important for establishing spatial expression of target genes along the dorsal-ventral axis of Drosophila embryos. Dorsal levels are also dynamic as nuclear concentration builds in time. Surprisingly, expression of high-threshold target genes such as snail is supported before levels peak, raising the question why levels continue to build. Our data support the view that peak Dorsal levels act to preferentially support activity of one enhancer over another to effectively decrease snail expression. In addition, while the morphogen Dorsal acts early to support gene expression, later it effectively acts as a damper to limit gene expression. Our results suggest other morphogens also have effects on gene expression that change over time.