A Genetic Mosaic Screen Reveals Ecdysone-Responsive Genes RegulatingDrosophilaOogenesis

Abstract

Multiple aspects of Drosophila oogenesis, including germline stem cell activity, germ cell differentiation, and follicle survival, are regulated by the steroid hormone ecdysone. While the transcriptional targets of ecdysone signaling during development have been studied extensively, targets in the ovary remain largely unknown. Early studies of salivary gland polytene chromosomes led to a model in which ecdysone stimulates a hierarchical transcriptional cascade, wherein a core group of ecdysone-sensitive transcription factors induce tissue-specific responses by activating secondary branches of transcriptional targets. More recently, genome-wide approaches have identified hundreds of putative ecdysone-responsive targets. Determining whether these putative targets represent bona fide targets in vivo, however, requires that they be tested via traditional mutant analysis in a cell-type specific fashion. To investigate the molecular mechanisms whereby ecdysone signaling regulates oogenesis, we used genetic mosaic analysis to screen putative ecdysone-responsive genes for novel roles in the control of the earliest steps of oogenesis. We identified a cohort of genes required for stem cell maintenance, stem and progenitor cell proliferation, and follicle encapsulation, growth, and survival. These genes encode transcription factors, chromatin modulators, and factors required for RNA transport, stability, and ribosome biogenesis, suggesting that ecdysone might control a wide range of molecular processes during oogenesis. Our results suggest that, although ecdysone target genes are known to have cell type-specific roles, many ecdysone response genes that control larval or pupal cell types at developmental transitions are used reiteratively in the adult ovary. These results provide novel insights into the molecular mechanisms by which ecdysone signaling controls oogenesis, laying new ground for future studies.

A hormonal cue promotes timely follicle cell migration by modulating transcription profiles

“…Prior studies had demonstrated that the transcriptional regulators EcR and Tai are required in border cells after motile cell specification occurs through JAK/STAT/Slbo activation(Bai et al, 2000; Cherbas et al, 2003; Hackney et al, 2007; Jang et al, 2009). A role for transcriptional regulation during cell migration is underappreciated, aside from the transcriptional programs required for cell fate, possibly because many cell migration events have been studied in vitro and signaling regulation on the protein level is often sufficient for accurate movements (Ables et al, 2016; Riddiford et al, 1993). In vivo, however, transcriptional control adds a layer of adaptability in responding to environmental and growth cues, so it is likely to be essential within migrating cells during development.…”

“…Conversely, early expression of an activated form of tai ( taiΔB ) can cause abnormally early movements of the border cell cluster, but only in combination with ectopically early specification via STAT activation (Jang et al, 2009). Several key ecdysone targets have been identified during metamorphosis (Beckstead et al, 2005; Beckstead et al, 2007), and in ovary (Ables et al, 2015; Ables and Drummond-Barbosa, 2010; Ables et al, 2016; Buszczak et al, 1999; Terashima and Bownes, 2005), but few are known to have roles in cell motility. Furthermore, it is not clear how downstream targets for EcR, Tai, STAT, and Slbo coordinate their activities to result in proper temporal control of border cell movements.…”

Steroid Hormones and the Physiological Regulation of Tissue-Resident Stem Cells: Lessons from the Drosophila Ovary

“…Ecdysone signaling mediates the increase in GSC proliferation induced by the first mating of young females [26], and is necessary for sustained GSC proliferation and self-renewal [38]. Ecdysone signaling is also necessary for the differentiation of germ cells and the individualization of germline cysts into discrete follicles [39, 23, 40, 41]. As oogenesis proceeds, ecdysone signaling is required for the growth and survival of germline cysts at two important developmental stages.…”

“…A genetic screen designed to find ecdysone-responsive genes required for oogenesis identified several genes that modulate the fate and proliferative capacity of FSCs [41]. FSCs give rise to the follicle cell lineage, which is required for proper germline development and ultimately forms the eggshell [13].…”

“…Ecdysone signaling is well known to regulate follicle cell function at later stages of oogenesis [45–47, 35, 36, 48, 49, 37], but its roles in early somatic cell differentiation remain largely undescribed. While our data suggest that ecdysone signaling controls FSC function [41], it remains to be investigated whether this occurs through similar molecular mechanisms as those controlling GSCs. The screen identified a mixture of downstream targets, some common to GSCs and FSCs and some with separate effects, suggesting that ecdysone signaling may control the two stem cell populations largely independently.…”