Conditional knockout of retinal determination genes in differentiating cells in Drosophila

Hippo pathway and Bonus control developmental cell fate decisions in the Drosophila eye

The canonical function of the Hippo signaling pathway is the regulation of organ growth. How this pathway controls cell-fate determination is less well understood. Here, we identify a function of the Hippo pathway in cell-fate decisions in the developing Drosophila eye, exerted through the interaction of Yorkie (Yki) with the transcriptional regulator Bonus (Bon), an ortholog of mammalian transcriptional intermediary factor 1/tripartite motif (TIF1/TRIM) family proteins. Instead of controlling tissue growth, Yki and Bon promote epidermal and antennal fates at the expense of the eye fate. Proteomic, transcriptomic, and genetic analyses reveal that Yki and Bon control these cell-fate decisions by recruiting transcriptional and post-transcriptional co-regulators and by repressing Notch target genes and activating epidermal differentiation genes. Our work expands the range of functions and regulatory mechanisms under Hippo pathway control.

Glass promotes the differentiation of neuronal and non-neuronal cell types in the Drosophila eye

Transcriptional regulators can specify different cell types from a pool of equivalent progenitors by activating distinct developmental programs. The Glass transcription factor is expressed in all progenitors in the developing Drosophila eye, and is maintained in both neuronal and non-neuronal cell types. Glass is required for neuronal progenitors to differentiate as photoreceptors, but its role in non-neuronal cone and pigment cells is unknown. To determine whether Glass activity is limited to neuronal lineages, we compared the effects of misexpressing it in neuroblasts of the larval brain and in epithelial cells of the wing disc. Glass activated overlapping but distinct sets of genes in these neuronal and non-neuronal contexts, including markers of photoreceptors, cone cells and pigment cells. Coexpression of other transcription factors such as Pax2, Eyes absent, Lozenge and Escargot enabled Glass to induce additional genes characteristic of the non-neuronal cell types. Cell type-specific glass mutations generated in cone or pigment cells using somatic CRISPR revealed autonomous developmental defects, and expressing Glass specifically in these cells partially rescued glass mutant phenotypes. These results indicate that Glass is a determinant of organ identity that acts in both neuronal and non-neuronal cells to promote their differentiation into functional components of the eye.

Pleiotropy in Drosophila organogenesis: Mechanistic insights from Combgap and the retinal determination gene network

Master regulatory transcription factors cooperate in networks to shepherd cells through organogenesis. In the Drosophila eye, a collection of master control proteins known as the retinal determination gene network (RDGN) switches the direction and targets of its output to choreograph developmental transitions, but the molecular partners that enable such regulatory flexibility are not known. We recently showed that two RDGN members, Eyes absent (Eya) and Sine oculis (So), promote exit from the terminal cell cycle known as the second mitotic wave (SMW) to permit differentiation. A search for co-factors identified the ubiquitously expressed Combgap (Cg) as a novel transcriptional partner that impedes cell cycle exit and interferes with Eya-So activity specifically in this context. Here, we argue that Cg acts as a flexible transcriptional platform that contributes to numerous gene expression outcomes by a variety of mechanisms. For example, Cg provides repressive activities that dampen Eya-So output, but not by recruiting Polycomb chromatin-remodeling complexes as it does in other contexts. We propose that master regulators depend on both specifically expressed co-factors that assemble the combinatorial code and broadly expressed partners like Cg that recruit the diverse molecular activities needed to appropriately regulate their target enhancers.

Antagonistic regulation of the second mitotic wave by Eyes absent-Sine oculis and Combgap coordinates proliferation and specification in the Drosophila retina

The transition from proliferation to specification is fundamental to the development of appropriately patterned tissues. In the developing Drosophila eye, Eyes absent (Eya) and Sine oculis (So) orchestrate the progression of progenitor cells from asynchronous cell division to G₁ arrest and neuronal specification at the morphogenetic furrow. Here, we uncover a novel role for Eya and So in promoting cell cycle exit in the second mitotic wave (SMW), a synchronized, terminal cell division that occurs several hours after passage of the furrow. We show that Combgap (Cg), a zinc-finger transcription factor, antagonizes Eya-So function in the SMW. Based on the ability of Cg to attenuate Eya-So transcriptional output in vivo and in cultured cells and on meta analysis of their chromatin occupancy profiles, we speculate that Cg limits Eya-So activation of select target genes posterior to the furrow to ensure properly timed mitotic exit. Our work supports a model in which context-specific modulation of transcriptional activity enables Eya and So to promote both entry into and exit from the cell cycle in a distinct spatiotemporal sequence.

Mutant analysis by rescue gene excision: New tools for mosaic studies in Drosophila

A host of classical and molecular genetic tools make Drosophila a tremendous model for the dissection of gene activity. In particular, the FLP-FRT technique for mitotic recombination has greatly enhanced gene loss-of-function analysis. This technique efficiently induces formation of homozygous mutant clones in tissues of heterozygous organisms. However, the dependence of the FLP-FRT method on cell division, and other constraints, also impose limits on its effectiveness. We describe here the generation and testing of tools for Mutant Analysis by Rescue Gene Excision (MARGE), an approach whereby mutant cells are formed by loss of a rescue transgene in a homozygous mutant organism. Rescue-transgene loss can be induced in any tissue or cell-type and at any time during development or in the adult using available heat-shock-induced or tissue-specific flippases, or combinations of UAS-FLP with Gal4 and Gal80^ts reagents. The simultaneous loss of a constitutive fluorescence marker (GFP or RFP) identifies the mutant cells. We demonstrate the efficacy of the MARGE technique by flip-out (clonal and disc-wide) of a Ubi-GFP-carrying construct in imaginal discs, and by inducing a known yki mutant phenotype in the Drosophila ovary.