Abstract
During organogenesis, inductive signals cause cell differentiation and morphogenesis. However, how these phenomena are coordinated to form functional organs is poorly understood. Here, we show that cell differentiation of the Drosophila trachea is sequentially determined in two steps and that the second step is synchronous with the invagination of the epithelial sheet. The master gene trachealess is dispensable for the initiation of invagination, while it is essential for maintaining the invaginated structure, suggesting that tracheal morphogenesis and differentiation are separately induced. trachealess expression starts in bipotential tracheal/epidermal placode cells. After invagination, its expression is maintained in the invaginated cells but is extinguished in the remaining sheet cells. A trachealess cis-regulatory module that shows both tracheal enhancer activity and silencer activity in the surface epidermal sheet was identified. We propose that the coupling of trachealess expression with the invaginated structure ensures that only invaginated cells canalize robustly into the tracheal fate.
Highlights
A fundamental question in biology is how cells coordinately shape functional organs with complex architecture during embryogenesis
We showed that during Drosophila tracheal morphogenesis, (1) the master regulator trh is essential for maintaining the invaginated structure of the trachea but is dispensable for driving invagination of the placode, and (2) trh expression is maintained in invaginated cells, while its expression in surface epidermal cells is actively repressed
We propose that under these two mechanisms, the only successfully invaginated cells establish tight coupling between different hierarchies, tracheal cell fate and tubular architecture. trh-positive placode cells that do not take part in tubules lose their trh expression and adopt the epidermal fate with a flat sheet architecture
Summary
A fundamental question in biology is how cells coordinately shape functional organs with complex architecture during embryogenesis. R15F01 activation was detected in the trh mutants before invagination, but the number of b-gal-positive cells (29.4 ± 6.7) was smaller than that of the control (47.3 ± 6.1) (Figure 3—figure supplement 2E,G), indicating that trh is involved in the initial activation of R15F01 but is not essential. After mitosis cycle 16, transient invagination and disappearance of invaginated architecture in the trh mutants, 19.1 ± 6.8 cells still maintained R15F01 reporter expression at stage 15 (Figure 3—figure supplement 2F,H) Because this number is smaller than the number of initially activated cells, R15F01 activity could be maintained in a trhindependent manner. Embryos harboring only the 3UAS-R15F01 reporter without arm-GAL4 showed the tracheal GFP signal but not hindgut GFP expression (Figure 5I), indicating that the hindgut activity was driven by arm-GAL4 These results are consistent with the notion that R15F01 silences enhancer activities in the surface epidermis but not in internal tubular organs
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