The PR factor Hamlet controls heterotypic epithelial tissue assembly in Drosophila reproduction system

Epithelial tissue fusion is essential for animal development and wound healing, yet its regulatory mechanisms remain elusive. The Drosophila reproductive system (RS) offers a model to study such fusion events, as it integrates independently formed gonadal- and genital disc-derived structures for efficient sperms and egg delivery. In this study, we unveil the pivotal role of Hamlet (Ham), a PR-domain containing transcription factor, in orchestrating sequential epithelial assembly steps in the Drosophila RS. Even mild reductions in ham expression lead to sterility and disruption of the connection between the testis (TE) and seminal vesicle (SV). To elucidate the regulatory network governing RS epithelial assembly, we systematically identified Ham-downstream effector genes, encompassing cytoskeletal, metabolic regulators, Wnt2, Toll signaling pathway genes, and numerous uncharacterized molecules. Notably, we discovered that Drosophila RS epithelial tube assembly represents a process of heterotypic epithelium fusion, with distinct gene expression profiles in TE terminal and SV epithelial cells. Furthermore, TE epithelial cells exhibit high sensitivity to E-Cadherin levels, as reduction of E-Cad induces epithelial-to-mesenchymal-transition-like behaviors. Our findings present a comprehensive gene network crucial for heterotypic epithelial tissue fusion. Given the mammalian Ham orthologs in epithelial tissues, our results suggest an evolutionarily conserved role for these factors across species.