Myogenesis of the smooth muscles surrounding the testes of Drosophila melanogaster males. Jessica Kuckwa¹, Christina Hornbruch-Freitag¹, Loreen Susic-Jung¹, Uwe Lammel², Renate Renkawitz-Pohl¹. 1) Developmental Biology, University of Marburg, 35043, Marburg, Germany; 2) Neurobiology, University of Muenster, 48149, Muenster, Germany.

   Myoblast fusion and myotube differentiation has been extensively studied in the Drosophila embryo, but limitations occurred due to functional redundancies and maternally supplied components. We here focus on the male reproductive system as a model to study myogenesis of different types of muscles. The inner genitalia of males consist of the testes, which emerge from the gonads and the remaining genital organs, i.e. paragonia, ejaculatory duct, sperm pump and seminal vesicles, developing out of the genital imaginal disc. We identified the muscles of the testes, seminal vesicles and sperm pump to be multinucleated. Paragonia and ejaculatory duct are encircled by a mononucleated muscle layer. All muscles derive from myoblasts of the male genital disc. Upon all characterized Drosophila muscles, the testes musculature is very special: their unique filament arrangement is reminiscent of vertebrate smooth muscle fibers. Smooth muscles have not been described in Drosophila, so far. Analyzing the developing muscle fibers, we revealed that the myoblasts building up the muscles of the seminal vesicles and/or the testes begin to get multinucleated on the male genital disc. The nascent myotubes then migrate over the seminal vesicles onto the testes where they spread out and elongate. This process is accompanied by expression of the mesodermal transcription factor DMef2, needed for specification and differentiation of embryonic myoblasts. The immunoglobulin proteins Duf and Sns are similarly expressed. Duf and Sns function during embryonic myoblast fusion as well as in ommatidia spacing during eye development. knocking down either sns or duf by RNAi in the developing testes muscles lead to some scattered filaments, arranged like the little rods in the Mikado game (Mikado-phenotype). All in all, we currently suppose that the multinucleated smooth muscles surrounding the testes develop by myoblast fusion during migration of the myoblasts/myotubes on the genital disc.