Abstract
The testis of Drosophila resembles an individual testis tubule of mammals. Both are surrounded by a sheath of smooth muscles, which in Drosophila are multinuclear and originate from a pool of myoblasts that are set aside in the embryo and accumulate on the genital disc later in development. These muscle stem cells start to differentiate early during metamorphosis and give rise to all muscles of the inner male reproductive system. Shortly before the genital disc and the developing testes connect, multinuclear nascent myotubes appear on the anterior tips of the seminal vesicles. Here, we show that adhesion molecules are distinctly localized on the seminal vesicles; founder cell (FC)-like myoblasts express Dumbfounded (Duf) and Roughest (Rst), and fusion-competent myoblast (FCM)-like cells mainly express Sticks and stones (Sns). The smooth but multinuclear myotubes of the testes arose by myoblast fusion. RNAi-mediated attenuation of Sns or both Duf and Rst severely reduced the number of nuclei in the testes muscles. Duf and Rst probably act independently in this context. Despite reduced fusion in all of these RNAi-treated animals, myotubes migrated onto the testes, testes were shaped and coiled, muscle filaments were arranged as in the wild type and spermatogenesis proceeded normally. Hence, the testes muscles compensate for fusion defects so that the myofibres encircling the adult testes are indistinguishable from those of the wild type and male fertility is guaranteed.
Highlights
Function and formation of multinuclear myofibres are largely conserved within the animal kingdom
These results showed that myoblasts amplify during metamorphosis and first become multinuclear on the prospective seminal vesicles of the male genital disc at around 28 h after puparium formation (APF)
The cellular adhesion mediated by the immunoglobulin superfamily (IgSF) proteins and their signalling in myoblasts is essential for myoblast fusion and formation of striated muscles
Summary
Function and formation of multinuclear myofibres are largely conserved within the animal kingdom. Numerous studies in vertebrates and Drosophila melanogaster have revealed that multinuclear striated myotubes arise by myoblast fusion (Abmayr and Pavlath, 2012). In Drosophila, the stem cells most similar to satellite cells are the adult muscle precursor cells (Figeac et al, 2007), which allow modification of larval muscles into templates for dorsal longitudinal indirect flight muscles during metamorphosis (Roy and VijayRaghavan, 1998) These stem cells are set aside during embryogenesis and amplify mitotically in the larvae
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