The syncytial larval musculature of Drosophila melanogaster develops during embryogenesis by fusion of two different cell types, the founder cells (FCs), which determine the identity of an individual muscle, and the fusion competent myoblasts (FCMs). During the fusion events in the somatic mesoderm, which gives rise to the body wall muscles, a Fusion-restricted Myogenic-Adhesive Structure (FuRMAS) is established, consisting of a ring of adhesion molecules and their adaptor proteins as well as of an actin-rich plug on the side of the FCM and an actin sheet on the side of the FC. This FuRMAS is supposed to act as a signaling center, linking cell adhesion to downstream fusion steps.
The actin cytoskeleton is supposed to be important in processes like vesicle transport, fusion pore expansion and force generation at the FuRMAS. This thesis provides analyses of transport processes via microtubules during myogenesis by analyzing the expression of -Tubulin isoforms, as the structural subunits of microtubules. Although strongly expressed in the mesoderm, the somatic and visceral musculature develops independently of the 3-Tubulin isoform. Furthermore, a low level of maternally supplied 1-Tubulin is sufficient for body wall muscle formation. Thus, it is concluded that newly synthesized microtubules and microtubule-based transport processes are less important for Drosophila myogenesis.
Rolling pebbles 7 (Rols7) is an essential adaptor protein at the FuRMAS on the side of the FCs, which interacts in vitro with Myosin heavy chain-like (Mhcl). In this thesis, it is shown that Mhcl is also expressed in FCs, and localizes at the contact sites towards the adhering FCM. This unconventional myosin might act as a motor protein for F-actin at the FuRMAS, being involved in vesicle transport or widening of the fusion pore, most likely redundantly to other myosin heavy chains.
The syncytial visceral muscles of the embryo surround the gut as a network of binucleated circular muscles and perpendicularly arranged multinucleated longitudinal muscles. The longitudinal FCs migrate from the caudal visceral mesoderm and are shown in this thesis to fuse with a different FCM type than the circular FCs do. Furthermore, Rols7 is needed during myoblast fusion giving rise to the longitudinal gut muscles, while proteins regulating actin polymerization are involved either already in migration of the longitudinal FCs or also in the fusion processes itself. In conclusion, this muscle type develops distinct to the circular gut muscles and the body wall musculature of Drosophila.