In Drosophila melanogaster the visceral mesoderm (VM) develops during embryogenesis in a process where myoblasts become specified to generate two distinct cell types, the founder cells (FCs) and the fusion competent myoblasts (FCMs) that consequently fuses. The cell specification is dependent on cell signaling mediated by the receptor tyrosine kinase (RTK) Anaplastic lymphoma kinase (Alk) and its ligand Jelly belly (Jeb), how this further sets up different identity programs that drive myoblasts to differentiate into FCs and FCMs is still not well understood. We have analysed whether the Midkine (MDK)/Pleiotrophin (PTN) homologues in Drosophila, Miple1 and Miple2 activate the Alk RTK in vivo. Earlier results from cell culture experiments suggested that vertebrate MDK/PTN is capable of activating ALK, findings that have become controversial with other studies showing contradictory results. We wanted to use Drosophila that have conserved homologues of both MDK/PTN and ALK, to address the question in vivo. We analysed the contribution of Miple in Alk dependent developmental processes such as visceral mesoderm (VM) specification during embryogenesis and in body size regulation of adult flies. Specification of VM as well as body size are not effected by loss of Miple proteins, and over expression of Miple proteins do not effect VM specification or body size. All together we conclude that there is no evidence that Miple1 or Miple2 can activate Alk in vivo. We found that loss of Miple protein effect the median lifespan of the fly which is reduced, interestingly the over expression of Miple proteins can promote an increased median life span in Drosophila. We have also analysed how Alk RTK signaling regulates the Gli-like transcription factor Lame duck (Lmd) in vivo on a post-translational level. It has already been reported that Lmd plays an essential role in specification of FCMs in the somatic mesoderm during embryogenesis. We detect Lmd protein exclusively in FCMs of VM in control embryos, but in Alk mutants Lmd protein is present in all cells of VM and opposite to this when Alk is activated in all cells in VM by over expression of Jeb this results in total loss of Lmd protein. This suggests that Alk signaling is regulating Lmd, and we additionally show that Lmd persist in FCMs in mutants where VM is specified but where myoblast fusion do not occur, supporting that Alk activity in FCs is regulating the downregulation of Lmd in FCMs upon fusion. Finally we have characterised the Rap1GEF C3G in vivo in Drosophila. In cell culture systems, the GTPase Rap1 has been identified to mediate Alk signaling and that this is regulated by the GEF C3G and interestingly the Drosophila C3G is expressed in the FCs of VM. We generated deletion mutants of C3G which exhibit semi-lethality and reduced life span, but no defects in visceral mesoderm development during embryogenesis. Instead we detected distinct phenotypes in somatic muscles of 3rd instar mutant larvae, with detachment and mistargeting of muscles, which effect localisation of integrins. We suggest that Drosophila C3G regulates Rap1 via inside out signaling of integrins which in turn effects cell adhesion in vivo in Drosophila larval muscles.

The fully developed and functional Drosophila midgut comprises two layers, the visceral mesoderm and the endoderm. The visceral muscle of the midgut is formed by the fusion of founder cells with fusion competent cells to form the muscle syncytia. The specification of these cells and thus the fusion and the formation of the midgut muscle is dependent on the  Receptor tyrosine kinase (RTK) Alk (Loren et al., 2003). The endoderm underlies the visceral muscle and is formed from cells that originate from the anterior and the posterior parts of the embryo. These cells use the visceral mesoderm as a substrate for their migration. Using Alk mutant animals, we have studied endoderm migration during embryonic development. While the initial migration of the endoderm is not affected in the absence of the visceral mesoderm, we observe that the later dorsal-ventral endodermal migration does not take place. The development of the visceral muscle and its dependence on the endoderm is poorly understood.  We have analysed gürtelchen (gurt) mutant animals, originally identified in a genetic screen for mutations affecting visceral muscle formation. Gurt mutants are so named due to their belt-like phenotype of the visceral muscle (gürtelchen is German for belt). Mapping of the genomic locus identified gurt as a mutation in a previously described gene - huckebein (hkb) which is known to have an important function in endoderm development. Gurt (hkb) mutants were used to further study the interaction between the endoderm and the visceral muscle during development. The initial specification of founder cells and fusion competent myoblasts as well as fusion events are unaffected in gurt (hkb) mutants, however, the elongation and stretching of the visceral muscle does not proceed as normal. Moreover, ablation of the visceral mesoderm disrupts endoderm migration, while ablation of the endoderm results in a delayed disruption of visceral muscle formation. Signaling between the two tissues was investigated in detail. Since Alk is a critical player in visceral muscle development, we employed Alk mutant embryos for this task. In addition to the role of Alk in specifying the founder cells and initiating the visceral muscle fusion, we have shown that Alk mediated signaling has a role in the induction of the midgut constriction process by regulating dpp expression in the developing embryonic gut.  Finally, we wished to identify genes in the founder cells/fusion competent myoblasts that might be regulated by Alk. C3G is a gunaine nucleotide exchange factor expressed in the visceral muscle founder cells. Deletion of the Drosophila C3G locus resulted in the generation of null mutants in C3G which are viable, but display decreased longevity, fitness and are semi-lethal. Further analysis of C3G mutants indicated that C3G is essential for normal larval musculature development, in part by regulating integrin localization at muscle attachment sites.