The Drosophila gene Anaplastic lymphoma kinase (Alk) is homologous to mammalian ALK, a member of the Alk/Ltk family of receptor tyrosine kinases. In Drosophila Alk is crucial for development of the embryonic visceral mesoderm, where it is the receptor for Jelly Belly (Jeb) ligand. Jeb binding stimulates an Alk-driven, extracellular signal-regulated, kinase-mediated signaling pathway, which results in the expression of the downstream gene duf/kirre. The visceral mesoderm is made up from two different cell types, founder cells and fusion competent myoblasts. The Jeb-Alk signal transduction pathway drives specification of the founder cells of the Drosophila visceral muscle. In this work we aimed to identify genes specifically expressed in the founder cells and/or fusion competent myoblasts. Four genes from a number of candidiates were investigated further. These genes are Hand, expressed in founder cells, goliath, parcas and delilah, which are expressed in fusion competent myoblasts. Hand is a basic helix-loop-helix (bHLH) transcription factor. Alk-mediated signal transduction drives the expression of the bHLH transcription factor hand in vivo, and loss of Alk function results in a complete lack of hand expression in the visceral mesoderm, while Alk gain of function results in an expansion of hand expression. There are no obvious defects in the visceral muscle fusion process hand mutant animals, suggesting that Hand is not critical for visceral muscle fusion per se. I have studied another molecule Goliath, a putative RING finger E3 ligase. goliath is specifically expressed in the FCM of visceral and somatic muscles. goliath mutant animals do not display any obvious muscle phenotypes, perhaps reflecting a redundant role with CG10277, which encodes a second Goliath family protein in Drosophila. Deletion mutation of the CG10277 locus does not result in muscle defects either, and generation of double mutants of goliath and CG10277 will be required to determine their function in vivo. In addition, I have studied another bHLH transcription factor Delilah and its role in muscle development. We show that delilah is expressed in visceral muscle, somatic muscles and in tendon cells. Delilah mutant animals display a held out wing phenotype and are unable to fly. Inducible RNAi against delilah results in a similar phenotype. Delilah is transcriptionally regulated by mef2 and biniou, early regulators of muscle development. While delilah appears to function in tendon cells, we were unable to find any obvious phenotype in either visceral or somatic muscles. In order to further investigate the underlying mechanism of Delilah function we have used Tandem affinity purification (TAP) methodology followed by mass spectrometry to identify Delilah binding partners. This analysis suggests a number of candidate functional partners for the Delilah protein.

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.