Collective cell migration is critical for morphogenesis, homeostasis, and wound healing. Migrating mesenchymal cells form tissues that shape the body's organs. We developed a powerful model, exploring how Drosophila nascent myotubes migrate onto the testis during pupal development, forming the muscles ensheathing it and creating its characteristic spiral shape. To define genes regulating this, we used RNAseq to identify genes expressed in myotubes during migration. Using this dataset, we curated a list of 131 ligands, receptors, and cytoskeletal regulators, including all Rho/Ras/Rap1 regulators, as candidates. We then expressed 279 shRNAs targeting these genes and examined adult testes. We identified 29 genes with diverse roles in morphogenesis. Some have phenotypes consistent with defective migration, while others alter testis shape in different ways, revealing the underlying logic of testis morphogenesis. We followed up the Rho-family GEF dPix in detail. dPix knockdown drastically reduced migration and thus muscle coverage. Our data suggest different isoforms of dPix play distinct roles in this process and reveal a role for its partner Git. We also explored whether dPix regulates Cdc42 activity or cell adhesion. Our RNAseq dataset and genetic analysis provide an important resource for the community to explore cell migration and organ morphogenesis.