REQUIREMENTS OF DFR1 HEARTLESS, A MESODERM-SPECIFIC DROSOPHILA FGF-RECEPTOR, FOR THE FORMATION OF HEART, VISCERAL AND SOMATIC MUSCLES, AND ENSHEATHING OF LONGITUDINAL AXON TRACTS IN CNS/

DFR1 encodes a mesoderm-specific fibroblast growth factor receptor in Drosophila. Here, we identified and characterized a protein-null mutan t of DFR1 and examined DFR1 expression in embryos using anti-DFR1 anti body. Mutant phenotypes were completely rescued by a genomic fragment from the DFR1 locus. After imagination, mesodermal cells expressing DF R1 undergo proliferation and spread out dorsally to form a monolayer b eneath the ectoderm. In mutant embryos, however, the mesoderm is not c apable of extending to the normal dorsal limit and consequently mesode rmal cells fail to receive ectodermal signals and thus rendered incapa ble of differentiating into primordia for the heart, visceral and soma tic muscles. DFR1 is also required for normal development of the centr al nervous system. The absence of DFR1 resulted in the failure of long itudinal glia to enwrap longitudinal axon tracts. DFR1 mutant phenotyp es were partially mimicked by the targeted expression of activated Yan , thus demonstrating the MAP kinase pathway to be involved in differen tiation of mesoderm.