HEARTLESS, A DROSOPHILA FGF RECEPTOR HOMOLOG, IS ESSENTIAL FOR CELL-MIGRATION AND ESTABLISHMENT OF SEVERAL MESODERMAL LINEAGES

A Drosophila FGF receptor homolog (DFGF-R2/DFRI) termed Heartless (Htl ) is expressed in the embryonic mesoderm. The phenotypes of null mutan t embryos demonstrated that Htl is a central player that is required f or the development of several mesodermal lineages. No abnormalities in the primary specification of the mesoderm were observed. The first de fects were seen as irregular migration and spreading of the mesoderm o ver the ectoderm. Subsequently, cell fates were not induced in several lineages including the visceral mesoderm, heart, and the dorsal somat ic muscles. The defects in the induction of cell fates are likely to r esult from failure of the mesoderm to spread over the ectoderm and rec eive patterning signals. The defective spreading could be circumvented in htl mutant embryos by providing an ectopic Dpp patterning signal, leading to the formation of heart and dorsal muscle cells. Htl appears to be required also subsequently during the migration and morphogenes is of the different lineages. Expression of a dominant-negative htl co nstruct after the initial induction of eel fates gave rise to aberrant migration and organization of the visceral mesoderm, heart, and somat ic muscles. Thus, a common role for Htl in cell migration and tissue o rganization may account for the pleiotropic defects of the htl mutatio n.