Heartbroken is a specific downstream mediator of FGF receptor signalling in Drosophila

Drosophila possesses two FGF receptors which are encoded by the heartless a nd breathless genes, HEARTLESS is essential for early migration and pattern ing of the embryonic mesoderm, while BREATHLESS is required for proper bran ching of the tracheal system. We have identified a new gene, heartbroken, t hat participates in the signalling pathways of both FGF receptors, Mutation s in heartbroken are associated with defects in the migration and later spe cification of mesodermal and tracheal cells. Genetic interaction and epista sis experiments indicate that heartbroken acts downstream of the two FGF re ceptors but either upstream of or parallel to RAS1. Furthermore, heartbroke n is involved in both the HEARTLESS- and BREATHLESS-dependent activation of MAPK, In contrast, EGF receptor-dependent embryonic functions and MAPK act ivation are not perturbed in heartbroken mutant embryos. A strong heartbrok en allele also suppresses the effects of hyperactivated FGF but not EGF rec eptors, Thus, heartbroken may contribute to the specificity of developmenta l responses elicited by FGF receptor signalling.