FGF signalling and the mechanism of mesoderm spreading in Drosophila embryos.

FGF signalling is needed for the proper establishment of the mesodermal cell layer in Drosophila embryos. The activation of the FGF receptor Heartless triggers the di-phosphorylation of MAPK in the mesoderm, which accumulates in a graded fashion with the highest levels seen at the dorsal edge of the mesoderm. We have examined the specific requirement for FGF signalling in the spreading process. We show that only the initial step of spreading, specifically the establishment of contact between the ectoderm and the mesoderm, depends upon FGF signalling, and that unlike the role of FGF signalling in the differentiation of heart precursors this function cannot be replaced by other receptor tyrosine kinases. The initiation of mesoderm spreading requires the FGF receptor to possess a functional kinase domain, but does not depend upon the activation of MAPK. Thus, the dispersal of the mesoderm at early stages is regulated by pathways downstream of the FGF receptor that are independent of the MAPK cascade. Furthermore, we demonstrate that the activation of MAPK by Heartless needs additional cues from the ectoderm. We propose that FGF signalling is required during the initial stages of mesoderm spreading to promote the efficient interaction of the mesoderm with the ectoderm rather than having a long range chemotactic function, and we discuss this in relation to the cellular mechanism of mesoderm spreading.