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Evolutionarily conserved developmental pathways

Ectoderm/mesoderm interaction - Ectodermal FGF involvement in development of respiratory and muscle systems

Two fibroblast growth factor receptors have been characterized in Drosophila: one, Breathless, involved in tracheal development and a second, called Fibroblast growth factor receptor 1 (FR1), involved in heart and somatic muscle development. The ligand for Breathless, called Branchless, is produced in a highly localized and stereotyped fashion by ectodermal cells. Branchless, acting through Breathless, controls branching morphogenesis of tracheal precursor cells. Although a ligand for FR1 has not yet been characterized, the effect of this ligand and its interaction with FR1 on the spreading of muscle precursor cells can be observed. FR1 is involved in the spreading of mesoderm over ectoderm. In mutants, cell fates are not induced in several lineages, including the visceral mesoderm, heart and the dorsal somatic muscles. The defects in the induction of mesodermal cell fates are likely to result from failure of the mesoderm to spread over the ectoderm and receive patterning signals.

In vertebrates, FGF and its receptors are involved in the branching morphogenesis of lung precursor cells, comparable to the involvement of Branchless and Breathless involvement in tracheal development in Drosophila. In limb outgrowth, the role of FGF is comparable to the involvement of an uncharaterized FGF in Drosophila myogenesis. For example, FGF-8 in chicken development is produced by the apical ectodermal ridge and appears to be akey signal involved in initiation, outgrowth and patterning of the developing vertebrate limb. FGF has an early role in vertebrate morphogenesis. FGF acts during gastrulation in anterior/posterior patterning. This function has a primitive evolutionary origin. Basic fibroblast growth factor induces notochord formation and the expression of a Brachyury homolog during ascidian embryogenesis. It is likely that this role of FGF is related to the involvement of FR1 and its ligand in the early spread of mesoderm coating the overlying ectoderm in Drosophila development. In many if not most cases, vertebrate FGF secretion is not confined to ectoderm, nor is ectoderm its primary site of origin. Production of FGF by non-ectodermal tissues promotes epithelial mesenchymal transformation as a prelude to cell migration.

In vertebrates, basic fibroblast growth factor plays an important role in development of the central nervous system and is neurotropic for a variety of neurons. A similar function has not been documented in Drosophila.

Drosophila                        Homologs in other species ----------                        ----------------------------Branchless                        Mammals: KGF/FGF-7Breathless                        Mammals: FGFR-2unknown ligand for FR1            Xenopus: basic FGF                                  Mammals: FGF-4 and FGF-8 FGF receptor 1 (FR1)              C. elegans: EGL-15                                       Xenopus: FGF receptor 1                                                       Mammalian: FGFR-2, FGFR-3 and FGFR-4

date revised: 24 FEB 97

Developmental Pathways conserved in Evolution

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