breathless, a Drosophila FGF receptor homolog, is essential for migration of tracheal and specific midline glial cells.

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Abstract
A Drosophila homolog of the fibroblast growth factor (FGF) receptor was isolated and structurally characterized. After EMS mutagenesis or imprecise excisions of marked P elements inserted upstream to the gene, a phenotypic series of mutations in the locus was isolated. The mutants exhibit defects in the two embryonic tissues in which the receptor is expressed: the tracheal system and the midline. The tracheal cells fail to migrate in severe mutants and remain within the tracheal pits. Hypomorphic alleles exhibit partial migration of all tracheal branches; thus, the locus was termed breathless (btl). In the midline of the mutant embryos, the posterior pair of midline glial cells begins to migrate anteriorly, but fails to reach the posterior commissure. Abnormalities in cell migration appear to be a common denominator for the btl defects in these two disparate tissues. In hypomorphic mutants the cells exhibit partial migration but follow the normal tracts, suggesting that the presence of this receptor is essential for the ability of the migrating cells to recognize external guiding cues.