Fig. 4. The Egfr mediates midline repulsion of GB1. (A,B) A ventral view of late stage 16 embryos stained for the tracheal lumen (by mAb2A12). Many GB midline crosses are observed in spi mutants (A). (C,D) spi mutants also lack a functional midline. (C) In spi, longitudinal fascicles collapse on the midline (stained by mAb1D4; compare with D, wild type; all panels show ventral views, anterior towards the left). (B,E) SRF-Gal4 drives gene expression specifically in tracheal tip cells. (E) A single GB is shown that expresses UAS-NLS:lacZ in the GB1 nucleus (arrow) under the control of SRF-Gal4 (mAb2A12 stains the tracheal lumen in red, anti-ß-gal is green). (B) A ventral view of a late stage 16 embryo expressing dominant-negative Egfr, showing misroutings and midline crosses.