Fig. 2. Tracheal tube size defects do not result from trans-epithelial diffusion barrier defects. The integrity of the septate junction diffusion barrier in tracheal and septate junction mutants was assessed by a dye permeability assay. For each mutant, DIC images of the trachea are shown in black and white (A-H) and the matched fluorescence image taken in the same focal plane is shown in color (a-h). Wild-type trachea exclude fluorescently labeled 10 kDa dextran dye injected into the body cavity (a; dotted lines outline tracheal tubes), while the trachea of Na⁺/K⁺ ATPase and other mutants are permeable to the dye which enters and fills their lumens (b-g). Permeability defects do not cause the observed tube-size defects as cor¹⁴* mutants have defective diffusion barriers (e) but have normal tracheal morphology (E). Furthermore, convoluted (conv) mutants have the identical tube-size defects as nrv2 (compare H with B) but do not have permeability defects (h). All mutants that failed to exclude dye from the trachea also failed to exclude it from the salivary gland. Genotypes: nrv2^nwu3, ATP^DTS1R2, cor¹⁴* is coracle¹⁴ plus additional unidentified genetic background, cystic^k13717b, megatrachea^EA97, convoluted^k6507b. In addition, coracle⁵, varicose^3953b, neurexin IV¹⁴, neuroglian¹⁷, gliotactin^J29-41bl, nrv2^l(2)k04223 btl-Gal4/nrv2^l(2)k04223 UAS-nrv2.1 and nrv2^l(2)k04223 btlGal4/nrv2^l(2)k04223 UAS nrv2.2 animals have tracheal diffusion barrier defects, while coracle¹⁵, hindsight¹¹⁴², nrv2^k13315 e22C-Gal4/nrv2^l(2)k04223 UAS-nrv2.2, nrv2^k13315 e22C-Gal4/nrv2^l(2)k04223 UAS nrv2.1 and coracle^{14(backcrossed)} do not (data not shown). Scale bar: 10 µm.