Yki is required to restrict tracheal tube length without affecting SJs and luminal matrix. (A) WT, ykiB5, and ykiB5;btl>yki late stage 16 embryos, stained with the chitin-binding probe (CBP). Note the convoluted DT tubes in ykiB5 mutant embryos. Tracheae-specific expression of yki rescues this phenotype. Scale bar: 50 µm. (B) DT length is significantly increased in ykiB5 mutants (n = 20) compared with that of WT (n = 9) embryos or ykiB5 mutant embryos expressing yki under the control of the tracheal driver btl-Gal4, ykiB5;btl>yki (n = 15). Tube length is expressed as the ratio of DT length (metamere 6–10) to body length, normalized against WT embryos (ratio taken as 1). Tube length of mtf mutants (n = 20), a gene encoding a bona fide SJ component, is increased. (C) WT, ykiB5, and ykiB5;btl>yki late stage 17 embryos. Gas filling of the DT lumen is observed in WT and ykiB5;btl>yki, but not in ykiB5 mutant embryos. Scale bar: 20 µm. (D) Plot showing the percentage of ykiB5 mutant (n = 99) embryos with gas-filling defects, which is significantly different from WT (n = 86) and ykiB5;btl>yki embryos (n = 106). (E) The relative tube length of ykiB5 mutant embryos is rescued to different extents by expression of either yki or Diap1 by btl-Gal4. WT (n = 20), ykiB5 (n = 11), ykiB5;btl>yki (n = 12), ykiB5;btl>Diap1(n = 12). Error bars represent SEM. (F–H′) Airyscan confocal images showing the DT (tracheal metamere 7) of WT (F and F′), ykiB5 (G and G′) and mtf (H and H′) mutants of late stage 16 stained for the core SJ components Yurt and Cora (green), the luminal matrix protein Serp (magenta), and the nucleus with DAPI (blue). Note that Cora and Yurt are apically restricted in WT and ykiB5 mutant tracheae, but not in mtf mutants. Scale bar: 10 µm. (I–K′) Transmission electron microscopy of stage 16 embryonic tracheae of WT (I and I′), ykiB5 (J and J′), and mtf (K and K′) mutants. Electron-dense septa (arrows) are comparable in WT and ykiB5 but invisible in mtf mutants. Scale bars: 0.5 µm. Error bars represent SEM.
