The tracheal system, the gaseous exchange network of the fly, is formed during mid embryogenesis from 10 placodes of cells on each side of the embryo that invaginate from lateral ectoderm. The cells in the placodes undergo cell shape changes and migrations, without further cell divisions, to form branching structures which eventually interconnect to form a continuous tubular network which fills with gas at late embryogenesis.
In the course of our work on germ cells, we noticed that embryos lacking the lipid phosphate phosphatase (LPP) Wun have trachea that do not fill with gas prompting us to investigate the role of Wunens in tracheal development.
wun mutant embryos have breaks in the trachea and non-uniform tracheal luminal diameters. The tracheal lumen is surrounding by a chitin containing cuticle. We see that crucial luminal components such as the chitin deacetylases Serpentine and Vermiform do not accumulate in the tracheal lumen as in wild type. A failure to accumulate these 2 proteins is reported for mutants in septate junction (the occluding junctions of insects) components. We tested septate junction component localization and barrier function in wun mutant embryos and see that these are indeed defective, implicating LPP products in SJ function.
Based on the localization of fluorescently tagged secretion reporters, we further suggest that the failure to accumulate crucial luminal components in wun mutant embryos, reflects their leakage out of the lumen into the hemolymph due to defective septate junctions.