Figure 1Schematics illustrate the fly accessory glands and associated secondary cells, highlighting their acidic and non-acidic exosome-forming compartments. First panel shows male fruit fly and its accessory glands. Boxed region is enlarged in second panel, revealing secondary cells (SCs; black dots), with red line indicating the plane of section through the AG lumen, which is used to generate the transverse view through a SC and surrounding main cells within the epithelial layer in the third panel. The unusually large, acidic late endosomes and lysosomes (Rab7-positive; magenta) and non-acidic compartments, characteristic of SCs, which we demonstrate here contain intraluminal vesicles (ILVs; some of which are Rab11-positive; yellow), are labelled. The red line in the third panel shows the basal SC plane of section in the fourth panel and images in this figure. (B-F) Basal views through living SCs, with dashed white circles approximating the outline of a single SC, and acidic compartments marked by the vital dye LysoTracker® Red (magenta). In merge images, a single non-acidic (C, D, F) and acidic (E) compartment containing intraluminal vesicles (ILVs) is boxed and magnified in the right panel (Zoom). ILVs appear as membrane-delineated vesicles, using super-resolution 3D-structured illumination (3D-SIM) microscopy for the brighter overexpressed GFP-tagged constructs (yellow; C and F). However, ILVs appear only as puncta, using lower resolution wide-field microscopy for the fainter endogenously expressed YFP-tagged Rab GTPases (yellow; D and E). (B) Wide-field fluorescence image, including differential interference contrast (DIC), of SC expressing a GFP-tagged version of human CD63 (CD63-GFP). CD63-GFP expression is apparent on the limiting membranes of non-acidic compartments and their ILVs, and also on the limiting membranes of the enlarged acidic compartments. Most large non-acidic compartments are Rab11-positive (D) and contain dense-core granules, which have a 'fried egg' appearance with DIC (arrowheads) (Corrigan et al., 2014; Redhai et al., 2016). (C) 3D-SIM image of CD63-GFP-expressing SC. Arrow highlights CD63-GFP-marked ILVs (Zoom). Many more ILVs are apparent in non-acidic compartments in a complete Z-stack of a non-acidic compartment (Movie EV1). (D) Wide-field fluorescence image of an SC expressing a YFP-Rab11 gene trap. YFP-Rab11 marks the limiting membranes of most non-acidic compartments and internal puncta (arrow in Zoom), but not the surface of acidic compartments (Appendix Fig S1B). (E) Wide-field fluorescence image of SC expressing a YFP-Rab7 gene trap. YFP-Rab7 marks the limiting membranes of acidic compartments and internal puncta (arrow in Zoom). Enlarged acidic compartments are also present in adjacent main cells. (F) 3D-SIM image of SC expressing a GFP-tagged version of Breathless (Btl-GFP). Btl-GFP marks the limiting membranes of non-acidic compartments and their ILVs (arrow in Zoom), but not the surface of acidic compartments (Appendix Fig S1C). Images from six-day-old male flies shifted to 29°C at eclosion. This induces GAL4/UAS-dependent SC transgene expression in (B), (C) and (F). The genotypes of flies carrying multiple transgenes are: w; P[w+, UAS-CD63-GFP] P[w+, tub-GAL80ts]/+; dsx-GAL4/+ (B, C); w; P[w+, tub-GAL80ts]/+; dsx-GAL4/P[w+, UAS-btl-GFP] (F).