Fig. 2. The ER-membrane protein Rtnl1 is a marker of fusomal membranes during oogenesis. Examination of the fusome in the germaria during oogenesis reveals that the transmembrane protein Rtnl1 is highly enriched in fusomal membranes and very specific for the germline compared with PDI-GFP and Sec61-GFP. (A) Scheme of a germarium. (B) Scheme of Rtnl1, showing the two proposed topologies for reticulon proteins. (C-C'",D-D'", E-E'") Comparison of Rtnl1-GFP, PDI-GFP and Sec61-GFP, respectively, with antibody staining for HDEL (C",D",E") and -Spectrin (C'",D'",E'"). (F-F") Comparison of Rtnl1-GFP with labeling for acetylated -tubulin that marks the stable microtubule array, which is associated with the fusome and points into the oocyte (Röper and Brown, 2004). Note that Rtnl1 becomes very concentrated in the oocyte and at its posterior in region 3 of the germarium. Bars, 10 µm.