Multiple overlapping SNARE complexes drive endosome maturation in Drosophila nephrocytes.

Endosomal maturation determines whether internalized cargo is recycled or degraded, yet the logic governing early endosomal fusion remains incompletely defined. Although commonly depicted as a linear Rab5-to-Rab7 transition mediated by a single SNARE pathway, we show that early endosome maturation is driven by multiple parallel, non-interchangeable SNARE complexes. Using Drosophila nephrocytes as a tractable in vivo model, we define a Syx12L-Snap29-Ykt6 complex that mediates homotypic early endosomal fusion and identify two related assemblies-Syx7L-Snap29-Ykt6 and Syx7L-Snap29-Vamp7-that promote later endosomal and lysosomal fusion with distinct Rab requirements. When the canonical Syx12L pathway is disrupted, alternative complexes remain active and generate aberrant endolysosomal swirls which also form in the absence of Rab5 or early endosomal tethers. Thus, Rab5-independent membrane progression toward lysosomes persists but is insufficient to produce fully functional degradative organelles. Together, our findings reveal a network of parallel fusion pathways underlying endosome maturation.