Interommatidial cells build a tensile collagen network during Drosophila retinal morphogenesis

Drosophila SPARC collagen IV chaperone-like activity essential for development is unique to the fat body

Drosophila fat body-derived SPARC acts as a chaperone for collagen IV (Col(IV)), enabling their diffusion and incorporation into distal tissue basement membranes (BMs). Disruption of SPARC or Col(IV) production by the fat body is lethal, despite expression by other tissues such as imaginal discs. Wing disc-derived SPARC does not associate with Col(IV) in BMs and is not essential for survival. We show that differential association of fat body- and wing disc-derived SPARC with Col(IV) is not due to differences in SPARC glycosylation nor to the absence of SPARC and Col(IV) co-expression. Further, we demonstrate that SPARC domain II/III produced by the fat body is sufficient for Col(IV) diffusion to both proximal and distal BMs, and rescues lethality associated with loss of SPARC. However, SPARC domain II/III does not diffuse beyond the hemolymph. Thus, the essential Col(IV) chaperone-like activity specific to fat body-derived SPARC is not required beyond the hemolymph.

Inter-plane feedback coordinates cell morphogenesis and maintains 3D tissue organization in the Drosophila pupal retina

How complex organs coordinate cellular morphogenetic events to achieve three-dimensional (3D) form is a central question in development. The question is uniquely tractable in the late Drosophila pupal retina, where cells maintain stereotyped contacts as they elaborate the specialized cytoskeletal structures that pattern the apical, basal and longitudinal planes of the epithelium. In this study, we combined cell type-specific genetic manipulation of the cytoskeletal regulator Abelson (Abl) with 3D imaging to explore how the distinct cellular morphogenetic programs of photoreceptors and interommatidial pigment cells (IOPCs) organize tissue pattern to support retinal integrity. Our experiments show that photoreceptor and IOPC terminal differentiation is unexpectedly interdependent, connected by an intercellular feedback mechanism that coordinates and promotes morphogenetic change across orthogonal tissue planes to ensure correct 3D retinal pattern. We propose that genetic regulation of specialized cellular differentiation programs combined with inter-plane mechanical feedback confers spatial coordination to achieve robust 3D tissue morphogenesis.