Hybridization Chain Reaction FISH in the Developing Drosophila Optic Lobe

Understanding neural development requires precise characterization of when and where genes are expressed. Techniques such as in situ hybridization have long enabled visualization of gene expression in developing tissues. Although in situ hybridization is widely used in Drosophila melanogaster embryos, its application to visual system development has been limited by several technical challenges. For example, long probes often penetrate poorly into the optic lobe, and protocols that rely on diffusible catalytic reporters, such as horseradish peroxidase, often yield low signal-to-noise ratios in whole-mount preparations. Hybridization chain reaction fluorescent in situ hybridization (HCR-FISH) overcomes many of these limitations and enables more robust signal detection. As in conventional FISH, DNA probe monomers hybridize to RNA sequences within tissues. In HCR-FISH, however, fluorescently conjugated amplifier probes form stable hairpin structures that trigger polymerization of additional monomers, thereby amplifying signal intensity. The use of multiplexed probe sets also permits the use of shorter oligonucleotides (~45 nt, compared with up to 1,000 nt in conventional FISH), improving probe penetration and producing more uniform staining in whole-mount samples. This article describes an HCR-FISH protocol optimized for the Drosophila larval optic lobe. The protocol adapts existing HCR-FISH approaches by optimizing fixation conditions and increasing probe and hairpin concentrations to improve signal intensity and tissue penetration. Reagent lists and step-by-step procedures are provided. Potential applications in developmental neurobiology are also presented, including visualization of newborn neurons, and characterization of gene expression patterns in cases where conventional methods, such as endogenously tagged proteins or antibody staining, fail to provide sufficient information.