Raeppli: a whole-tissue labeling tool for live imaging of Drosophila development.

Observation of how cells divide, grow, migrate and form different parts of a developing organism is crucial for understanding developmental programs. Here, we describe a multicolor imaging tool named Raeppli (after the colorful confetti used at the carnival in Basel). Raeppli allows whole-tissue labeling such that the descendants of the majority of cells in a single organ are labeled and can be followed simultaneously relative to one another. We tested the use of Raeppli in the Drosophila melanogaster wing imaginal disc. Induction of Raeppli during larval stages irreversibly labels >90% of the cells with one of four spectrally separable, bright fluorescent proteins with low bias of selection. To understand the global growth characteristics of imaginal discs better, we induced Raeppli at various stages of development, imaged multiple fixed discs at the end of their larval development and estimated the size of their pouch primordium at those developmental stages. We also imaged the same wing disc through the larval cuticle at different stages of its development; the clones marked by Raeppli provide landmarks that can be correlated between multiple time points. Finally, we used Raeppli for continuous live imaging of prepupal eversion of the wing disc.