FlEx-based transgenic reporter lines for visualization of Cre and Flp activity in live zebrafish

Site-specific recombinases such as Cre and Flp are invaluable tools for genetic manipulations, but their usage in zebrafish has been limited. Incorporating recently developed flip-excision (FlEx) design that allows stable inversions, we have established zebrafish reporter lines that express bright and ubiquitous EGFP, but switch to express mCherry in the presence of Cre or Flp. Here, we demonstrate the stable inversion in the reporter lines, both in somatic cells and in the germ line by Cre or Flp, and the subsequent reinversion using the other recombinase. Using the reporter lines, we characterized cardiomyocyte-specific Cre lines and neuronal progenitor-specific and tamoxifen-dependent Cre lines. We also used the reporter lines for screening Cre- and Flp-based enhancer trap lines. Similar to the widely used Cre reporter lines in mice, these FlEx-based reporter lines will facilitate the use of recombinases for genetic manipulations in zebrafish.