Colored medaka and zebrafish: Transgenics with ubiquitous and strong transgene expression driven by the medaka β‐actin promoter

Conditional cell labeling, cell tracing, and genetic manipulation approaches are becoming increasingly important in developmental and regenerative biology. Such approaches in zebrafish research are hampered by the lack of an ubiquitous transgene driver element that is active at all developmental stages. Here, we report the isolation and characterization of the medaka fish (Oryzias latipes) β‐actin (Olactb) promoter, which drives constitutive transgene expression during all developmental stages, and the analysis of adult organs except blood cell types. Taking advantage of the compact medaka promoter, we succeeded in generating a zebrafish transgenic (Tg) line with unprecedentedly strong and widespread transgene expression from embryonic to adult stages. Moreover, the Tg carries a pair of loxP sites, which enables the reporter fluorophore to switch from DsRed2 to enhanced green fluorescent protein (EGFP). We induced Cre/loxP recombination with Tg(hsp70l: mCherry‐t2a‐Cre^ERt2) in the double Tg embryo and generated a Tg line that constitutively expresses EGFP. We further demonstrate the powerful application of Olactb‐driven Tgs for cell lineage tracing using transplantation experiments with embryonic cells at the shield stage and adult cells of regenerating fin. Thus, the use of promoter elements from medaka is an alternative approach to generate Tgs with stronger and even novel expression patterns in zebrafish. The Olactb promoter and the Tg lines presented here represent an important advancement for the broader use of Cre/loxP‐based Tg applications in zebrafish.