Creation of colorless transparent tilapia using CRISPR/Cas9 mediated multi-gene mutation

Transparent mutant fish have been obtained from small-sized fish (medaka, zebrafish, and killifish), and are often used as experimental models in biological and medical research. However, transparent fish have never been created in medium- or large-sized fish through gene editing. In this study, mutants without xanthophores, erythrophores and pigmented melanophores were first obtained by crossing to form tyrb;csf1ra double mutant tilapia. Subsequently, single mutants lacking reflective platelets without visible iridophores were obtained by mutation of pnp4a and tfec using CRISPR/Cas9 gene editing, the recovery of iridophores were observed in pnp4a mutants but not in tfec mutants in the later stage. In addition, we also found that tfec mutation led to a decrease in number of melanophores and diameter of melanophores/erythrophores. Finally, transparent triple mutant homozygotes without visible pigment cells were obtained by triple mutation of tyrb;csf1ra;pnp4a (named amber) and tyrb;csf1ra;tfec (named ruby), through crossing of double and single mutants. The two mutant lines were no longer transparent at 60 dpf (days post fertilization) and 120 dpf, respectively, due to the recovery of iridophores in the former, and thickening of the body wall in the latter. In summary, this study created two transparent lines without visible pigment cells through aggregation of multiple gene mutation and crossing. The transparent fish is suitable for in vivo imaging, and is currently the only medium-sized transparent fish obtained through gene editing.