Simple embryo injection of long single-stranded donor templates with the CRISPR/Cas9 system leads to homology-directed repair in Xenopus tropicalis and Xenopus laevis.

We report model experiments in which simple microinjection of fertilized eggs has been used to effectively perform homology-directed repair (HDR)-mediated gene editing in the two Xenopus species used most frequently for research: X. tropicalis and X. laevis. We have used long single-stranded DNAs having phosphorothioate modifications as donor templates for HDR at targeted genomic sites using the Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated protein 9 (CRISPR/Cas9) system. First, X. tropicalis tyr mutant (i.e., albino) embryos were successfully rescued: partially pigmented tadpoles were seen in up to 35% of injected embryos, demonstrating the potential for efficient insertion of targeted point mutations. Second, in order to demonstrate the ability to tag genes with fluorescent proteins (FPs), we targeted the melanocyte-specific gene slc45a2.L of X. laevis to label it with the Superfolder green FP (sfGFP), seeing mosaic expression of sfGFP in melanophores in up to 20% of injected tadpoles. Tadpoles generated by these two approaches were raised to sexual maturity, and shown to successfully transmit HDR constructs through the germline with precise targeting and seamless recombination. F1 embryos showed rescue of the tyr mutation (X. tropicalis) and tagging in the appropriate pigment cell-specific manner of slc45a2.L with sfGFP (X. laevis).
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