Gene knock-ins in Drosophila using homology-independent insertion of universal donor plasmids

Abstract

Targeted genomic knock-ins are a valuable tool to probe gene function. However, knock-in methods involving homology-directed repair (HDR) can be laborious. Here, we adapt the mammalian CRISPaint homology-independent knock-in method for Drosophila melanogaster, which uses CRISPR/Cas9 and non-homologous end joining (NHEJ) to insert universal donor plasmids into the genome. This method is a simple and fast alternative to HDR for certain strategies such as C-terminal tagging and gene disruption. Using this method in cultured S2R+ cells, we efficiently tagged four endogenous proteins with the bright fluorescent protein mNeonGreen, thereby demonstrating that an existing collection of CRISPaint universal donor plasmids is compatible with insect cells. In addition, we inserted the transgenesis marker 3xP3-RFP into seven genes in the fly germ line, producing heritable loss of function alleles that were isolated by simple fluorescence screening. Unlike in cultured cells, indels always occurred at the genomic insertion site, which prevents predictably matching the insert coding frame to the target gene. Despite this effect, we were able to isolate T2A-Gal4 insertions in four genes that serve as in vivo expression reporters. Finally, we apply this fast knock-in method to uncharacterized small open reading frame (smORF) genes. Therefore, homology-independent insertion is a useful genome editing technique in Drosophila that will better enable researchers to dissect gene function.\n\nArticle summaryWe report a fast and simple genomic knock-in method in Drosophila to insert large DNA elements into any target gene. Using CRISPR-Cas9 and non-homologous end joining (NHEJ), an entire donor plasmid is inserted into the genome without the need for homology arms. We demonstrate its usefulness in cultured cells to fluorescently tag endogenous proteins and in the fly germ line to generate heritable insertions that disrupt gene function and can act as expression reporters.