A highly efficient knock-in strategy using fast-annealed 3′-extended dsDNA templates for emerging model insects

CRISPR-Cas genome editing toolkits have expanded the scope of genetic studies in various emerging model organisms. However, their applications are limited mainly to knockout experiments due to technical difficulties in establishing knock-in strains, which enables in vivo molecular tagging-based experiments. Here, we investigated knock-in strategies in the harlequin ladybug Harmonia axyridis, a model insect for evolutionary developmental biology, which shows more than 200 color pattern variations within a species. We tested several knock-in strategies using synthetic DNA templates. We found that ssDNA templates generated founder knock-in strains efficiently (2.5-11%), whereas the 5′ regions of ssDNA templates were frequently scraped when the insert length exceeded ~40 bases. To overcome this limitation, we designed several 3′ extended DNA templates. Of those, fast-annealed 3′-extended double-stranded DNA templates showed extremely high founder generation efficiency (32-67%) and accuracy (67-100%). This strategy is also applicable to the two-spotted cricket Gryllus bimaculatus, demonstrating that the fast-annealed 3′-extended dsDNA template is a highly potent and versatile DNA template for generating knock-in strains in emerging model insects for developmental genetic studies.