The fruit fly Drosophila melanogaster is extremely amenable to genetic manipulation and genetic analysis. Transgenic fruit flies are widely used in biological research. Since it was developed in the early 1980s, P-element transposon-mediated transgenesis has been indispensable for Drosophila research¹. In some scenarios, other transposons, such as piggyBac and Minos have been used for transgenesis in Drosophila as well². Transgenes via transposon are randomly inserted into the Drosophila genome, and the expression levels of transgenes at different genomic loci may vary due to position effects and thereby can not be compared². These drawbacks were overcome by the phiC31-mediated site-specific transgenesis³. The bacteriophage phiC31 gene encodes an integrase that mediates sequence-specific recombination between the attB and attP sites in Drosophila³. Many attP docking sites have been characterized and are available in the Bloomington Drosophila Stock Center³^,⁴^,⁵^,⁶^,⁷^,⁸^,⁹.

The phiC31 transgenesis system for Drosophila has been widely used by the fly community. Among the attP docking sites, attP18 on the X chromosome, attP40 on the second chromosome, and attP2 on the third chromosome are usually the preferred sites for transgene integration on each chromosome because transgenes at the three sites show high levels of inducible expression while having low levels of basal expression⁵. Recently, however, van der Graaf and colleagues found that the attP40 site, close to the Msp300 gene locus, and transgenes integrated at attP40 cause larval muscle nuclear clustering in Drosophila¹⁰. In another recent study, Duan and colleagues found that the homozygous attP40 chromosome disrupts the normal glomerular organization of the Or47b olfactory receptor neuron class in Drosophila¹¹. These findings suggest that rigorous controls should be included when designing experiments and interpreting data.

Drosophila is an ideal model organism for in vivo interrogation of gene function due to its short life cycle, low cost, and easy maintenance. Genome-wide genetic screening relies on the construction of genome-wide transgenic libraries in Drosophila¹²^,¹³^,¹⁴^,¹⁵. We previously generated 5551 UAS-cDNA/ORF constructs based on the binary GAL4/upstream activating sequence (GAL4/UAS) system, covering 83% of the Drosophila genes conserved in humans in the Drosophila Genomics Resource Center (DGRC) Gold Collection¹⁶. The UAS-cDNA/ORF plasmids can be used for the creation of a transgenic UAS-cDNA/ORF library in Drosophila. Efficient embryo microinjection technology can accelerate the creation of transgenic fly libraries.

For embryo microinjection, the needle tip is normally broken against a coverslip¹⁷. Thereby, needles frequently become clogged or cause leakage of large amounts of cytoplasm, and when these issues arise, new needles must be employed. Although beveling needles can enhance microinjection efficiency, the grinding solution enters the beveled tip during the grinding process. The grinding solution in the beveled tip does not quickly evaporate naturally; therefore, needles can not be utilized for microinjection directly after beveling. These factors reduce the efficiency of embryo microinjection procedures. Here, using the phiC31-mediated site-specific transgenesis in Drosophila as an example, we present a protocol for embryo microinjection for transgenesis in Drosophila. To prevent the grinding solution from entering the needle, we utilize an air pump to exert air pressure within the needle, thereby preventing the grinding solution from entering the beveled tip. Beveled needles are ready for sample loading and microinjection directly after beveling.

Microinjection of Drosophila Embryos for phiC31 Integrase Mediated Embryo Transgenesis