Bacteriophage integrase-directed insertion of transgenic constructs into specific genomic loci has been widely used by Drosophila community. A second chromosome-located attP40 landing site gains popularity because of its high inducible expression levels of transgenes. Here, unexpectedly, we found that homozygous attP40 chromosome leads to defects in the glomerular organization of Drosophila olfactory receptor neurons (ORNs). This effect is not likely to be caused by the loss of function of Msp300, where attP40 docking site is inserted. Moreover, attP40 site seems to genetically interact with a second chromosome GAL4 driver, which also results in a similar ORN axon terminal defect. Though it remains elusive so far whether the ORN phenotype is caused by the neighboring genes around Msp300 locus in the presence of attP40-based insertions or a second unknown mutation in the attP40 background, our finding raises the critical issue with using this popular transgenic landing site. Rigorous controls are needed in the relevant experiments to rule out the attP40-associated background effects.

Bacteriophage integrase-directed insertion of transgenic constructs into specific genomic loci has been widely used by Drosophila community. The attP40 landing site located on the second chromosome gained popularity because of its high inducible transgene expression levels. Here, unexpectedly, we found that homozygous attP40 chromosome disrupts normal glomerular organization of Or47b olfactory receptor neuron (ORN) class in Drosophila. This effect is not likely to be caused by the loss of function of Msp300, where the attP40 docking site is inserted. Moreover, the attP40 background seems to genetically interact with the second chromosome Or47b-GAL4 driver, which results in a similar glomerular defect. Whether the ORN phenotype is caused by the neighboring genes around Msp300 locus in the presence of attP40-based insertions or a second unknown mutation in the attP40 background remains elusive. Our findings tell a cautionary tale about using this popular transgenic landing site, highlighting the importance of rigorous controls to rule out the attP40 landing site-associated background effects.