TRANSGENE COPLACEMENT AND HIGH-EFFICIENCY SITE-SPECIFIC RECOMBINATIONWITH THE CRE LOXP SYSTEM IN DROSOPHILA/

Studies of gene function and regulation in transgenic Drosophila are o ften compromised by the possibility of genomic position effects on gen e expression. We have developed a method, called transgene coplacement , in which any two sequences can be positioned at exactly the same sit e and orientation in the genome. Transgene coplacement makes use of th e bacteriophage P1 system of Cre/loxP site-specific recombination, whi ch we have introduced into Drosophila. In the presence of a cre transg ene driven by a dual hsp70-Mos1 promoter, a white reporter gene flanke d by loxP sites is excised with virtually 100% efficiency both in soma tic cells and in germ cells. A strong maternal effect, resulting from Cre recombinase present in the oocyte, is observed as white or mosaic eye color in F-1 progeny. Excision in germ cells of the F-1 yields a s trong grand-maternal effect, observed as a highly skewed ratio of eye- color phenotypes in the F-2 generation. The excision reactions of Cre/ loxP and the related FLP/FRT system are used to create Drosophila line s in which transgenes are at exactly allelic sites in homologous chrom osomes.