Germ-line expression of a functional LINE from Drosophila melanogaster: fine characterization allows for potential investigations of trans-regulators.

The I factor (IF) is a functional non-viral retrotransposon, or LINE, from Drosophila melanogaster. It is mobilized in the germ-line of dysgenic SF females during I-R hybrid dysgenesis. In previous papers (Lachaume et al., Development 115: 729-735, 1992; Lachaume and Pinon, Mol. Gen. Gen. 240: 277-285, 1993) we used a transgenic fusion between the 5' part of the IF and the lacZ gene to characterize IF expression and its regulation. This I-lacZ transgenic fusion expresses beta-galactosidase activity during oogenesis. We established a Drosophila line bearing four transgenic insertions (the 4I-lacZ line) and got new insights about IF expression: (1) I-lacZ expression is proportional to the copy number of transgenes present in the genome, (2) the expression occurs just before or when meiosis begins, (3) this expression seems to be subjected to a variegation effect within the germ-line cells, (4) the transgenic activity is mainly directed toward the decondensed chromatin of nurse cells. The close relationship between I factor expression and oogenesis led us to investigate the role played by genes expressed during oogenesis on I factor expression. We present recent data indicating that mutants which interfere with oogenesis can also affect I factor expression. From this data we propose an original screen using the 41-lacZline to detect identified mutations which also affect I factor expression.