Abstract
The I factor (IF) is a LINE-like transposable element from Drosophila melanogaster. IF is silenced in most strains, but under special circumstances its transposition can be induced and correlates with the appearance of a syndrome of female sterility called hybrid dysgenesis. To elucidate the relationship between IF expression and female sterility, different transgenic antisense and/or sense RNAs homologous to the IF ORF1 have been expressed. Increasing the transgene copy number decreases both the expression of an IF-lacZ fusion and the intensity of the female sterile phenotype, demonstrating that IF expression is correlated with sterility. Some transgenes, however, exert their repressive abilities not only through a copy number-dependent zygotic effect, but also through additional maternal and paternal effects that may be induced at the DNA and/or RNA level. Properties of the maternal effect have been detailed: (1) it represses hybrid dysgenesis more efficiently than does the paternal effect; (2) its efficacy increases with both the transgene copy number and the aging of sterile females; (3) it accumulates slowly over generations after the transgene has been established; and (4) it is maintained for at least two generations after transgene removal. Conversely, the paternal effect increases only with female aging. The last two properties of the maternal effect and the genuine existence of a paternal effect argue for the occurrence, in the IF regulation pathway, of a cellular memory transmitted through mitosis, as well as through male and female meiosis, and akin to epigenetic phenomena.