MATERNAL INHERITANCE OF P-CYTOTYPE IN DROSOPHILA-MELANOGASTER - A PRE-P CYTOTYPE IS STRICTLY EXTRA-CHROMOSOMALLY TRANSMITTED

In Drosophila melanogaster, transposition of the P element is under th e control of a cellular state known as cytotype. The P cytotype repres ses P transposition whereas the M cytotype is permissive for transposi tion. In the long-term, the P cytotype is determined by chromosomal P elements but over a small number of generations it is maternally inher ited. In order to analyse the nature of this maternal inheritance, we tested whether a maternal component can be transmitted without chromos omal P elements. We used a stable determinant of P cytotype, linked to the presence of two P elements at the tip of the X chromosome (IA sit e) in a genome devoid of other P elements. We measured P repression ca pacity using two different assays: gonadal dysgenic sterility (GD) and P-lacZ transgene repression. We show that zygotes derived from a P cy totype female (heterozygous for P (1A)/balancer devoid of P copies) an d which inherit no chromosomal P elements from the mother, have, howev er, maternally received a P-type extra-chromosomal component: this com ponent is insufficient to specify the P cytotype if the zygote formed does not carry chromosomal P elements but can promote P cytotype deter mination if regulatory P elements have been introduced paternally. We refer to this strictly extra-chromosomally inherited state as the ''pr e-P cytotype''. In addition, we show that a zygote that has the pre-P cytotype but which has not inherited any chromosomal P elements, does not transmit the pre-P cytotype to the following generation. The natur e of the molecular determinants of the pre-P cytotype is discussed.