The P cytotype in Drosophila melanogaster: a maternally transmitted regulatory state of the germ line associated with telomeric P elements.

The incomplete P elements TP5 and TP6 are inserted in the TAS repeats near the left telomere of the Drosophila melanogaster X chromosome. These telomeric P elements repress P-induced gonadal dysgenesis and germ-line hypermutability in both sexes. However, their capacity to repress hypermutability is lost when they are transmitted patroclinously in a cross. TP5 and TP6 do not repress P-element activity in somatic cells, nor do they alter the somatic or germ-line phenotypes of P-insertion alleles. In the germ line, these elements suppress the phenotype of a P-insertion allele of the singed gene that is evoked by other P elements, presumably because these other elements encode repressor polypeptides. This suppression is more effective when the telomeric P elements are inherited maternally. Regulation by telomeric P elements parallels that of the P cytotype, a state that represses P-element activity in some strains of Drosophila. This state exists only in the germ line and is maternally transmitted along with the P elements themselves. Regulation by known repressor P polypeptides is not restricted to the germ line and does not require maternal transmission of the relevant P elements. Regulation by telomeric P elements appears to be epistatic to regulation by repressor P polypeptides.