Cytotype regulation by telomeric P elements in Drosophila melanogaster: interactions with P elements from M' strains.

P strains of Drosophila are distinguished from M strains by having P elements in their genomes and also by having the P cytotype, a maternally inherited condition that strongly represses P-element-induced hybrid dysgenesis. The P cytotype is associated with P elements inserted near the left telomere of the X chromosome. Repression by the telomeric P elements TP5 and TP6 is significantly enhanced when these elements are crossed into M' strains, which, like P strains, carry P elements, but have little or no ability to repress dysgenesis. The telomeric and M' P elements must coexist in females for this enhanced repression ability to develop. However, once established, it is transmitted maternally to the immediate offspring independently of the telomeric P elements themselves. Females that carry a telomeric P element but that do not carry M' P elements may also transmit an ability to repress dysgenesis to their offspring independently of the telomeric P element. Cytotype regulation therefore involves a maternally transmissible product of telomeric P elements that can interact synergistically with products from paternally inherited M' P elements. This synergism between TP and M' P elements also appears to persist for at least one generation after the TP has been removed from the genotype.