test for enhancement of cytotype regulation in Drosophila melanogaster by the transposase-encoding P Element ∆2-3

Transposable P elements are regulated in the germ line by piRNAs, which are small RNAs that associate with the Piwi class of proteins. This regulation, called the P cytotype, is enhanced by genetic interactions between P elements that are primary sources of these RNAs and other P elements. The enhanced regulation is thought to reflect amplification of the primary piRNAs by cleavage of mRNAs derived from the other P elements through a mechanism called the ping-pong cycle. We tested the transposase-encoding P element known as ∆2-3 for its ability to enhance cytotype regulation anchored in P elements inserted at the telomere of the left arm of the X chromosome (TP elements). The ∆2-3 P element lacks the intron between exons 2 and 3 in the structurally complete P element (CP). Unlike the CP element, it does not markedly enhance cytotype regulation anchored in TP elements, nor does it transmit transposase activity through the egg cytoplasm. However, mRNAs from both the CP and ∆2-3 elements are maternally deposited in embryos. These observations suggest that maternally transmitted CP mRNA enhances cytotype regulation by participating in the ping-pong cycle and that it encodes the P transposase in the embryonic germ line, whereas maternally transmitted ∆2-3 mRNA does not, possibly because it is not efficiently directed into the primordial embryonic germ line. Strong transposon regulation may, therefore, require ping-pong cycling with maternally inherited mRNAs in the embryo.