Transposase titration in Drosophila melanogaster: a model of cytotype in the P-M system of hybrid dysgenesis.

In the P-M system of hybrid dysgenesis of Drosophila melanogaster, some M strains possess chromosomal P elements. The chromosomes from one of these pseudo-M strains reduced the instability of a P element-insertion mutation of the singed bristle locus. We suggest that this reduction is an indication of competition for a transposase that binds to the P elements on the chromosomes from the pseudo-M strain as well as to the P elements at the singed locus; the pseudo-M strain's P elements might therefore be said to titrate the transposase, reducing its availability to interact with the P elements at the singed locus. We hypothesize that a similar mechanism regulates the movement of P elements in the P strains of D. melanogaster, although in this case we propose that the titrating elements are extrachromosomal and that they are generated by the action of the transposase itself.