Why the different results from different M and P strain crosses?
Essentially, it is an example of maternal effect. The transposase mRNA can be alternately spliced in the somatic cells to form a transposase inhibitor, so that P-element hopping does not occur. In order to prevent P-element hopping in the germline (where the inhibitor is not made), the inhibitor must be maternally provided by a P strain mother (maternal effect). Therefore, in the cross of an M female to a P male, hybrid dysgenesis occurs, which is essentially P-element hopping in the germline of the progeny (F1). Since HD occurs in the germline, the F1 are phenotypically wt, but produce lots of mutant progeny (F2). Any other cross of M and P does not result in HD, either because the mother provides inhibitor (P cytotype) or because there are no P-elements (M cytotype)!
