When complete Pelements are injected into embryos from an M strain, they transpose into the chromosomes of the germ line, and progeny reared from these embryos can be used to establish new P strains. However, when complete Pelements are injected into embryos from insects that lack these elements, such as mosquitoes, they do not transpose into the chromosomes of the germ line.
What does this failure to insert in the chromosomes of other insects indicate about the nature of Pelement transposition?