Plausible ways for the alien species to be able to impregnate every and any species?

In many movies, alien species often have the ability to impregnate "every and any" species and have their mutant offspring.

I have a story setup where some, not so conscientious, scientists decide to make that happen, for whatever reasons.

But I wish to explain "how it worked", what's the biology mechanic behind it that make such thing "plausible"!?

Because in real world, crossbreeding often failed. Even if they do succeed, they often produce an infertile offspring.

What possible ways for the alien species to "guarantee" a success and produce, preferably fertile, offspring!?

posted almost 5 years ago

PiggyChu001‭

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