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An alien creature has secretly landed on earth, seeking to propagate itself among the local species. It studies the individual sexes of a race called humans and determined that the males would be more suited for the purpose. They produce millions of genetic material a day involuntarily with little recovery time, making them a perfect factory for making more of creatures like itself. They are also wired to desire to spread their material, making them easily influenced by certain methods (allegedly).

After taking control of one individual, it soon realizes that it has access to a faster method. Unlike the medieval past, where demons had to take control of humans to pass on progeny one by one, technology as developed to the point where this can be done far easier through the use of sperm banks. These locations store genetic material from donating males, keeping them in storage for long term use to keep them applicable. It determined that it can use this as a way to spread itself to potentially billions of people, creating children all across the world for more quicker and easier than before. These children would operate as a hive mind, providing fresh bodies for the creature to possess if it is ever killed.

The creature cannot fully possess the human, as doing that would change the victim biologically and be a clue that something was wrong. It decides to remain dormant and allow the human to retain its form. However, I need the sperm material of the human to be the genetic material of the creature itself when held under scrutiny. This 👽 sperm will be able to "hijack" eggs through biological story-plot shenanigans, converting the sperm and egg combination into an alien embryo.

Plot holes aside, would it be possible for a human male to biologically produce sperm for another species ? How can I make this work?

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    $\begingroup$ What makes you believe that sperm banks accept any sample of milky fluid without examining it? It doesn't work this way. Sperm is accepted from donors only after qualified people look at it carefully, e.g. measuring the number of spermatozoa, their motility, and after a genetic screen to ward off congenital defects. You cannot even pass sperm from another mammal. $\endgroup$
    – AlexP
    Commented Dec 28, 2019 at 13:50
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    $\begingroup$ Uh, whether or not men could produce alien sperm seems irrelevant if they don't have any alien females to fertlise with it. If it works on regular human females, then what you have is human sperm, presumably with a slightly tweaked genome, but still human nonetheless. $\endgroup$
    – Starfish Prime
    Commented Dec 28, 2019 at 14:30
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    $\begingroup$ Can the alien code be hidden in unused junk DNA regions, activated when exposed to a particular secondary enzyme? $\endgroup$
    – SRM
    Commented Dec 28, 2019 at 16:36
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    $\begingroup$ @AlexP Well, to be pedantic, that just suggests how many people you'll need to deceive, bribe, blackmail, and/or threaten before they'll "accept" your "donation". $\endgroup$
    – Cadence
    Commented Dec 28, 2019 at 16:37
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    $\begingroup$ Using sperm banks for a creature to produce more of its own kind, by breeding with humans isn't necessarily faster. Certainly than a one by one method, but not much more. Not unless you add some aspect to prioritize the use of sperm banks for reproduction. Say, an infertility plague. Even so, it will take generations to be effective. $\endgroup$
    – a4android
    Commented Dec 28, 2019 at 23:41

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The kind of effect you are looking for seems to be Larry Niven's Pak Protector species where there is a second puberty like alteration for humans at menopause that is triggered by dietary etc changes (not available to humans on Earth). Such a change can be triggered in a number of ways once its encoded in the human genome and might not be that obvious and be specific. Shingles for example affects all humans but is normally only a concern in adults so your trigger might just be an annoying rash in children or old people but react properly at certain oestrogen or testosterone levels.

"Plot holes aside, would it be possible for a human male to biogically produce sperm from another species ? How can I make this work?"

Short answer... yes you could probably alter human testicles to produce different sperm with a bit more difficulty than current work successfully growing human tissue in other animals for transplants. That would mean they are basically a seperate system grafted onto a human. The real question is Why?

"The creature cannot fully possess the human, as doing that would change the victim biologically and be a hint to anyone that something was wrong."

If you are implying Mental possession then yes it might lead to a slow corruption as brain chemistry issues are triggered. These can be dealt with by alterations to the human genome to make possession easier, rather than creating a new species or forcing the recreation of your alien species. Your functional hosts might then manifest with something like autism or epilepsy but with different markers for those issues or just a propensity for mental health issues eg anxiety or sociopathy.

If you are implying physical possession then the more we learn, the more obvious it becomes that War of the Worlds virus saviours or Invasion of the Body Snatchers takeovers are unlikely. You are apparently assuming your alien genetics uses same chemistry as Earth ... same 4 rna/dna chemicals, same chirality etc or your idea has a massive incompatibility problem.

Even with the above as a given, there are going to be a wide range of compatibility issues eg proteins and even if they are minimal, you will still have hybrid sterility issues like horse + donkey = sterile mule not a new species. You would basically be creating a new, almost human species.

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It cannot, for a simple reason: as soon as a cell is not bearing the fingerprint of the individual whose body it is inside, the immune system of that individual will foght to take rid of it.

For these humans it would be an inflammation of the testicles to kill all the non self sperm cells. I guess it would be rather painful.

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  • $\begingroup$ This seems “simple” to get around: any sort of AIDS-like virus can disable the immune system. Or, more handwavy, teach the body to accept the entity. $\endgroup$
    – SRM
    Commented Dec 28, 2019 at 16:17
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Not only is it possible. Humans can do it with a technology exists right now.

There is a relatively new gene editing technique called crispr which among other things allows scientists to modify the DNA of living cells while the cell(s) are inside of a living breathing creature. Clinical trials to see if it is possible to fix a certain type of genetic blindness will soon be started here in the USA.

So Assuming a sufficiently an alien, kmows what to change I the human reproductive system to get the desired result I don't see why not.

Making your modified males remain fertile with unmodified females would be harder.

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Bone Marrow

When a person gets a bone marrow transplant, the bone marrow starts producing cells with the donor's DNA which then flows onto the cells around the body when they get replaced including sperm cells

See Bone Marrow Transplant

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  • $\begingroup$ It would be preferable if the source was from a peer reviewed scientific journal or other reputable source rather than a tabloid. $\endgroup$
    – Andrew Fan
    Commented Dec 28, 2019 at 23:33
  • $\begingroup$ Of course it would but I haven't found one. As pointed out by the article, it hasn't really been studied. Doesn't make it untrue and it makes sense if you think about it. The bone marrow is producing the stem cells used to grow new cells. Said new cells is going to contain the new DNA. The cells which get replaced the quickest are the ones to show the new DNA the quickest AKA cheek cells and sperm $\endgroup$
    – Thorne
    Commented Dec 29, 2019 at 0:06

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