how can a developing organism affect the DNA of its host?

Question

A group of scientists have been commissioned to create a new kind of super soldier. As this project includes illegal human experimentation, the government wants to limit the risk of anything being traced back to them, selecting disposable people for the process. Illegal human immigrants from Latin America, fleeing from violence and poverty in their home countries and hoping to obtain a green card, are chosen for the project with promises that they would earn passage into the country and given permanent residence. This is provided they cooperate and agree to participate in the experiment.

In order to create a new hybrid species, a group of scientists combined eldritch DNA with human embryos and inserted them into these surrogate mothers.This decision was a mistake in retrospect, as the unborn child was able to alter the DNA of its mother, twisting the body into horrific new forms. The affects on the host varied depending on the fetus's goals. These hybrids manipulated and molded their hosts to better accommodate its size and shape, outfitting it with natural weapons in order to to better defend itself and the host. Whatever dark goals these alien children had in mind are unknown, as all the scientists were slaughtered by the brood mothers, who left the facility to sire their children in parts unknown.

After the incident, the government sent a special team in to try to piece together the events and what led up to them. How would a fetus be able to change or manipulate the DNA of its host?

Answer 1

Fetal cells do enter the mother, and take up long term residence.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2633676/

Fetal cells migrate into the mother during pregnancy. Fetomaternal transfer probably occurs in all pregnancies and in humans the fetal cells can persist for decades. Microchimeric fetal cells are found in various maternal tissues and organs including blood, bone marrow, skin and liver. In mice, fetal cells have also been found in the brain. The fetal cells also appear to target sites of injury. Fetomaternal microchimerism may have important implications for the immune status of women, influencing autoimmunity and tolerance to transplants.

The result: many women who have given birth are turned into chimeras. It is not clear whether this process is adaptive in some way, or accidental. It is a very cool area of research - if one could purposefully create microchimeras like this it might be possible to effectively treat a broad range of inherited genetic diseases.

For your scenario, the alien fetal cells use this process. They do not change the DNA that they find, but change the cellular composition and so the DNA balance of the organism. As with humans, the alien cells migrate to sites of "injury" and establish themselves, then differentiate into the alien organs and augmentations that you want for your story.

Answer 2

It is known that a mother's immune system can attack her fetus if their blood types don't match.

In response, a fetus with a strong self-defense inducing mutation could naturally try to understand why it's being considered as a foreign body and alienate the mother's blood type. This wouldn't work as planned, of course, and cause all types of side effects resulting in said horrific new forms.

[Note: this is science fiction of course, as suggested by the tag.]

Answer 3

Your mention of DNA suggests that your "xeno aliens" aren't actually that alien after all, which is presumably how the scientists were able to "combine" it with human cells.

Well, if the "alien" DNA is that compatible with humans, then one issue is the presence of endogenous retroviruses in the inserted DNA sections. These are usually passive and inert if not outright broken, but the engineering process that created the chimerae could have reactivated them, and the hybrid's immune system might not have been able to keep them under control. Viruses are known to cross the placental barrier in the mother-to-child direction, but the only reason they don't pass the other way is that humans don't have (currently) active retroviruses in germ cells and so the unborn child is effectively disease-free.

What you've ended up with, though, is not the hybrid you were expecting but some hideous diseased mutant with a congenital illness that can spread across the placenta and infect the mother. This is of course not deliberate or targeted on the part of the foetus... both mother and unborn child are warped by the virus, just in different ways. Possibly the virus itself was deliberately engineered for this purpose.

brood mothers, who left the facility to sire their children in parts unknown.

Given the meaning of the verb "sire", this suggests some fairly extensive modification of the host mothers to the point where "mother" is probably no longer the correct noun.

A group of scientists have been commissioned to create a new kind of super soldier

What really needs to be done is to create a terrifying alien hybrid budget manager, who is kept in an armoured cell in the accounting department, who remains placid until someone proposes starting up another supersoldier project, at which point it turns into an implacable killing machine, devours them and hunts down their sponsors. It would save everyone so much trouble.

Answer 4

Viruses do this all the time in real life. They enter the cells of an organism and reprogram the cell's DNA to produce more copies of the virus. As a result of this, they can do other things too which is the basis of gene therapy.

It just so happens that this is usually not in the host's best interest, but some viruses co-exist fairly well with their host (lots of ways to go wrong, and not many ways to go right, after all, but one real life example is the cold sore virus, for the most part). It's not a stretch for a virus to be beneficial to its host in a symbiotic relationship. It would barely be recognized as a virus if it was.

All you would need is for the developing organism to have inherently have its cells produce an appropriate virus (aka it's own cells are "infected" with the virus). But the virus would just be part of the organism's normal function in a symbiotic relationship.

The virus could serve another incidental purpose in the organism and just so happen to also have this effect on the host's host or the host (the host of the virus, the developing organism) could be able to manipulate the virus's DNA so it affects the DNA of the host's host in a specific way. Gene therapy, if you will.

It also leaves the door open for this to happen without a developing organism since it could also be made to be contagious if needed be.

This is pretty much what the black oil is from X-files. Or the Zerg spores from StarCraft. Or the flood from Halo.