How can an immortal surrogate pass on genes to a fertilized embryo?

Question

I am part of a group of immortals from the planet Ziest. From the dawn of time we came…moving silently down through the centuries. Living many secret lives, struggling to reach the time of the gathering, when the few who remain will battle to the last.

We immortals unfortunately cannot create progeny, as our bodies cannot produce the sex cells necessary to make offspring. However, recent technology have allowed immortals to become surrogates, allowing them to pass on their immortal genes to the next generation. A fertilized egg from two mortals is put into the womb of a female immortal, allowing it to gestate there.

During pregnancy, placenta separates the baby’s and the mother’s DNA. When the baby forms in utero, the placenta forms along with it. The primary purpose of the placenta is to be a gatekeeper. It provides the growing baby with nourishment and sustenance, and is one of the main protectors of the integrity of the fetus. Only specific matter can go through to the baby, meaning that the DNA of the carrier stay on the other side of the placenta. This is meant to protect both parties, as too many cells passing between mother and child can be dangerous to both.

How can this be modified to allow the surrogate to pass on their genes without endangering the lives of the carrier or child?

Answer 1

You could change the premise of infertility in the immortals of Zeist from an inability to produce sex cells, to the cells being produced being dud/defective due to an inherited disease of the nuclei.

Once you establish the above, you can then use the technique of three parent baby (used in real life) to allow for transfer of the nuclei in the reproductive cell, which will ensure that the mitochondrial DNA of the immortals of Zeist gets transferred to the babies.

Next, you need to establish that the mitochondrial DNA is source for some of the pecular traits of the immortals of Zeist, which can now be inherited with advances of technology.

Answer 2

The children are chimeras.

In our world, cells from a fetus can move thru the placenta and take up long term residence in the mother.

https://www.scientificamerican.com/article/scientists-discover-childrens-cells-living-in-mothers-brain/

The physical connection between mother and fetus is provided by the placenta, an organ, built of cells from both the mother and fetus, which serves as a conduit for the exchange of nutrients, gasses, and wastes. Cells may migrate through the placenta between the mother and the fetus, taking up residence in many organs of the body including the lung, thyroid, muscle, liver, heart, kidney and skin. These may have a broad range of impacts, from tissue repair and cancer prevention to sparking immune disorders.

It is remarkable that it is so common for cells from one individual to integrate into the tissues of another distinct person. We are accustomed to thinking of ourselves as singular autonomous individuals, and these foreign cells seem to belie that notion, and suggest that most people carry remnants of other individuals...

It is easy to detect male cells in a female because you can look for cells with a Y chromosome. There is no reason to doubt that the exchange is reciprocal - mother cells moving in and taking up residence in the fetus.

This is how your immortals pass on their genes. Their cells move thru the placenta and take up residence in the growing fetus. Immortal cells in residence then gradually and gently compete for space with the cells of the native human, taking advantage of the scaffolding grown by the fetus and gradually edging out the natives over time. It might take a lot of time but eventually the individual is effectively a clone of the "surrogate", with the mortal cells of the original having died out over the decades.

Answer 3

Have you considered somatic cell nuclear transfer aka cloning? No need for sex cells from two mortals, just take a mortal egg cell, remove nucleus and replace it with a nucleus from an immortal somatic cell. The capability to use this technique of course depends on the scientific level of your immortals, we humans have so far only cloned Dolly the Sheep and some macaques. Also, cloning of humanoids might be a moral issue.

Second thing that came to mind reading your question is the viability of immortals to be surrogates. Lacking the ability to produce sex cells, why would female immortals have a menstrual cycle or even any capability to form uteral lining to grow a fetus? This is something you might want to consider.

Considering your question aka how can a surrogate mother give the fetus she is carrying her immortality. As previously @Willk has pointed out, microchimerism is known to happen both fetal to mother and mother to fetus in placental mammals, including humans. However, little is yet known about this phenomenon. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5532073/ This article might be a heavy read, but the key points in the end summarise it nicely. (The article also touches the issue of immune tolerance in expecting mothers, which might also be of interest to you considering your surrogate mother carrying a different species, but this was not the question). The conclusion is that we don't know what types of cells are passed on to the fetus during pregnancy. After quickly glancing over a few other abstracts on the matter, the scientific evidence seem to point to the conclusion that mother to fetus microchimerism happens to help the child's immune system mature into something more complex than it would without this phenomenon (sorry, english third language). For me, as a person who likes to pretend to know something about human biology and immunology, it seems like a little bit of a stretch to think that the surrogate mother could pass on stem cells to the fetus it's carrying, and that the stem cells could replace all the cells of a fetus without causing a immune reaction and downright rejection by the child's immune system. But, we also have a story with immortals and as I said, the research is incomplete, so what the hell.

However, while researching this, I also came across the info that different types of immune cells are passed on to a baby during breastfeeding... at least in mice. So I thought that if you formed your reason for the immortals' immortality around their different immune system, you could explain that the effect of the surrogate mother on the child's immune system both during pregnancy and while breastfeeding would compound into the child getting immortality through immortal-like immune system. This system could also cause a different level of immortality.

TL;DR

1. cloning
2. maternal microchimeric stem cells in fetus/child somehow evading immune system and replacing all other cells over time
3. immortality because of immune system + motherly effect on child's immune system during pregnancy, breastfeeding (and a tad growing up)

I hope I didn't fall to much into the realm of discussion, I'm new to this website. :)