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I had an idea for a planet which, millions of years ago, was accidentally contaminated with various animals, plants and microbes from Earth, and - via the planet's native aliens' bizarre reproductive biology - the two independent lineages hybridized and produced fertile descendants.

Is there any conceivable reproductive mechanism, however alien, which could allow these organisms to interbreed with Earthly ones? One of the first problems is that of DNA - however, it's not statistically impossible that it would be used by the aliens too, coincidentally. Or maybe you can think of a way to do it without DNA.

One very hazy idea I had was something loosely based on the reproduction of lysogenic viruses, so you can try that if you want, but I didn't expand on it much more.

EDIT: In response to a comment on L.Dutch's answer; horizontal gene transfer, though not technically interbreeding, is fair game, as is any other method which produces chimeras rather than true hybrids.

Life on Earth reproduces using nucleic acids (DNA or RNA). Thus a prerequisite for this to happen with an extraterrestrial life form is that it also used nucleic acid.

On Earth bacteria are able to exchange genetic information among them, regardless of their species, by exchanging plasmides. Think of them like "macro" which serve specific scopes (i.e. degrade an antibiotics molecule to make it non toxic) and which can be executed under different software.

Most of the times, when it happens with higher life forms, genetic instruction can be executed (that's how bacteria and viruses infects), but rapidly removed from the system, either by healing or by death of the host. Sometimes they are kept and transmitted.

If somehow your alien has:

• capability of taking genetic sequences from the outside


• capability of integrating at will said sequence in its own nucleic acid and executing them


• capability of not destroy the carriers of the imported sequences

then it becomes viable that the alien organism, after importing and embedding genetic sequences from an other organism, becomes a chimera and, upon reproducing, transmits those sequences to the offspring.

Mind that the second bullet can backfire if not controlled properly: you don't want to embed the genetic material of everything you eat, for example.

Something to consider are aliens roughly akin to something like Tardigrades, which are both cool and freaky. They are believed to have survived the last five mass extinctions. They can survive in crazy conditions, even temporarily in dead space.

Ignoring the title of this article, it states

In the case of these tardigrades, when they're dried out, their cell membranes are often broken and leaky. This allows DNA from the environment to sneak inside, allowing them to make it part of their DNA when they rehydrate and repair themselves. It’s possible that it’s this successful DNA acquisition that’s allowed them to become so adaptable to so many environments.

In such a situation, blood or other sources of human DNA could "leak into" the dried Tardigrades. When they re-animate, so to speak, the human DNA would be integrated into them, and therefore replicate during reproduction.

The alien has a complex hybridized reproductive and immune system. Using reproductive cells (or not) from the donor female or male, it uses stem cells to systematically copy the host cell(s) thousands of times for systematic evaluation. It randomly splices segments of its own DNA into the host's cell. The alien checks if the cloned organism is a viable, healthy being (capable of sexually reproducing like either the host or the alien itself). What's a healthy, viable being? The alien DNA has a complex corpus of "match" and "reject" cells that show if a certain genome is likely to produce a viable offspring. By matching RNA sequences from the alien's corpus to the candidate zygote, the immune system will reject or keep that particular copy. Polybirth is common.

Hybrid: 2. a thing made by combining two different elements; a mixture.
"the final text is a hybrid of the stage play and the film"

Is it possible to make a "hybrid" creature which combines prokaryotic and eukaryotic methods of handling DNA? Or a hybrid which combines evolved fungal and algal elements? Yes - this is possible via symbiosis. Mitochondria are prokaryotic organisms that developed a symbiotic relationship with ancient eukaryotes. It is possible to accidentally kill your mitochondria with antibacterial antibiotics. Lichens are ancient symbiotic organisms combining elements of fungus and algae. In both these situations, the symbiote cannot exist without both partners.

A stable symbiosis sidesteps very different methods of handling the genome, DNA, cellular housekeeping or what have you. In a symbiotic relationship, earth beasts keep ours and aliens keep theirs. But we move in together and work together for the common fitness of the symbiotic organism. Perhaps an earth creature takes up alien symbiotes that interface with the cells and organs of the earth creature. Or vice versa: an alien creature picks up commensal earth creatures which contribute to the alien creature's fitness.