I'm workshopping a sapient fantasy species, that, in lore, was designed by external forces to be a biological weapon. This species accomplishes this with a form of sexual parasitism that enables them to outcross with several other unrelated sapient species, while still propagating their own; i.e: an alien interbreeds with a human, but the offspring is a member of the alien's species only, rather than a true hybrid.

The problems I'm encountering are my own hang ups of making this still a dual sex system, as well as mental roadblocks with figuring out if this is a sustainable system or if the species would die off in a few generations (or less). I also do not want the offspring to be genetic clones of the parasitic parent, and, ideally, I would want them to incorporate at least some of the genes from the other parent, even if those genes are only responsible for superficial characteristics like eye color. I also want this set up where two opposite sex members of the same parasitic species can sexually reproduce with each other and create offspring with an even 50/50 combination of their genes.

My questions specifically are:

  1. Can this system exist while still retaining a dual sex system (male/female) where both sexes gametes overpower the gametes of their mate? Something similar has already been established for female 'aliens' in fantasy, such as the Asari from Mass Effect, but how would males be able to accomplish this? One concern is fetal rejection and gestation times in other species, and how they would be able to 'force' the 'incubator' (for lack of better words) to carry their young.

  2. Does this system exist in nature? From searching, I was brought to a lot of different terms, from Hybridogenesis to Kleptons, etc, but with my limited understanding of biology, nothing seemed to fit, but it's possible the jargon was just lost on me.

  3. Is this system sustainable, or will it collapse from inevitable genetic bottlenecking or other structural weaknesses?

I have considered alternative solutions to this system, and I am okay with making some concessions as well. One such consideration was the parasitism being more similar to sacculina barnacles and their relationship with crabs (where they effectively castrate their host and substitute it with their own reproductive organs), but I'm not sure how this would scale up on a sapient species that otherwise reproduces sexually. I also considered having one sex be haploid, and one sex be diploid, or potentially just hybrids being haploid and 'pure' offspring being diploid, but I don't know what this would entail for any future generations. If it comes down to it, I am okay if the male/female sex distinction is purely superficial, and both 'sexes' on technicality have ovum and testes (albeit less developed or more developed depending on 'sex'), but I want to avoid the temptation for this species to just propagate via cloning. I do want the species to utilize some level of recombination for all of it's offspring.

Some details about the rules of the world:

  • There is magic, though the magic is alchemical, and at a basic level, only capable of transmuting elements. Additional confines to this magic are that atomically similar elements transmute cleaner and consume less magic than transmuting elements on very different sides of the atomic table. Larger structures can be altered, like complex molecules and compounds, though they consume a proportionately larger amount of magic than just altering a single atom. Nonetheless, this magic system, in it's handwavium, cleanly absorbs any kind of radioactive decay that would ordinarily be produced.

  • The sexually parasitic species gives live birth, is warm blooded, has a placenta, and their young are precocial and do not need to breastfeed. Every sapient species they can reproduce with is also warm blooded and gives live birth. The parasitic species does not need to be able to reproduce with everything like a xenomorph would, just other warm blooded species that give live birth.

If more clarification is needed, please let me know! As I said before, my understanding of this facet of biology is very limited, and it's very possible I'm missing a few key details or completely misunderstanding certain concepts. I appreciate the help and consideration!


2 Answers 2


Let me give your biologically weaponized aliens a menacing name for convenience. I'll call them cheese (pronounced "cheece"; singular female "choose"; singular male "chander").

Now when a chander and a choose love each other very much, they give each other a special hug. What they don't tell the choslings is that this special hug is pre-mating. They exchange gametes, with both the chander and the choose getting some fertilized eggs for themselves.

Once they have these fertilized eggs, the cheese get the itch. This happens within hours, not after seven years like in humans. The chander and the choose each go out to find another warm mammaloid to seduce. The chander will seduce a host. The choose will seduce a donor.

The chander deposits his fertilized eggs in his host. These eggs (usually only one, unless you want multiple births) perform phagocytosis on the host gamete. Some of the host genes are incorporated into the egg's chromosomes via crossing over, and the remaining genetic material is expelled and digested. The host's mitochondria signal to the egg that it is within a host, causing it to produce immunosuppressants that prolong the pregnancy until gestation is complete.

The choose receives a gamete from her donor. Again, phagocytosis and prophase, metaphase, and anaphase I of meiosis is used to incorporate some genetic material from the donor. Since no mitochondria from the spermatozoon's tail survive the phagocytosis process, there is no signal to the egg to produce immunosuppressants. The egg develops as its mother's womboid is designed to accommodate.

The immunosuppressants don't really cause too much of a difference between hosted and donated choslings, but this doesn't stop them from making up insults for each other. Things like: "Hosted go to planet Thumb, because they're dumb! Donated go to planet Drool, because they're cool!" The planets in their solar system are named after their creation myth about some great being spitting in its hand and rubbing them together really fast.

  • $\begingroup$ I absolutely love this. A great, detailed answer that was very interesting to read, and also I loved the "cheese (pronounced cheece)" thing. We all know geese are the apex predator of our world. $\endgroup$
    – Katy
    Jun 8, 2023 at 10:16
  • $\begingroup$ So if I understand correctly, this species would sort of have a 3 sex system -- specifically in that the outcrossed host's genetic material is still utilized somewhat? That's really fascinating if so. It's not 3 sex as we know it, but it seems similar! $\endgroup$
    – Tardigreat
    Jun 8, 2023 at 16:09
  • 1
    $\begingroup$ Yes, but really you have 3 = 2 + 1/2 + 1/2 sexes. 2 for chander and choose, 1/2 for donor, 1/2 for host. This also raises the question: what if a chander/choose chooses a choose/chander for its host/donor? Presumably the choose's womboid will be designed to handle the immunosuppressants, so perhaps these pure cheese will just look extra cheesy compared to their friends who got external genes. $\endgroup$
    – skeep
    Jun 8, 2023 at 16:59
  • 2
    $\begingroup$ @Katy Geese have a natural mortal enemy: cheese. I recommend extra sharp cheddar for inflicting critical damage. $\endgroup$
    – skeep
    Jun 8, 2023 at 17:01

They need a biological supercomputer

If you want them to take genetic traits from their host species, they need some way to analyze new species. Give them an organ that can take a sample from any species and work out a way to mesh their genetics together somewhat. You can't just slot a random protein into an organism normally without a lot of trial and error, a special organ to do this is needed.

For example, their stomach might contain an offshot to this organ, so if they can get a sample from the species they can merge.

This doesn't exist in nature

The closest you get is parasites who consume a host. They don't take any traits from the parent, and it's generally a very unpleasant process. Your examples, Hybridogenesis and Kleptons, involve closely related species, like a lion and a tiger.

Inbreeding is going to be an issue

If they were created they can be fairly biologically pure, but they only have major genetic input from a single parent. So long as their genetics are pure enough they'll be fine for a while, but in the long run they may face mutations and issues if their creator doesn't renew them.

  • $\begingroup$ "This doesn't exist in nature": It does. Klepton. The edible frog is a well know example. $\endgroup$
    – AlexP
    Jun 8, 2023 at 2:58
  • $\begingroup$ It's a hybrid of two closely related species, not two radically different lifeforms combining. I even noted that in my post, mentioning Kleptons. $\endgroup$
    – Nepene Nep
    Jun 8, 2023 at 3:01
  • $\begingroup$ It is not a hybrid. That's the entire point. And nobody consumes anybody. $\endgroup$
    – AlexP
    Jun 8, 2023 at 3:02
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    $\begingroup$ "Pelophylax kl. esculentus is the fertile hybrid of the pool frog (Pelophylax lessonae) and the marsh frog (Pelophylax ridibundus). It reproduces by hybridogenesis (hemiclonally)." from your article. The weird thing is that half of its genetics is hybrid, and half is clone. $\endgroup$
    – Nepene Nep
    Jun 8, 2023 at 3:04

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