Silly sci-fi biology idea: haploid/diploid alternation of generations

In episode 89 of the podcast @DrBob mentioned her frustration with silly biology in science fiction. I have a confession to make:

I can’t remember when this first occurred to me, but certainly I used the idea for the natives of the inhabited moon Tether in my Flat Black freeform “Uninvited Guests”, and I ran that at SydCon in 1998. Regardless, I have never got it to work out right, and invite comment:

An alien kingdom or domain of ~multicellular organisms displaying an alternation of generations between

  • Diploid, sessile, autotrophic forms that reproduce by asexual meiosis, producing
  • haploid, motile, heterotrophic forms reproduce sexually to produce diploid seeds, which they cultivate.

I guess that most of the haploid forms would hatch from pods at a precocial stage of development. The diploid forms might produce food bodies to nourish their young. The motile offspring would defend their parent from ~vores that sought to eat its foliage, food bodies, and unhatched fruit; until a plant had offspring to do that for it it might rely on its parents for protection.

Problem: a motile form is as closely related to its sessile parent as it would be to its own offspring, its siblings are as evolutionarily valuable as its possible grand children. So when and why do motiles seek mates and breed?

Question: all the diploid offspring of a given pair of motiles are genetically identical. What does that do to reproductive strategy?


Translation: on this planet the animals grow on plants, and mate to produce seeds.

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It sounds approximately like the lifecycle of jellyfish, with a benthic sessile stage that produces the pelagic medusa.


There’s a species of jellyfish that do something like this, but the concept of parent & child gets a bit blurry…

  • The larval stage float through the water column as tiny plankton
  • These eventually settle on the sea floor where they grow, much like plants in a sessile stage
  • After reaching a certain size, the ‘plants’ start releasing several jellyfish stage off the top of their ‘stem’; reproducing asexually
  • The jellyfish do all the normal jellyfishy things, reporducing sexually and releasing eggs
  • The eggs hatch into the larval stage
  • Rinse & repeat

Edit: ninja @Scribbs got there first


(Without the nice diagram though!)

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This blew my mind when I first learned it. Gotta be in my top 3 lifecycles (along with aphids that are born pregnant and axolotl which just kind of stop in the middle)


I can’t see or download that image. Could you please supply a link?

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Sure, it came from here:


All cnidarians (jellyfish, corals, anemones and hydroids) have a medusoid and polyp stage to their lifecycle. The polyp stage can proliferate asexually into colonies, whilst sexual reporduction only happens in the medusoid stage. Within the hydrozoa, the medusae bud off from polyps that is some species are specialised for that function. (Caution over some gross simplification here).

I think if you have a read around the lifecycle of the Cnidaria you’ll find that the idea in your RPG isn’t massively far fetched.


One of James Tiptree, Jr.'s early stories was about an alien race with alternation of generations. In Tiptree’s treatment, both generations were humanoid, and they were mutually hostile.

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The alternation between haploid and diploid generations is a feature that I cribbed from vascular plants, and the alternation of sessile and motile generations from cnidarians. Alternation of autotrophs with heterotrophs I’m relaxed about. The point where woodsmoke starts coming out my ears is when I try to figure out reproductive and nurturing strategies.

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Well, if they don’t seek to breed for some reason they aren’t a concern by the time the PCs find them. :slight_smile:

How about some leakage in the diploid assembly process? So if the hap parents are aaa and bbb, the standard diploid offspring would be


but sometimes it might be


which starts to give you a reason to seek out genetically interesting mates.

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In terms of the way it works in real world animals, I believe that usually happens not in the diploids, but during the production of the next generation of haploids. The chromosomes swap bits before they pull apart.

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Sure, the offspring of a given diploid display a degree of genetic variety, especially in species with numerous chromosomes. They have an index of relatedness of 0.5, like full siblings among diploid species.

The issue arises because the haploid “animals” have only one gene at each locus. All of each one’s gametes are identical, and contain a complete set of its genes. So for a given pair, all their offspring are genetically identical, each containing all the genes of both parents. They then produce a variety of offspring, each containing about half the genes of each parent, but a different half in each case. These grandchildren are just like siblings.


The logic seems to be the same as that of sperm and ova. All of a given ovum’s diploid offspring are genetically identical (that’s often a trivial result, but some ova undergo twinning). The major difference seems to be that you’re envisioning a haploid/haploid pair producing multiple offspring, not just one. That seems to invite comparison with increased r-selection, which suggests that the diploid offspring would be less likely to survive and might be produced opportunistically during a favorable season.

On the other hand, can the haploid generation live long enough to pair with more than one mate? If so, they can have diverse offspring.

If you want some funky life cycles, get a parasitology book. I had to learn some of them in Uni, and the flatworms were nearly always awesome ones.

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Okay, either I’m not understanding the Problem bit, or I need some clarification from you. My problem about your problem is that you can look at ‘relatedness’ by looking back at the previous generation or forward at the next one.

So the haploid motile:

1) Gets ALL its genes from its parent. But the parent has a bunch of other genes. So a motile is 50% related to a single parent, whereas you and I are 50% related to Mum and 50% related to Dad. I think that’s going to throw a spanner into the maths of working out how related a motile is to its siblings.

In theory I can be from 0% to 100% (identical twin) related to my sibling, average 50%. But I suspect the 0% is far rarer than identical twins, for various reasons. Like identical twins happen because an already existing embryo splits, not because by an amazing coincidence the exact same genes were in that second egg and second sperm! Also I I’m a mammal and mammalian sex determination involves genes on sex chromosomes, and genes scattered across other chromosomes, plus switching on and off of some of those genes depending on whether I got them from dad’s sperm or mum’s egg. So Janet knows she does not share a whole bunch of genes with her brother John, because he got a Y chromosome from their dad, but she got an X.

So the mathematical reality of sibling relatedness in humans will be:

  • The average is over 50% (because of identical twins)
  • The average will be different when comparing a brother and sister, than it will be when comparing two sisters.

(If Janet and John were crocodiles, their sex is nothing to do with genes, so the second point would be irrelevant. Actually, thinking about it, I don’t know of any egg-laying creature which can successfully have twins - embryos do split but there is only one yolk in the egg to sustain them, so they both die).

Now, I have no ideal how your fictional motiles sort out their chromosomes; if they mix up the genes during asexual reproduction (like whiptail lizards); or how they do sex determination; or if twins are possible. 0% related to a sibling might be a real possibility for them. 100% may be an impossibility.

If there is a high probability of a sibling being 0% (or some other low number) related to you, then suddenly inbreeding is not a problem. That might be a strategy in its own right, if you can accurately detect how similar/different you are to your siblings. Find a mate on your doorstep and stay home with your sister-wife and your Mum, versus decide you are too similar to the siblings and set out looking for new territory and new mates.

2) Gives ALL those genes to its offspring. Every single one. So you are sort of 50% and sort of 100% related to your offspring, depending on how you look at it. Certainly protecting your son is protecting 100% of your genes, whereas you can only protect 100% of your genes in a sibling if they are an identical twin! So investing in offspring makes perfect sense.

Genetically Identical Offspring
If all of Janet and John’s offspring are genetically identical, the solution is promiscuity. Or mass spawning for species that haven’t got around to inventing copulation and internal fertilisation. Dump your eggs and sperm out into the ocean currents in the mass spawning event, and everything gets mixed up by water currents and sheer luck. There is a non zero chance that all 1000 of Janet’s eggs are fertilsed by different males.

In fact, if genetic diversity takes a kicking when you do get around to inventing copulation and internal fertilisation, then the selective advantage for said internal fertilisation will have to be much, much higher than it was in Earth’s history. Shagging might not have evolved at all! :slight_smile:

Meanwhile… you’ve got a whole Kingdom or Domain which reproduces the same way? Surely something would have exploited neoteny to dump the sessile stage and become a totally motile species. Like our sea squirt ancestors dumped the sessile adult stage to become free-swimming proto-chordates.

I don’t think that’s biologically accurate. In alternation of generations, the sessile and motile forms are not different stages in the life cycle of a single organism, any more that an unfertilized ovum or a motile sperm is a different stage in the life cycle of the female or male that produced it. They’re two distinct “organisms” (we don’t think of a sperm cell as an organism, but that’s a bias produced by its small size and short lifespan). So the concept of neoteny doesn’t really apply. You can’t dump the sessile stage entirely, any more than humans can dump having sperm and ova (well, unless we become parthenogenetic!). You might reduce its size and duration but if you eliminate it the species doesn’t reproduce.

I suppose you might have the sessile stage lead a parasitic existence attached to or inside the motile stage, the way a fetus leads a parasitic existence during pregnancy. But that’s contrary to the original hypothesis of this thread, I think.

Yeah, but we are taking about a fictitious biosphere where a whole domain/kingdom operates under the principles posited by Agemegos original post. Here you, I and the other posters are rather scrabbling and head-scratching to find real-world examples to fit the bill. On an actual planet, with a 3.75 billion year history of life, there would be all sorts of niches, mutations and opportunities. I mean it is not like everything in the real world Kingdom Animalia does things the same way, is it?

Back to real world biology… the cnidarians, as pointed out by other folk in this thread, do the alternation of generations thing. I’ll add that Hydra has dumped the medusa phase. Probably because (a) it lives in freshwater, and having planktonic larvae ain’t nearly as useful in rivers as it is in the sea, because all currents go downstream, so each generation gets swept further away from the nice place your ancestors lived and closer to horrible salty death; and (b) upwelling and mixing works differently in lakes, ponds and rivers than it does in the sea, plus tides are a thing, so niches are different.


Well, true, but when Agemegos said haploid/diploid he was assuming things worked more or less as they do on Earth. And the Earth trait of alternation of generations is not the same as the Earth trait of developmental metamorphosis in each generation: polyp:medusa ≠ caterpillar:butterfly. If a caterpillar mates and lays eggs that hatch into little caterpillars, that’s neoteny, but if Hydra doesn’t produce medusae, I don’t think that’s the right word. And if you apply the concept of alternation of generations to an alien biosphere I still don’t think it’s the right word.

Incidentally, if I understand things right, all Plantae do alternating haploid/diploid generations.