Alternatives to Sex?

This is kind of beside the point for Junction, since the aliens in question are non-sapient animals, but I guess I got into this habit writing New Frontiers. Just can’t stop thinking about alien sex!

Specifically, why have it? Why not asexual reproduction? Well, there have been whole books written on the subject, but the upshot is that it seems to be a good long-term strategy to scramble your genome a bit and to allow individual animals to choose who their genes will be scrambled with.

Does that mean that sex is inevitable? Greg Egan postulates an alternative, in which individuals swap genetic information with their neighbors, which they then incorporate into their offspring.

Alternate quantum physics weirdness aside, this sort of horizontal gene transfer should be familiar to us from the way real-life bacteria exchange plasmids (and eukaryotes occasionally exchange exogenous retroviruses). Why not scale up this system to work for large animals?

Animals accept retroviruses from other animals and put them in a “library.” The next time they accept a retrovirus, they compare it to the ones they’ve already captured. Is the animal getting a lot of copies of the same gene from different donors? If so, perhaps that gene is useful and should be expressed.

That system is vulnerable to abuse, though. Retroviruses may code for nothing more than making other retroviruses. In other words, they are parasites, and waste the host’s resources. I can also imagine donors sending out trojan-horse retroviruses containing destructive code to kill off their competition. The solution would be for the host to watch its potential donors while they compete with each other and judge which donor seems healthiest and best suited for its environment. Then select the genes from that one…and I’ve  re-invented sex.

Okay, what if, in order to swamp out the parasitic genes that might otherwise hijack the swap, organisms would copy and swap ALL of their genes? And to avoid disrupting organs already in operation, these new genes are sequestered and saved until they can make a contribution to the next generation…hell, I’ve re-invented sex again.

OKAY. Here’s yet another possibility, this one I know works because bdelloid rotifers have been doing it for millions of years. When a rotifer is stressed, it expels the water from its body and forms a cyst. The cyst sits there for however long it takes for conditions to improve, at which point it fills with water again and comes back to life. During its hibernation, its DNA was damaged (since the mechanisms that usually protect and repair it were dormant), so it takes DNA fragments from other rotifers that it sucked up along with the water and uses them to repair its own genome. This strategy has the added advantage of keeping the genome stable while times are good and everyone is reproducing parthenogenically, but introducing variation when times are bad and some new survival strategies might be in order.

Abuse of the system could be more viruses, and there may be something like sexual selection where rotifers huddle together with healthy individuals in order to make sure they get access to good gene-fragments. I can see how this system might evolve in time back into something like sex (it’s just such a good strategy) but bdelloid rotifers have been doing okay since the Jurassic, so maybe asexual reproduction works after all :). At least the system’s good enough for me to use for my Martians.

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  • Sara3346 .

    Why use your own species to reproduce at all? I can imagine some sort of multicellular organism the sex cells of other multicellular organisms within the same Class to replicate for them?

    I’m such a system I can even seen the labyrinth arms race going on between different species parasitizing one another and trying to make sure that their offspring are the ones produced leading to some crazy bizarre immune system equivalence and all sorts of trickery!

    I’m such a system I can even seen the labyrinth arms race going on between different species parasitizing one another and trying to make sure that their offspring are the ones produced leading to some crazy bizarre immune system equivalence.

    Also what if you store your replication information, in something other than DNA, some other molecule or repeatable but modifyable pattern, how might this affect any particular species?

    • I’m not sure what you mean. Do parasites mix DNA with their hosts in this scenario, or do their young just steal energy from the host? If the second, you have exactly described parasitism and re-invented the immune system 🙂

      If the first, there is a reason you can’t exchange with a musk ox or a blue whale. They are adapted for very different environments from you, and any hybrid would most likely be a much worse survivor than either of its parents. Hybridization does sometimes occur in disturbed environments, where any change would probably be an improvement, but usually there are mechanisms in place to prevent exactly that thing.

      Sure, there could be all sorts of way to store genetic information other than DNA, in the same way there are lots of other things you can write on aside from paper. Exactly what molecule you’re using will make a difference to your biochemistry, but it won’t have much effect on how a given organism looks.

      • Sara3346 .

        No, in this scenario parasites attempt to sto steal little in the way of he tic waterfall just trying to co-opt the host produce idenfinite number of the offspring by messing with their coding and sexual cells..

        And I was imagining, that in the scenario I was thinking of the creatures but only trying to harass the ties those closest enough to them we in which their offspring could actually have a viable environment for growing, at the same time we might have some of the same cells attempting to Hyjack nervous system, like with parasitoid wasp larvae exept with adult moths giving birth too a different species for a while.¡

        So wouldn’t be like a muskox trying to have a blue whale give birth to its babies it wouldn’t be more like a musk ox running up to an Indian Zebu, *rape* running away and creating a new musk ox factory.

        • autocorrect mangled your text, but I can understand the last paragraph.

          But it’s important to separate parasitism (like what ichneumon wasps do) where a female lays her eggs in a host, and sex, where a male and female split and recombine their genomes to produce an offspring.

          So a parasite like an ichneumon wasp could implant her eggs in a host (perhaps even hijacking the host’s normal pregnancy and child-care instincts, as with parasitic barnacles on crabs) and cause an animal of whatever species to “give birth” to one of her offspring. A less extreme form of this behavior is brood parasitism, where a bird (say a cuckoo) lays its eggs in the nests of birds of other species, tricking them into raising its chicks.

          However, that parasite can’t (or at least shouldn’t) try to mix its DNA with its host, because the result will be a useless hybrid. Even animals as close as zebus and musk-oxen don’t produce ecologically viable offspring (even if those offspring might be genetically viable). Zebus and musk-oxen are different species in the first place because they are adapted to different niches.

          Now, what the parasite could do is mate with other members of its species, then delete the genes of its partner so that its offspring are clones of it. This occasionally happens in nature, but then you’re not talking about sex any more, but parthenogenesis.

      • Spugpow

        I’m guessing that they’re talking about organisms that reproduce like viruses, e.g. by hijacking a host’s genetic machinery to produce the spores/gametes of the infector. I could see that strategy leading to a lot of cross-clade genetic exchange, especially since the infector would probably want to reuse the host’s own genes wherever possible. Why inject a gene for collagen production when the host already has a perfectly good one? There could be radical polymorphism among infectors depending on what hosts they sprang out of. Perhaps infectors could make use of different hosts according to environmental conditions in order to “steal” advantageous genes.

        Perhaps some co-evolution could occur between parasites and hosts, to the degree that an entire ecological guild of “factory organisms” arises that specializes in manufacturing the young of injectors in exchange for nutritional gifts. The genes of an entire ecosystem could mingle in these factories, leading to massive genetic exchange and perhaps even the evolution of Gaian “superorganisms.”

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