This is a reply to Lindenare’s and Beth’s posts a few days back. You guys are awesome. SF bio, hee….
WARNING: In-depth discussion of biological fertility here. Should be PG-13 at most, but there will be technical language. A glossary will be provided for the most esoteric terms, but I take no responsibility for any injuries incurred by frantically excavating your high school bio book out of storage. 😉
Glossary: Haploid: having 1 n set of chromosomes. Most higher organisms we’re familiar with are diploid, with a 2n set of chromosomes, and produce haploid gametes (sperm and eggs in most animals) to reproduce. Though technically salmonids (salmon, trout, others) are actually tetraploid, with one set of 2n chromosomes active in fresh water and another in salt (or wherever they spend growing up). And then there’s fungi and that can get really scary….
Macrocephalic: With a very large head. In this case, the “sperm” head.
Meiosis: Specialized cell division that, instead of producing 2 daughter cells that are 2n duplicates of the original cell, produces 4 haploid (1n) gametes. It often involves crossover, where chromosomes of each pair will literally twist around each other and trade pieces of DNA; anywhere from a tiny bit of a gene to most of a chromosome. This doesn’t always work correctly, and some gametes will end up with incomplete chromosomes or duplicate chromosomes. That… tends not to have good results. Really.
Methylation is the process of adding methyl groups to some areas of the DNA that serve to, among other things, either promote or block RNA transcription, thus affecting how the gene is expressed. (DNA transcribed to RNA translated to proteins is the common way to express a gene, though it can get much, much more complicated than that.)
Motile means “it can move”. It specifically refers to cells that can move, as opposed to larger organisms.
Polar bodies: Egg cells are formed by a specialized type of meiosis that boots out 3 sets of DNA while keeping almost all the cell organelles and energy in the remaining egg cell. Polar bodies are the 3 booted-out sets.
Tritiya gametes are haploid and motile. I think Beth has a great idea in amoeboid sperm, because, well, this dialogue comes to mind….
Isshin: “Wait, even those have tentacles?”
Isshin: “Tentacles. I’m calling it now.”
> Also, if fertilization can occur in tritiya + egg and tritiya + sperm combinations, why don’t tritiya + tritiya combinations work?
Wrong methylation on the parental chromosomes. Same reason you can’t currently make a viable zygote with two sperm cells or two eggs. Chromosomes get “marked” with what sex they’re from. Anything with two sets of tritiya chromosomes would get tossed as nonviable.
Also, it’s not a case of “can occur in this combo”, it’s a case of “has to have all of these combos to work”. Splicing in your DNA via a virus into existing chromosomes is not an exact science. There might be insertions that are viable enough in the somatic case (the infected person) but will fail utterly to generate a new organism from zygote on. It does take a lot of energy, but by using this method the Arrancar do 2 things: they guarantee low fertility (they want more spreading than natural offspring), but they also give resulting offspring the best chance by “weeding out” all the nonviable chromosomes.
No, I’m not sure exactly how the weeding out happens. It’s a combo of natural measures with the regular microtubules (protein structures that hook onto the chromosomes to move them), and “checking” the psychokinetic database Arrancar have set up for their new spawn. Complicated stuff like flash-step isn’t just instinct!
> For that matter, are the initial zygotes (diploid, 2n) motile at all,
They are motile. As for fusion – I suspect the sperm-egg 2n puts out the right signals for the other two to find it. Which means there are 6 copies of each chromosome in the superzygote, and they will pair up in three pairs, crossover indeed possible. Shinigami and youkai are three-sexed, but they are 2n chromosome count. From there, yes, splitting three ways; some of which hopefully would be viable.
>If we are assuming that the egg is still like a human egg and both the sperm and tritiya are small and sperm-like, then only the egg cell carries mitochondria.
That seems highly likely. After all, why mess with what works?
A tritiya cell might be carrying an X, Y, or W chromosome. It’s also likely carrying other necessities – not just other plasmid DNAs, but particular enzymes that say “right, not human, this is how splitting up the superzygote is going to work, people.”
>The entry of a sperm into an egg causes the membrane of the egg cell to change to exclude any additional sperm from entering. If this “new” membrane is what attracts the tritiya cell, then lack of sperm in the equation means that the triyata cell would be incapable of merging with the egg, again creating no offspring.
Ooo. That’s a good idea! At least for one of the combos to attract the other two. As for IVF tritiya-female offspring… I warn you the bunnies took a look at Soi Fon and Yoruichi and hid.
> If a single egg can create three viable fetuses in this AU than the two copies of maternal DNA are both needed.
*Nod* I’m actually thinking that this is a more a case of “tendency to fraternal twins” ramped Up to Eleven rather than saving one of the discarded polar bodies. As in, two eggs released. So there are actually 3 (1n+1n) combinations involved, that then fuse.
I do agree on the tritiya cell attracting sperm, although it’s likely to be a bit more… aggressive in actively finding it. Macrocephalic “sperm” definitely works.
>There will still be mistakes when packaging the DNA, and DNA imprinting will still come into play.
And how. Ouch. Once they realize fertility is possible, I suspect our geneticists will be wanted to check. Check everything.
> Both sides divide again, hopefully putting 3 DNA-sets in their own cell and leaving one empty ghost-cell. Any cells with more than 1 DNA set will not be viable. After this 4-cell division all 4 cells break apart via signalling from one of the tritiya cell plasmids and become up to three viable embryos.
>Now take a moment to stop and pity the poor mother who would have given birth to fraternal triplets if she had stayed fully human.
*G* Masaki had twins. I’m guessing twins could be relatively common.