Is it possible that a complex humanoid creature to somehow still use Mitosis as a method for reproduction, and if so, what would it look like?
1$\begingroup$ Do you mean reproduction instead of breeding? Breeding is generally taken to be the act of sexual intercourse of some kind, whereas reproduction involves the whole process up to and including giving birth. $\endgroup$– userApr 14, 2016 at 12:28
While BlindKungFuMaster is sort of correct, I don't think that is what you had in mind.
You want a humanoid body that splits in half, and then the resulting two forms are viable individuals.
The problem with this is that splitting a human in half makes us non-functional, so first you need to duplicate all the vital organs. But that's still not enough, how do you do the split? The most likely way would be a vertical split from the top of the head to the crotch, this has the least "surface area" to create and reseal.
So this would probably start with the growth of vestigial limbs at the front and back of the body, these would grow gradually, becoming more and more viable. At the same time internally the vital organs are duplicating and splitting, until eventually you have two hearts, the two lobes of the lung have separated and are starting to form a new pair.
The head would start to expand outwards and new brain cells fill in as memories are copied between the two halves as they gradually become more and more separate.
After a few months you have a body with two heads, and extra limbs front and back. The two systems would continue to separate and new skin grow in to cover them all until eventually they fall completely apart and rise as two separate individuals. They then have to argue over who gets which possessions.
During early stages they would be able to function as normal. Once the separation grew advanced enough this would become harder and the last few weeks would involve bed rest most likely. The splitting and reforming of the brain would most likely cause confusion and mood swings, and the two resulting people while being genetically identical would not have fully identical memories or personalities.
2$\begingroup$ You could also have a full set of redundancy in internal organs (unlike our case where some are singular) and the missing half grows back after splitting. $\endgroup$ Apr 14, 2016 at 12:58
Doubling every part of the body and then splitting into two separate persons strikes me as unrealistic. Humans (or any creature really) need a long time to build up so much tissue, after all this isn't just filling fat cells until you have added 50, 60 kilos. So this would basically amount to a several year pregnancy in which you add another hundred percent of your body weight!
Instead I would consider something akin to the power of a planaria. These flatworms can be cut into up to 50 pieces and every single piece turns into a fully functional worm again. They do this by turning cells back into stem cells and then basically rearranging the complete layout of the little part until a small but viable worm is recreated.
For humanoids this sounds farfetched, but less so if your humanoids have organs that are decentralised and redundant. In that case they might just drop an arm and wait until it turns into a baby …
Let's say if a body part is dropped or cut from the body the skin secretes a kind of cocoon in which the transformation into a baby occurs. That would be similar to the metamorphosis of an insect, which also often involves a switch to a completely new body layout.
$\begingroup$ Ovum etc. sounds like you are thinking of parthenogenesis, not mitosis. There is no egg or whatnot developing into a new body starting with a single cell. He's asking about the adult body splitting in half. $\endgroup$ Apr 14, 2016 at 12:55
$\begingroup$ No, an adult body splitting in half would have nothing to do with mitosis. Except of course, if this adult body is a single cell, not very plausible for humanoids. The difference between parthenogenesis and sexual reproduction is (usually) that there is mitosis instead of meiosis+fertilisation. But you might be right that TrEs-2b was going for full body splitting, in that case "mitosis" was just an unfortunate choice of words. $\endgroup$ Apr 14, 2016 at 13:14
$\begingroup$ I assume the term is adapted by anology, organs for organelles. It's sometimes used this way in science fiction. $\endgroup$ Apr 14, 2016 at 13:23
1$\begingroup$ You are probably right, maybe I'll adapt my answer. $\endgroup$ Apr 14, 2016 at 13:26
Mitosis is a process specific to single-celled organisms. However, there are some multicellular animals, such as planaria and starfish, that are capable of breaking into fragments and regenerating into multiple individuals. Some starfish will fragment proactively as their main means of reproduction.
The main reason why this ability is absent in 'higher' animals is due to the complexity of the central nervous system. Vertebrates have a highly regulated growth pattern with very specific stages from zygote to adult that does not work well unless it develops from start to finish. While some vertebrates have limited regenerative abilities, we cannot grow a whole central nervous system unless we start from scratch.
If a vertebrate were to 'split' in order to reproduce, it would need to grow a new body as a 'bud', starting from a lump of cells and passing through the standard growth stages before breaking off of the parent body. Of course, this would be a lot more inconvenient than just growing a clone inside the body... which is just parthenogenesis.
$\begingroup$ I would probably de-specify away from CNS as we are just complicated all around but this is a good answer. $\endgroup$– JamesApr 14, 2016 at 13:58
I asked a somewhat related question in May 2015: What needs to be different in order for a parthenogenetically reproducing species to evolve sentience?
The answers might be useful to you, although they focus on the conditions in which asexual reproduction might dominate rather than on what the mechanics would be. As BlindKungFuMaster has said above there is no obvious reason why the humanoid shape, or sentience, should be a barrier to it occurring. It is rare but not unknown in large animals. It sometimes occurs in komodo dragons and turkeys.
Note that parthenogenesis can occur with or without mitosis, though I confess I don't really understand what "Parthenogenesis can occur without meiosis through mitotic oogenesis" means.
1$\begingroup$ Lost this seems more commentary and doesn't really address the questions being asked. I'd recommend reworking it a bit. $\endgroup$– JamesApr 14, 2016 at 14:00
Well, I'm assuming you mean like a bacteria, instead of making gametes through mitosis. The main problem would be, well, the complex part. The more complex you are, the more genetic information you need to make yourself, meaning higher risk of screw ups during the division process, a.k.a cancer, which is also why we don't regenerate lost limbs. However, if your humanoid has a way to never make such mistakes and could multiply its cells at a much faster rate, I'm assuming preparing would need to be the main part. Just like cells shut down their usual metabolism to grow in size and duplicate everything, I'd assume your being would enter a process like preparing for hibernation. After that, I'm unsure. Perhaps it could start making an external "sack" with a lot of stem cells, having it mature into adulthood at a fast pace, while remaining linked to it on the process, acting as the nutrient reserve part of an egg, but without the shell. Or perhaps it could enter a stage where it temporarily become Symbian twin, starting by slowly splitting its head in half like so: ! After that it would duplicate its spine and slowly continue the fission towards the end. But in both cases, I'd assume it enters a lethargic state, as mitosis is already a costly process for a single cell, let alone an entire organism. Hope it helped