To start off -I only made an account today so dont shoot me if i haven't got all the guidelines quite right (but let me know). I am not a scientist and happy to admit my knowledge is limited on this so if you see something that is wrong id love to know about it especially if you have reference material. Please take everything i say about a-sexual reproduction with a grain of salt as it was very hard to find any material about how that occurs at a cellular level and with the several different types of a-sexual reproduction i feel i have made a biiiig generalization on that side of the reproduction field. Also the following info might not all specifically answer the question at hand but i think is all information that is helpful to the understanding
Saw this and bugged me that as a science teacher i didn't know so after a fair amount of research i found this website which was incredibly helpful;
www.the-scientist.com/?articles.view/articleNo/30827/title/From-Simple-To-Complex/2nd video was especially insightful thou heads up its about reproduction and not pg13
It states that the transition between single cells and multicellular organisms is still not fully understood and being researched
In my clear understanding with heavy support by the site;
-multi cellular organisms and heavily reliant on chemical messaging between cells and that signalling starts while the organism(s) are still single cellular
-a huge priority in multi cellular organisms is to have specialized germ cells (Cells that produce gametes or offspring not make the organism sick haha) because it reduces the incentive for cells to take advantage of the organism and beat out the other cells like a cancer or that lazy kid in your group that rocks up and gets an A+ for a project that he hasn't helped
- germ cells do not have to be present from birth and can be made from somatic cells later on in the case of plants (found this on the wiki sited later in the post)
- multi cellular organisms tend to be the same base cell (ie same DNA) that but different groups of cells have been signaled via the chemical messaging system to specialize into different roles hence why a germ cell can reproduce a whole complicated multi cellular being
In my FOGGY understanding from what i read from the site
- Reproductive germ cells specialization occurs very early in the evolutionary transition from single celled organism to multi celled organism
Germ cells work in the following way;
- they reproduce themselves via mitosis (generating a whole duplicate of a cell with all the DNA of a somatic cell or normal body cell)
- If the organism is a-sexual these mitotic duplicates start dividing into their own organism and when the new organisim is mature enough and has enough cells to survive the two divide (and i assume that verry early stage of multi cellular evolution this is what happens)
- OR for sexual reproduction a portion of the germ cells go through meiosis to generate haploid cells or gametes or egg and sperm which are held in an ovum or laid to to await fertilization
- also note that one does not exclude the other as some some animals like the copperhead snake can do both asexual or sexual reproduction with the female either being inseminated by a male or creating a "virgin birth" by inseminating itself as it produces both egg and sperm gametes (another assumption i made is that to go from a-sexual to sexual there would probably be a point of overlap)
a couple articles supporting the germ cells work this way section and what they show
www.ncbi.nlm.nih.gov/pubmed/26134407 - asexual fissioning occurs using gonads (germ cells)
www.ncbi.nlm.nih.gov/books/NBK21049/ - asexual budding and hydra reproduction relies on mitosis only not meiosis
en.wikipedia.org/wiki/Germ_cell - general info on germ cells
education.seattlepi.com/five-examples-organisms-use-asexual-reproduction-5849.html - 5 different versions of multi cellular reproduction with examples (where i got the snake virgin birth from)
I think (with probably little or no evidence feel free to completely disregard)
- the change from asexual to sexual is purely dependent on the capability of the organism in that it must be able to regularly be able to use the resources to locate a second member of its race with the opposite gamete type and preform whatever mating ritual is required
- im not sure if 2 cells would be enough for a multi-cellular organism so much as two of the same cell that happen to be helping each other out due to the aforementioned signalling system. I think that for it to reach single organism status that role specialization to occur in which case for a 2 celled one would have to be the germ cell to produce eggs (or/and in the case of sexual reproduction a DNA fertilization system for eggs) and the other would have to be able to take care of every other need of the organism including taking care of the other cells requirements.