I will make a separate comment here, as I want to summarise theory on one specific topic (that is one of the earlier goals on the roadmap for this year): sexual reproduction / reproduction types in general.
Since this is all fairly well-described stuff (and this forum’s audience is less demanding than peer review), I will just be linking to relevant Wikipedia pages without going through the effort of referencing specific journal articles except if I feel like it.
Sexual Reproduction
The term “sexual reproduction” itself simply refers to two gamete cells (potentially from different organisms) fusing to produce a zygote that in this way ends up with double the chromosomes. This requires a meiosis (Mitosis without the DNA duplicating) event at some point to balance things out. Most typically for animals, normal diploid cells undergo meiosis to produce haploid (one copy of each chromosome) gametes, which then must merge merge to produce a new diploid (two copies of each chromosome) zygote, which then continues to divide normally. But there are all sorts of exceptions to this, for example species that spend most of their lifecycles in the haploid form. As always with Thrive, I believe different cases are fun, but less important than handling one case well.
Sexual reproduction itself is very much not a multicellular-only invention. It is very common in Eukaryotes, including single-celled ones. You don’t need a multicellular structure in order to split a cell into gametes and then merge with others. In fact, the last common ancestor of all eukaryotes (the LECA) almost certainly had the whole system. (So only unlocking it in the Multicellular Stage is definitely going on the Thrive Sins!)
As for why it evolved, that’s a lot more hotly debated. There’s obvious suggestions of letting beneficial changes spread more easily (translating to faster evolution), but this is not really relevant to individuals or on short timescales. But the rapid-remixing of genes may also be beneficial on shorter timescales, such as for avoiding/repairing damaged functions, improving disease resistance, or rapid adjustment to changing (climatic) environment. DNA damage and diseases are not something we really model in Thrive at the moment, so better resisting them would just lead to overall better performance of the organism. There are even hypotheses that the evolution of sexual reproduction is primarily beneficial for the genes causing it, but that is far off from our organism-(and species)focused approach.
One final topic here is Isogamy versus Anisogamy: Whether the gametes look the same in size and function, or whether there is a difference between them. (animals have an extreme form of anisogamy known as Oogamy. Here are your typical egg and sperm cells!) The vast, vast majority of single-celled eukaryotes are isogamous, and that is assumed to be the “original” version. Quite unlike sexual reproduction in general, anisogamy evolved multiple times independently, and (almost) exclusively in multicellular organisms. (The exceptions I alluded to appear to be in that “colonial” grey area between single-celled and complex multicellularity). Anisogamy is universal in animals and true land plants, but some fungi and multicellular algae are isogamous.
Reproduction types
With the sexual question out of the way, here’s an overview of common “reproduction types” in microscopic multicellular organisms:
- Budding: This word can mean many different things for many different lifeforms, but indeed also applies to the type of organism that we’re talking about here. In the case of small multicellular organisms, “budding” means that there is an outgrowth on the colony that then breaks off, becoming an independent colony.
Note that this generally refers to outgrowths that reach a significant size and number of cells before they “break off”, which is quite different from the single cell break-off currently labelled “budding” in the Thrive Multicellular Stage. - Fragmentation: A piece of the original organism breaks off, before growing into a new individual. This can be a small or large fragment breaking off a still larger colony that remains as-is, or it can be one colony “falling apart” into many pieces. Since they don’t stay attached to the parent for a while until they are more developed, this might be a better analogy for current Thrive Multicellular reproduction (assuming a very small fragment breaking off).
This becomes more difficult to do properly (and requires larger fragments), the more complex the organism in question is. Because your chopped off pinky is not very effective at fending for itself, even if it had the ability to re-grow the rest of you. - Sporogenesis: Speaking honestly, “spore” as a term is overly broad and poorly defined, covering a lot of different things among different species. The best I can do for you is “an independent strucure that can be dispersed to then form a new individual somewhere else”. They might be resistant to environmental conditions while dormant. They’re also usually single cells (again, poorly defined term). Can be formed asexually or sexually (but again, we’re talking about very different processes and resulting “spores”) Meaning, current Thrive Multicellular reproduction is a spore also.
- Isogamy: as mentioned before, simply the fusing of two outwardly identical (might still have genetic opposites) gametes to form a zygote, that then becomes a new full organism. Can take the form of gametes simply being released freely into the water, but also can happen between two cells that are part of their “host” organism (though only one becomes a zygote, of course), in fungi or some multicellular algae for example. What does appear to be the case is that resulting zygotes are similar in size to normal cells, so the new organism has to grow from square 1.
- Anisogamy: As said before, gametes of different size and form. The smaller gametes are often (but not always) motile and released into the water. The larger non-motile gametes are either also released into the water, or kept inside the parent organism. The motile gametes seek out the non-motile gametes, which might require them seeking out the organism that is still holding them. Very importantly, the non-motile gamete can be very large, giving the zygote a substantial head start in making a full new organism. Of course, in slightly more advanced animals, the smaller gametes can be introduced directly into the body containing the larger gametes through copulation.
Remaining observations:
- I’ve been noting all these reproduction types separately, but the complicated reality is that many species can reproduce via multiple methods, often both sexually and asexually.
- Even on the microscopic scale, many organisms have specialised cell types or “organs” dedicated to making offspring, and this can be both for producing gametes for sexual reproduction internally, including full-on eggs held inside the body, but also asexual spore-forming locations or even specific spots where new budding takes place.