What evolutionary advantages would limiting cells per organism give?

Question

My previous question was how to classify a domain (now superdomain) of duocellular life named Duotorusa.

Anyone who has taken high school biology know that all life on earth are divided into three different domains: Bacteria, Archaea, and Eukaryota. On a planet in the outer habitable zone of a K-type star with approximately 1.5 Earth oceans of water, there is another domain: Duotorusa.

Given the nice color-coding of the scanning-microscope Duotorusa unit (just kidding, this is an image borrowed from TeX.SE), I will call the cell units red and blue respectively. These are my current ideas to their functions, those can be changed in the answers if necessary to increase the evolutionary advantage.

The red cell helps power the organism. Propulsion (cilia or flagella) on the red cell moves the organism, and it also provides (photosynthesizes/finds) food, which it eats. It is a prokaryote, which only carries the DNA/RNA necessary for finding/obtaining food and moving based on where the food is.

The blue cell is the reproductive center of the organism. It has a strong, cellulose backbone that provides structure to the unit, as well as a nucleus that stores DNA necessary for reproduction, regeneration of the red unit, which is expendable when harsh conditions occur and the blue cell must hibernate (oh yeah, it can do that too). The blue cell, when conditions allow (lots of food/space), produces a new Duotorusa blue unit which it expels with some cytotoxic waste (just to make sure the 'child' is safe).

My question is, what makes a Duotorusa with one red unit more advantageous than one with like, 100 units, or one that can have a near-infinite amount of red units? Why do only two units have to stick together?

Answer 1

Everyone's favourite indestructible critters, the tardigrades, are known as eutelic. Once they reach maturity they have a fixed number of cells. Any extra growth? It's just the cells getting bigger, not the cells replicating.

The biggest advantage I can think of is an immunity to cancers /cell division problems. Perhaps your organisms exist in an environment with high radiation or lots of potentially dangerous 'virus-like' strands of DNA, something to make cell division an extremely dangerous proposition.

In such an environment the best way to survive is to replicate cells no more than absolutely necessary. If your two celled critters can breed but then grow no larger it minimises the potential risk.

A secondary advantage might be tighter coupling of cell functions. As in programming, it's more efficient to streamline processes if you know you have one instance and one instance alone of a thing. If the blue cell knows it's only purpose is to build the red cells and copies of itself and the red cell knows it's only going to have to feed one blue cell then the two can form a tighter, potentially more efficient link. This is stretching a bit though, as the logical conclusion of that process is both becoming one unit.

The third advantage I can think of is efficiency in breeding. Your creature doesn't have to contain the blueprints for making blue cells that attach to blue cells, only blue cells on their own. Once a blue cell is copied that's it: no further integration needs to take place. The new blue cell knows it needs to make a red cell, and that's where the process ends. No complex control mechanisms are needed for stopping cell replication in the individual animal.

In the end it all comes down to balance: You have to balance the reasons for not having more cells against the reasons for having cells. It may be the best version is one blue with four red, or a yellow that makes two blues that each have two reds, but if replication is a concern, your creature is evolved for efficient cell linkage, and it's really simple, then you have reasons to not grow by replicating.

Answer 2

There are organisms with a relatively small and fixed number of cells, but in any case such as this where there are only two cells it seems more practical for the cells to eventually merge into one. The main advantage to multicellularity - the ability to increase in size without limiting food intake relative to volume - does not outweigh the costs when the difference in size is this small.

Unless: The organism is not born like this. Instead, what you're looking at is an interesting form of sexual reproduction, where red and blue cells are born separately and live independently for a while, but join together into a single organism when mature. The blue cell demonstrates "female-like" properties in that it contains the physical apparatus for reproduction, while the red cell is "male-like" and must provide the blue cell with energy in order to reproduce.

Now the advantages reflect those of other forms of sexual reproduction: genetic variation and the ability to choose a mate with good genes.

Monogamy is rare among species this simple, but since the cells physically merge perhaps it can be promoted by the fact that one of the cells is primarily responsible for motion. Multiple red cells may find it more difficult to coordinate, limiting the usefulness of joining with more than one partner.