Why are colonial organisms not considered multicellular

From single-cell to multi-cell

  • Unicellular organisms: Protozoa are organisms that only consist of a single functional cell. This term includes prokaryotes, but also protists, unicellular plants and fungi.
  • Multi-cell: Multicellular cells are organisms that consist of more than one functional cell, whereby specialization and differentiation lead to division of tasks and cooperation. This is associated with an increase in complexity.

Central question: How did single cells become our modern multi-cell?

There are several theories that have been more or less confirmed to answer this question. The most important aspects will be dealt with briefly below. From the point of view of evolutionary theory, there are basically three basic principles that have led to the development of multicellular organisms:

1) The increased cell nucleus division resulted in the formation of polykaryotic cells in a common cytoplasm.

2) Cell division without separation of the two daughter cells resulted in the formation of colonies.

3) The agglomeration of similar cells to form a colony initially resulted in multicellularity; from this later a multi-cell developed through differentiation and specialization.

The formation of a cell colony, that is, a grouping together of a number of identically differentiated single cells, initially grants a certain form of protection for each single cell; in the spirit of: “As a crowd, we are stronger!”. The classic example from today's world of the eukarya is Volvox, an alga.

Comparison of single-cell and multi-cell:

 

Unicellular organisms

Multi-cell

groups

Procyte: Bacteria, Archaea

Eucyte: Protozoa

No prokaryotes, only eukarya (animals, fungi, plants)

Cell count

1

> 1 to infinity

protection

Specialized cell organelles only

Effect as an organism> protection much greater!

Multiplication

Entire cell involved - division as an asexual form, gene transfer possible in different ways (sexual), e.g. conjugation between bacteria

Only a few specialized cells are involved

differentiation

-

+

specialization

+

+++

Cell types

1

>1

Extra chromosomal DNA

Sometimes present

Only in the mitochondrion or in the chloroplast

death

-

+

Apoptosis

-

+

Cell contacts

Conditionally

always

Extracellular matrix

Negative, but biofilm in prokaryotes may be comparable

+

Signal molecules

The entire cell reacts to it, but sometimes it does not exist

Special cells are addressed, an essential part of a functioning organism

Environmental conditions

Survival is possible under almost all conditions, special survival strategies have been developed, e.g. spore formation under heat, resistance present

Limited adaptability, fundamentally existing niche, resistance clearly decreases with increasing specialization

Examples

E. coli, Amoeba proteus, Paramecium spp.

Trichoplax adhaerens, Sponges, cnidarians, mammals, etc.

Today's hypotheses show different models:

1)    Gastraeal hypothesis by Ernst Haeckel: A simple, hollow, spherical cell colony, also known as Blastaea, was formed by the agglomeration of cells and developed in a further step through invagination into a two-layer, spherical cell colony. This then differentiated. A clear analogy to embryogenesis (blastula, gastrula) can be seen.

2)    Placula hypothesis according to Otto Bütschli: According to this hypothesis, multicellular cells emerged from a soil-living colony, which was differentiated from a lower, mobile and an upper part intended for food intake. Both parts later differentiated into two cell layers and through rearrangement led to multicellular cells. Nowadays this theory is mainly related to the smallest multi-cell, Trichoplax adhaerens.

3)    Synzoospore hypothesis according to Alexey Zakhvatkin: According to the hypothesis of this Russian biologist, the first stages would have been mobile, but not free-living organisms, but only a mobile larval form of a sessile cell colony. These would have arisen from initially unicellular swarming cells, so-called zoospores, which stuck to one another as a cell association.

If you look at all 3 theories in comparison, they initially appear logical and also connected to one another. Which theory will definitely prove to be correct remains to be seen. However, from a purely evolutionary theoretical point of view, it is to be expected that all 3 hypotheses are to be regarded as the path to multicellularity and that there is therefore no single solution for multicellular evolution.