The Meaning of The Nucleus:
The Green Alga Acetabularia, a New Model Organism

The importance of the nucleus for reproduction, its influence on which traits are developed and the mutual influence that plasma and nucleus exert on each other had already been discovered during the thirties with the model organism Acetabularia, a single-celled green alga.

Acetabularia is a single-celled green alga.

It has some properties that render it advantageous for experimental studies:

	it is several centimeters long.
	three different sections, hat, stalk and rhizoid are easily distinguished.
	the nucleus stays always in a certain, easily recognizable region of the cell, the rhizoid.
	several species exist that differ for example in the shape of the hat like Acetabularia mediterranea (lives in the Mediterranean Sea) or Acetabularia crenulata (Caribbean Sea).
	the division of the nucleus and the production of germ cells occurs only after the cells have reached their full size.
	Acetabularia has a great ability to regenerate.

Scanning electron micrographs of the hat center (Corona superior, a, b) and of a rhizoid (c) - Acetabularia mediterranea (a, c), Acetabularia crenulata. (b). (S. BERGER, 1986)

These advantages were first recognized by J. HÄMMERLING during the 1930s. He studied at first the chances for the survival of the single parts of an Acetabularia cell and looked which of them could propagate and which not. He found that only the part containing the nucleus is able to perform this task and that the sections without nucleus could be kept alive for weeks but did after this period inevitably initiate an irreversible stop of their metabolism. The question of Acetabularia's ability to regenerate was answered by the following experiment:

HÄMMERLING took cells that were not fully grown and found that the upper part of the stem could regenerate a new hat, that the middle piece lacked this ability and that the lower nucleus-containing part could regenerate a whole cell capable of reproduction. If he isolated the nucleus and implanted it into the middle piece, it, too, would gain the ability to regenerate into a whole cell.

The following experiment shows which influence the nucleus has onto the shape of the hat: Acetabularia mediterranea and Acetabularia crenulata differ in this property. HÄMMERLING grafted the stalk of Acetabularia crenulata onto the rhizoid of Acetabularia mediterranea. The chimera developed into a cell with a hat shaped like that of Acetabularia mediterranea. A second experiment was done just the other way round: the rhizoid of Acetabularia crenulata was combined with the stalk of Acetabularia mediterranea. The developing hat had the shape of the Acetabularia crenulata hat.

What influence has the plasma onto the nucleus? A further experiment answers this question: An 'old' hat of Acetabularia mediterranea was grafted onto the rhizoid of a young plant of the same species; a cell that had just begun to develop a stalk. Without further delay, the nucleus did divide and germ cells were developed.

We can now conclude the following:

The nucleus is absolutely necessary for propagation.
The nucleus determines which properties develop.
The activity of the nucleus can be controlled by the plasma. Accordingly, there have to exist substances that diffuse from the nucleus into the plasma and others that diffuse from the plasma into the nucleus.

Many Problems Have Been Solved, But New Questions Arise

How Is the Genetic Information Realized?
Does the 1 Gene > 1 Enzyme Hypothesis Help to Explain the Development of an Organism?
How Are Extern Signals Processed?

Nearly everything that is to follow has to be regarded only as a partial answer to these questions. Biochemical, physico-chemical and molecular biological knowledge serves as a basis, it is thus of little use to go into details here. To understand the meaning of the statements right, it is helpful to think a little about the information- and system theory, to know what control and regulation is, which quantities make up a system, how they may be interconnected and how disruptive factors are dealt with. The discipline that deals with such questions is called cybernetics.