Acetabularia
A giant unicellular green alga with a single nucleus.
Acetabularia is a genus of green algae belonging to the family Polyphysaceae, commonly found in subtropical waters. Despite being a single cell, it is enormous in size and intricately shaped, which makes it a valuable model for studying cell biology. A mature Acetabularia looks like the rounded leaves of a nasturtium, standing 4 to 10 centimeters tall, and consists of three parts: a rhizoid at the base that resembles short roots, a long central stalk, and an umbrella-like top made of branches that can fuse into a cap.
The name Acetabularia comes from the Latin *acetabulum*, a shallow cup used for dipping bread, because its upturned cap looks like such a cup. This is why it is sometimes called mermaid's wineglass. In the 19th century, the same name was proposed by George Edward Massee for a genus of fungi (now called *Cyphellopus*), but that usage is outdated and invalid since the algal name has priority.
Unlike other giant single-celled organisms that have many nuclei, members of this genus have just one nucleus, located in the rhizoid. This allows the cell to fully regenerate if its cap is removed. Caps from two different Acetabularia can also be swapped, even between species. If a piece of the stalk is removed without access to the nucleus in the rhizoid, that isolated stem piece will still grow a new cap.
In the 1930s–1950s, Joachim Hämmerling ran experiments showing that Acetabularia’s genetic information is stored in the nucleus. This was the first proof that genes are encoded by DNA in eukaryotes; earlier work by Oswald Avery and others had already shown this for prokaryotes.
Anatomically, Acetabularia is both unicellular and uninucleate. Its three main parts are the rhizoid (which holds the nucleus and anchors the cell to cracks in a surface), the stalk (which makes up most of its length), and the apex where the cap forms. Near the apex, there are usually several whorls of hair-like appendages. Acetabularia are among the largest single-celled organisms and have a remarkably large nucleus. During sexual reproduction, the nucleus undergoes many rounds of mitosis, producing many daughter nuclei all inside one nuclear membrane. These nuclei then undergo meiosis and are moved to the tips of the branches (the sporangia), where they are released as gametes.
Hämmerling’s experiments involved exchanging caps between two species: *A.
Quick Facts
- Taxon
- Acetabularia
Facts from the source article.
Lore & Background
The name Acetabularia derives from the Latin word acetabulum, a broad, shallow cup used for dipping bread; the upturned cap resembles such a cup, and it is sometimes called mermaid's wineglass. In the 19th century, the same designation was proposed by George Edward Massee for a genus of fungi (now Cyphellopus), but this usage is obsolete and considered invalid as the algal name takes precedence. Acetabularia, as well as being unicellular, is also a uninucleate organism, with its single nucleus located in the rhizoid. This allows the cell to regenerate completely if its cap is removed. The caps of two Acetabularia may also be exchanged, even from two different species. In addition, if a piece of the stem is removed with no access to the nucleus, this isolated stem piece will also grow a new cap.
Reader's Guide
In the 1930s–1950s, Joachim Hämmerling conducted experiments demonstrating that Acetabularia's genetic information is contained in the nucleus. This was the first demonstration that genes are encoded by DNA in eukaryotes; earlier studies by Oswald Avery and others had shown this for prokaryotes. Hämmerling exchanged caps between individuals from two species, A. mediterranea and A. crenulata. After the exchange, each transplanted cap gradually changed from its original form to the form typical for the species of the base it was now attached to, showing the nucleus controlled the cap form. In another experiment, inserting a nucleus from one species into an intact Acetabularia of a different species produced a hybrid cap with characteristics of both species. Acetabularia has also been used to study circadian rhythms; studies have shown it has a diurnal circadian rhythm maintained under constant conditions even with removal of the nucleus, though the nucleus is responsible for shifting cycles due to external changes. Acetabularia species occasionally enter the aquarium trade but are considered difficult to care for, being delicate, readily eaten by herbivorous fish, and growing slowly.
Did You Know?
- Acetabularia is a single-celled organism that can grow 4 to 10 centimeters tall.
- Its single nucleus is located in the rhizoid, allowing the cell to regenerate if the cap is removed.
- Joachim Hämmerling's experiments with Acetabularia provided the first demonstration that genes are encoded by DNA in eukaryotes.
- The name Acetabularia comes from the Latin word for a shallow cup used for dipping bread.
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