Acetabularia | |
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Scientific classification | |
(unranked): | Viridiplantae |
Division: | Chlorophyta |
Class: | Ulvophyceae |
Order: | Dasycladales |
Family: | Polyphysaceae |
Genus: | Acetabularia Lamouroux, 1812 [1] |
Species | |
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Acetabularia is a genus of green algae in the family Polyphysaceae. [4] Typically found in subtropical waters, Acetabularia is a single-celled organism, but gigantic in size and complex in form, making it an excellent model organism for studying cell biology. [5] In form, the mature Acetabularia resembles the round leaves of a nasturtium, is 4 to 10 centimetres (1.6 to 3.9 in) tall and has three anatomical parts: a bottom rhizoid that resembles a set of short roots; a long stalk in the middle; and a top umbrella of branches that may fuse into a cap. Unlike other giant unicellular organisms, which are multinucleate, members of this genus possess a single nucleus located in the rhizoid, which 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 in the rhizoid, this isolated stem piece will also grow a new cap. [6]
In the 1930s–1950s Joachim Hämmerling conducted experiments in which he demonstrated Acetabularia's genetic information is contained in the nucleus. [7] This was the first demonstration that genes are encoded by DNA in eukaryotes; earlier studies by Oswald Avery and others had shown that this was true for prokaryotes.
The name, Acetabularia, derives from the Latin word acetabulum , a broad, shallow cup used for dipping bread; the upturned cap of Acetabularia resembles such a cup. For this reason, it is also sometimes called mermaid's wineglass. [8]
In the 19th century, the same designation Acetabularia was proposed by George Edward Massee for a genus of fungi (now Cyphellopus ), but this usage is obsolete and considered invalid as the algal name takes precedence. [9]
Acetabularia, as well as being unicellular, is also a uninucleate organism. It has three basic parts: its rhizoid, a short set of root-like appendages that contain the nucleus and anchor the cell to fissures in a substrate; its median stalk, which accounts for most of its length; and its apex, where its cap forms. There are usually several whorls of hair-like appendages close to the apex.[ citation needed ]
Acetabularia are among the largest single-celled organisms, having also a remarkably large nucleus. During sexual reproduction, the nucleus undergoes multiple rounds of mitosis, forming many daughter nuclei all within one nuclear membrane. These nuclei undergo meiosis and are transported to the tips of the branches, the sporangia, where they are released as gametes. [10]
Each Acetabularia cell is composed of three segments: the "foot" or basal segment which contains the nucleus, the "stalk", and the "cap". Hämmerling exchanged caps between individuals from two species, A. mediterranea and A. crenulata. A. mediterranea has a smooth, disc-shaped cap, while A. crenulata has a branched, flower-like cap.
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. This showed that the nucleus controlled the form of the cap.
In another experiment, Hämmerling inserted a nucleus from one species of Acetabularia into an intact Acetabularia of a different species. The Acetabularia then produced a hybrid cap with characteristics of both species. This showed that both nuclei influenced the form of the cap. [7] Hammerling's results showed that the nucleus of a cell contains the genetic information that directs cellular development.
Although a single cell, Acetabularia exhibits a remarkably complex shape and has therefore long been a model organism for studying gene expression and morphogenesis. It seems to transport messenger RNA molecules (in an inactive riboprotein form) from the nucleus to its apical tips, where they are translated into proteins. These molecules may be activated by proteolysis of their protein carrier molecules, but this has not been verified as yet.[ citation needed ]
In addition to its gradient in specific mRNA molecules, Acetabularia exhibits concentration gradients in several types of molecules, such as ascorbic acid.[ citation needed ]
Acetabularia has been used to study circadian rhythms. [11] Studies have shown Acetabularia has a diurnal circadian rhythm. [12] These rhythmic changes in respiratory and photosynthetic activity are maintained under constant conditions, even with the removal of the nucleus, showing the regulation of the rhythm is independent of the nucleus. [13] However, the nucleus isn't completely uninvolved, as it is responsible for the shifting of the cycles due to external changes. In one experiment a nucleus from a specimen trained on one circadian rhythm was transplanted into a de-nucleated plant on a rhythm that differed by 12 hours, over a period of days the donated nucleus changed the circadian rhythm of the receiving organism to that of the donor organism. [13]
Acetabularia species occasionally make their way into the aquarium trade. They are generally considered to be more difficult or unappealing macroalgae to care for in the reef aquarium, a fish-only, or a FOWLR (Fish Only With Live Rock) system, as they are delicate, readily eaten by herbivorous fish, grow slowly, and do not have the high nutrient uptake that reef aquarium refugium species (such as Chaetomorpha and Caulerpa ) do. However, they are suitable for a macroalgae display tank, and thus macroalgae suppliers often carry species of Acetabularia.
The dikaryon is a nuclear feature that is unique to certain fungi. Compatible cell-types can fuse cytoplasms (plasmogamy). When this occurs, the two nuclei of two cells pair off and cohabit without fusing (karyogamy). This can be maintained for all the cells of the hyphae by synchronously dividing so that pairs are passed to newer cells. In the Ascomycota this attribute is most often found in the ascogenous hyphae and ascocarp while the bulk of the mycelium remains monokaryotic. In the Basidiomycota this is the dominant phase, with most Basidiomycota monokaryons weakly growing and short-lived.
Astrephomene is a genus of green algae in the family Goniaceae, order Chlamydomonadales. The genus was first described in 1937 by Pocock and named by Pockock in 1953.
Crustomastix is a genus of green algae in the class Mamiellophyceae.
Picocystis is a monotypic genus of green algae, the sole species is Picocystis salinarum. It is placed within its own class, Picocystophyceae in the division Chlorophyta.
Trebouxia is a unicellular green alga. It is a photosynthetic organism that can exist in almost all habitats found in polar, tropical, and temperate regions. It can either exist in a symbiotic relationship with fungi in the form of lichen or it can survive independently as a free-living organism alone or in colonies. Trebouxia is the most common photobiont in extant lichens. It is a primary producer of marine, freshwater and terrestrial ecosystems. It uses carotenoids and chlorophyll a and b to harvest energy from the sun and provide nutrients to various animals and insects.
Udotea is a genus of green algae in the family Udoteaceae.
Conceptacles are specialized cavities of marine and freshwater algae that contain the reproductive organs. They are situated in the receptacle and open by a small ostiole. Conceptacles are present in Corallinaceae, and Hildenbrandiales, as well as the brown Fucales. In the Fucales there is no haploid phase in the reproductive cycle and therefore no alternation of generations. The thallus is a sporophyte. The diploid plants produce male (antheridia) and female (oogonia) gametangia by meiosis. The gametes are released into the surrounding water; after fusion, the zygote settles and begins growth.
Korshikoviella is a genus of green algae in the family Characiaceae.
Bolidophyceae is a class of photosynthetic heterokont picophytoplankton, and consist of less than 20 known species. They are distinguished by the angle of flagellar insertion and swimming patterns as well as recent molecular analyses. Bolidophyceae is the sister taxon to the diatoms (Bacillariophyceae). They lack the characteristic theca of the diatoms, and have been proposed as an intermediate group between the diatoms and all other heterokonts.
Pediastrum duplex is a species of fresh water green algae in the genus Pediastrum. It is the type species of the genus Pediastrum.
Dr. Joachim Hämmerling was a pioneering Danish-German biologist funded by Nazi Germany who determined that the nucleus of a cell controls the development of organisms. His experimentation with the green algae Acetabularia provided a model subject for modern cell biological research, and proved the existence of morphogenetic substances, or mRNP.
Hildenbrandia is a genus of thalloid red alga comprising about 26 species. The slow-growing, non-mineralized thalli take a crustose form. Hildenbrandia reproduces by means of conceptacles and produces tetraspores.
The epithallium or epithallus is the outer layer of a crustose coralline alga, which in some species is periodically shed to prevent organisms from attaching to and overgrowing the alga.
Acetabularia acetabulum is a species of green alga in the family Polyphysaceae. It is found in the Mediterranean Sea at a depth of one to two metres.
Batrachospermaceae is a family of fresh water red algae (Rhodophyta). Genera within the Batrachospermaceae generally have a "Lemanea-type" life history with carpospores germinating to produce chantransia. Sporophyte phase with meiosis occurs in an apical cell to produce the gametophyte stage. Pit connections have two pit plug cap layers with the other layer enlarged. This family of freshwater red algae is uniaxial, meaning each filament with a single apical cell. The genera included within Batrachospermaceae are listed in the table below.
Pyrenomonadaceae is a family of cryptomonads which includes three or four known genera. They are distinguished from other cryptomonads by their nucleomorphs being imbedded into the pyrenoid, and the presence of distinctive pigment phycoerythrin 545.
Greta Albrecht Fryxell was a marine scientist known for her work on the biology and taxonomy of diatoms. In 1996, she was elected a fellow of the American Association for the Advancement of Science.
Linda Karen Medlin is a molecular biologist known for her work on diatoms. She is an elected member of the Norwegian Academy of Science and Letters.
Hans-Georg Schweiger was a German cell biologist and a former director of the Max Planck Institute for Cell Biology.
Kathleen "Kay" Margaret Cole was a Canadian phycologist, known as one of the world's leading experts in the cytology of marine algae. In 1998 the Canadian Botanical Society awarded her the George Lawson Medal for lifetime achievement.