Theca (single cell)

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A theca (also theka ) refers to single cells ( protists and single-celled algae) as a form of solid cover which, unlike a cell wall , usually does not completely cover the cell. This can be intracellular, i. H. sit beneath the cell membrane or be deposited extracellularly to the outside and enclose the cell. The different shells, called theca, in different taxa are completely different in their fine structure and also not homologous to one another, the expression only indicates a vague morphological similarity. Many researchers combine all forms of extracellular envelopes, including the theca of the prasinophytes, to form a "Type III". The shell-like shell of the Thecamoeben is occasionally called Theca.

Theca of the Choanomonada

In the case of the frilled flagellates (Choanomonada or Choanoflagellata), a theca is characteristic of some taxa from the order Craspedida . The other order, Acanthoecida , is characterized by a completely different shell structure made of silicate, which is called Lorica . The theca is an extracellular, thin, cup-like cover made of organic material, with a cup, bottle or funnel-like shape, which is usually anchored to the substrate with a long stem. The upper opening with the flagellum can be framed in the shape of a long collar, sometimes resulting in complex, multi-part shapes. There are also free-floating forms without a stem and with a short collar. A study on the phylogeny of the Craspedida has shown, however, that the shape of the Theca cannot be used to establish relationships. Species with a flexible, organic extracellular matrix that is not rigid and only forms a thin layer are referred to as “glycocalyx” instead. The theca, as a rigid, tightly fitting envelope, does not allow cells to divide in this form. In order to make this possible, the cell has to detach itself from it and partially move out in an "amoeboid" manner; after the division, one daughter cell returns to the theca, the other swims away. The theca is made up of fibers (microfibrils) which at least partially consist of carbohydrates.

Theca of the prasinophytes

The Prasinophytae form a (non- monophyletic ) group within the green algae, in which unicellular, planktonic algae species ( phytoplankton ) are combined. The genera Tetraselmis and Scherffelia (order Chlorodendrales), which are provided with four flagella, have a cell-wall-like extracellular envelope, which is also called theca. This is composed of individual, scale-like elements, which are each deposited individually; in the other prasinophytes such scales are also present, but do not fuse to form an envelope. The theca consists of several fused layers of such scales, and often it also has hair on top. The theca consists mainly of carbohydrates with an extremely low protein content and is not mineralized. The polysaccharides are made up of unusual ketoses .

Theca of the Dinoflagellates

Among the armored flagellants or dinoflagellates , theca is most often spoken of. The theca of the dinoflagellates is, unlike the forms discussed so far, intracellular. Under the cell membrane, a system of flat, plate-shaped vesicles (or alveoli) is formed, which form stable thecal plates in their interior, mostly made of cellulose . The outer layer of the cell plasma of the dinoflagellates is traditionally called the amphiesma, the alveoli with the thecal plates are a characteristic part of it. However, some authors have used the term amphiesma synonymously with Theca.

During cell division, in most groups the thecal plates are divided among the daughter cells. Alternatively, some genera shed their shell completely under adverse environmental conditions (such as the moulting of the arthropods called ecdysis) and instead form a permanent cyst with a shell consisting predominantly of proteins, which is called pellicula (the expression is, however, ambiguous and similar to Theca, used in different senses in different organisms). New thecal plates are then formed again later, under more favorable environmental conditions, after the pellicle has been stripped off. The shape and the pattern of the arrangement of the individual plates (which is pre-determined by the arrangement of the vesicles) are important features in the identification of the species of the dinoflagellates. There is a tendency for the groups with particularly thick panels to decrease in number in parallel. Thick plates often have rib-like structured surfaces or thorn-shaped projections. However, some genera do not have theca (they are athecat). The dinoflagellate cell is divided into two halves by a transverse furrow called the cingulum. The part of the theca that lies in front of the cingulum is called the epitheca, the part behind it is called the hypotheca (note, however, the elements with the same name, but completely differently formed, in the diatoms, cf. in the next section).

Thecae among the diatoms

In diatoms or diatoms (Diatomea or Bacillariophyta), the cell is enclosed in a complex cell envelope called Frustel, which is mainly made up of amorphous silicon dioxide . This usually consists of two halves, two parts that are not connected to each other, which fit into each other like a box with their lid. The somewhat smaller half-shell is called Hypotheca, the one that encloses and covers it on the sides is called Epitheca. Each of the two thecae usually has a complex shape with processes and complicated patterns of pores of different sizes, which are divided by belt-like zones (cingulum) with simpler pores. There is an external pleural ligament in the overlap area of ​​the thecae. In normal cell division, each of the daughter cells inherits a valve, which in both cases forms the epitheca of the new envelope. As a result, with each division an equally large and a slightly smaller individual is created, which means that the cell size continues to decrease on average over time. This development is stopped by activated phases with sexual reproduction, in which the gametes completely shed the shell, which is later formed anew in its old size. The theca is separated by special enzymes called silaffins.

Individual evidence

  1. ↑ a b H.R. Preisig, OR Anderson, JO Corliss, Ø. Meestrup, MJ Powell, RW Robertson, R. Wetherbee: Terminology and nomenclature of protist cell surface structures. Protoplasm 181: 1-28.
  2. ↑ Serguei A. Karpov: Flagellate phylogeny, an ultrastructural approach. In Barry SCLeadbeater & JCGreen (editors): The Flagellates, Unity, diversity and evolution. Taylor & Francis, London and New York 2000. ISBN 0-7484-0914-9 .
  3. ^ Burkhard Becker: The cell surface of flagellates. In Barry SCLeadbeater & JCGreen (editors): The Flagellates, Unity, diversity and evolution. Taylor & Francis, London and New York 2000. ISBN 0-7484-0914-9 .
  4. ↑ Alexandra Jeuck, Hartmut Arndt, Frank Nitsche (2014): Extended phylogeny of the Craspedida (Choanomonada). European Journal of Protistology 50 (4): 430-443. doi: 10.1016 / j.ejop.2014.06.001
  5. ↑ Barry SC Leadbeater: The Choanoflagellates, Evolution, Biology and Ecology. Cambridge University Press, 2015. 350 pages. ISBN 978-0-521-88444-0 .
  6. ↑ B. Becker, B. Marin, M. Melkonian (1994): Structure, composition, and biogenesis of prasinophyte cell coverings. Protoplasm 181 (1-4): 233-244.
  7. ^ Robert Edward Lee: Phycology. Cambridge University Press, 3rd edition 1999. ISBN 0-521-63090-8 . Chapter 7: Dinophyta.
  8. ↑ John D. Dodge: The fine structure of Algal Cells. Academic Press, London and New York 1973. ISBN 0-12-219150-1 . v. a. Pages 25–30.
  9. ^ Nils Kröger, Nicole Poulsen (2008): Diatoms — From Cell Wall Biogenesis to Nanotechnology. Annual Review of Genetics 42: 83-107. doi: 10.1146 / annurev.genet.41.110306.130109