Spiralia

from Wikipedia, the free encyclopedia
Spiralia

Obsolete systematic group

The taxon dealt with here is not part of the systematics presented in the German-language Wikipedia. More information can be found in the article text.

Spiral furrows in the Trochus snail.

Spiral furrows in the Trochus snail .

Systematics
Domain : Eukaryotes (Eucaryota)
without rank: Multicellular animals (Metazoa)
without rank: Tissue animals (Eumetazoa)
without rank: Bilateria
Paraphyletic taxon :
without rank: Spiralia
Scientific name
Spiralia
Schleip , 1929

A number of tribes of the Bilateria (bilateral animals) are summarized as Spiralia ( spiral furrows ) , which primarily have a special type of furrow , the spiral furrow. With spiral furrowing, four new cells are formed in a spindle shape at an angle to the axis of the egg.

Spiral grooves

The spiral furrow is characterized by the fact that the spindle apparatus is inclined to the animal-vegetative axis of the egg cell and the developing embryo during the cell division. This orientation of the egg cell results from the position of the cell nucleus relative to the yolk and, mostly, from the position of the polar bodies , whereby an animal and an opposite, vegetative pole of the egg cell can be distinguished. The orientation of the spindle apparatus changes from cell division to cell division, as a result of which the tied off daughter cells are arranged to the left or to the right, clockwise or opposite to it. When looking at the embryo in orientation towards the animal pole, a spiral arrangement of the planes of division results after the first five divisions, to which the name refers.

The cells are marked in the spiral groove according to a standardized notation that goes back to the biologist Edwin Grant Conklin . The descendants of the respective division step are marked with numbers (1,2,3,4 ...) and the respective cells with letters. The first division produces two cells (AB and CD), the second four (A, B, C, D). In the later, unequal (inequitable) divisions, the large cells (macromers) are designated with capital letters, the smaller cells (micromers) with small letters. 4d would accordingly be one of the two daughter cells that emerged in the fourth division step from the macromeric 3D that resulted in the third division step.

The fertilized egg cell first divides meridionally twice , the dividing furrows running from one pole to the other. This results in four cells lying in one plane (phase A of the taxo image). The second division creates four cells, A, B, C, D. The third division takes place equatorially. This creates two levels with four cells each, four larger blastomeres , the macromers (1A - 1D) and four smaller cells, the micromers (1a - 1d) (phase B of the taxo image). The micromers are each located in the furrows between two macromers and are rotated by 45 ° with respect to these. The four macromers divide again and form a second group of four micromers (phase C of the taxo image). The micromers also divide, so that the egg cell consists of 16 cells after four divisions (phase D of the taxo image).

The importance of the spiral furrow goes far beyond this arrangement of the planes of division, which is initially quite unimportant. Every cell of the embryo (which can be described according to the notation outlined in the previous paragraph) is cell-constant; its descendants form clearly defined tissues in the developing embryo. The back side (dorsal) of the embryo almost always goes back from the descendants of cell D. The macromers ultimately form the endoderm , while the micromers predominantly form the ectoderm . All cells of the mesoderm are descendants of cell 4d. Cell 2d, called the somatoblast, almost alone forms the entire outer ectoderm of the developing larva.

For this, some of the annelids and molluscs belonging model organisms belonging to species already end of the 19th century discovered the basic model concepts exist in many taxa variations that can but traced back to the same basic scheme. In some groups the first divisions are already unequal, in some the macromers and micromers are of the same size (very rarely even the micromers can be larger). In these cases, however, a clear homology of the cells of the embryo can be identified, so that the identity, position and fate of the individual cells in species belonging to different animal phyla can often be traced.

The spiral furrow as a taxonomic feature

The complicated, unique relationships involved in spiral furrowing make it extremely unlikely that this complicated apparatus could possibly have developed convergent to one another in several animal phyla . Therefore, shortly after the discovery, the idea arose that all forms with spiral furrows must be related to one another during embryonic development. They were first formally combined into a taxon called Spiralia in 1929 by the biologist Waldemar Schleip . The spiral furrow appeared to be a prime example of a feature developed early in tribal history and then conserved because of its fundamental importance in development.

The problem with the taxon Spiralia was that there are clearly recognizable cases in which the spiral division must obviously have been given up secondary. The cephalopods , whose membership of the molluscs is evident from numerous other characteristics, do not have a spiral division. Numerous exceptions are now also known for another of the model groups in which the phenomenon was discovered, the polychaeta or polychaeta. It must therefore be expected that it has also been transformed or completely regressed in other groups in which the connection is not so clear. Comparative developmental biologists found individual groups in other animal phyla that show a pattern of division that can be interpreted as deviating or altered spiral furrows. So embryonic that was barnacles long time interpreted as modified Spiralfurchung what constitutes support for the (now discontinued) Taxon Arthropods (Articulata) was evaluated.

Spiralia as a taxon - different approaches

It became clear early on that spiral furrowing is obviously restricted to the protostomia (or "primordial mouth"), while it never occurs in the other large group, the deuterostomia or "new mouths". For a time, spiral furrowing was considered by some taxonomists to be the autapomorphism of the protostomia, so it would have been pronounced in their ancestral form and, according to the theory, have been lost secondarily to all protostomies with other types of development. Later it turned out that all examples of spiral furrowing in one of the large groups of protostomia later recognized, the Ecdysozoa or "molting animals ", go back to misinterpretations. Spiral furrowing now seemed to be a convincing feature for another large group, the Lophotrochozoa . This interpretation was represented by the evolutionary biologist Gonzalo Giribet . But doubts also arose about this assignment. Some scientists considered their absence in the armpods (Brachiopoda) and horseshoe worms (Phoronida) (two small animal strains living in the sea) as primary, so that the Spiralia would now unite the other Lophotrochozoa.

Systematists working according to the methods of phylogenomics , who examine the relationship of the animal phyla and large groups of the animal kingdom by comparing homologous DNA sequences, have named numerous other groups of different sizes and compositions as "Spiralia" in the decades from around 2000 onwards (whereby The name-giving feature of the spiral furrow now essentially only serves as a label). These groups normally include all groups with classic spiral furrows, but also different, and each different, strains without this characteristic (or those that have not yet been examined at all). Many of these groups are synonymous with other names. Many, including the biologist Kenneth M. Halanych, therefore plead for avoiding the name Spiralia for a taxon entirely because of this confusion and the risk of misleading.

Sub-taxa

The following taxa clearly show a spiral furrow, so that their membership of the Spiralia is certain:

Other taxa have a spiral furrow, which is not yet certain whether it is homologous to that of the annelids and molluscs :

There is no spiral furrow in the arthropods . They were considered to be relatives of the annelids (concept of arthropods (Articulata)). The spiral furrowing of the ice in some crustaceans ( water fleas ("Cladocera"), barnacles (Cirripedia) and luminous shrimp (Euphausiacea)) is probably a secondary development.

In the horseshoe worms (Phoronida) a spiral furrow takes place, in which the cells are not offset by 45 °. Here, too, a secondary origin is assumed.

literature

  • Andreas Hejnol: A Twist in Time — The Evolution of Spiral Cleavage in the Light of Animal Phylogeny. In: Integrative and Comparative Biology. Volume 50, No. 5, 2010, pp. 695-706. doi: 10.1093 / icb / icq103
  • W. Westheide, R. Rieger: Special Zoology. Part 1: Protozoa and invertebrates. 2nd Edition. Spektrum Akademischer Verlag, Heidelberg 2007, ISBN 978-3-8274-1575-2 , pp. 203-208.
  • Donald P. Costello, Catherine Henley: Spiralian Development: A Perspective. In: American Zoologist. Volume 16, No. 3, 1976, pp. 277-291.
  • Claus Nielsen: Some aspects of spiral development. In: Acta Zoologica. (Stockholm). Volume 91, 2010, pp. 20-28. doi: 10.1111 / j.1463-6395.2009.00421.x

Individual evidence

  1. ↑ Elaine C. Seaver: Variation in spiral development: insights from polychaetes. In: International Journal of Developmental Biology. Volume 58, No. 6-7-8, 2014, pp. 457-467. doi: 10.1387 / ijdb.140154es
  2. ↑ Kenneth M. Halanych: How our view of animal phylogeny was reshaped by molecular approaches: lessons learned. In: Organisms Diversity and Evolution. Volume 16, No. 2, 2015, pp. 319–328. doi: 10.1007 / s13127-016-0264-8