What is the origin of biological symmetry

symmetry

symmetry, the ordered repetition of the same structural elements. It is observed wherever there are related, related similarities. S. manifests itself in the appearance of regular patterns. The pattern elements can be conceptually mapped onto one another by so-called cover operations. Most organisms show suspicious S. The systemic character of all organisms is reflected in the mutual dependence of the pattern elements.

Originally there were three basic forms of S in the theory of symmetry: metamerism, radial and mirror symmetry. In the Metamerism (segmentation) identical / similar structural elements are lined up along a line, always in the same orientation and at the same intervals. In the simplest case, this line is a straight line, and all structural elements are not only designed the same, but also have the same size (homonomous segmentation). Continuous enlargement or reduction of the metamers and / or their distances as well as their differentiation lead to heteronomous segmentation. For example, in the case of the branches of bushes and trees, the otherwise mostly homonomous metamerism of the shoot axes (nodes / internodes) is usually transformed into heteronomous metamerism by the different degrees of expulsion of axillary buds. Another example are the protofilaments (F-actin, protofilaments of the microtubules, etc.) formed by the metameric stringing together of globular protein units. The curvature of the metamerism line in one plane, together with heteronomous segmentation, leads to spiral patterns, as can be observed, for example, in the shells of molluscs.

Radial- or Rotational symmetry (Ray symmetry, Actinomorphism) is given if symmetry elements can be brought into congruence by rotating around an axis of symmetry. The most common examples of radially symmetrical biostructures are provided by leaf whorls and flowers. The fruiting bodies and mycelia of many mushrooms are also radial symmetry. This form of symmetry is rare in the animal kingdom; it is limited here above all to sessile or slowly moving forms (e.g. corals; thecaoeba; sea urchins and starfish) or to planktonic species (e.g. radiolaria, heliozoa, jellyfish).

The third of the classic forms of symmetry is that Bilateral- or Mirror symmetry. The cover operation of bilateral symmetry is mirroring, the number of symmetry elements is 2. Bilateral symmetry is predominant in the animal kingdom; more than 95% of the animal species belong to the Bilateria. The mirror symmetry is almost always paired with Dorsoventrality, i.e. different shaping of a top and bottom. Mirror symmetry also largely determines the shape of the human body. For many plant families zygomorphic flowers are typical (orchids; violets, labiates and pharynx, etc.). Leaf organs are almost always bilaterally symmetrical. Many biomolecules have enantiomorphic forms (enantiomers), of which only one is usually used in metabolism (e.g. α-amino acids, D-glucose).

Repetitions of processes along the time axis (temporal p.) are known as rhythms. Such temporal metameries can easily be represented as spatial (animal tracks; spider webs; segmentation as a result of developmental rhythms). Many biological processes are rhythmic, at the same time coordinated with the rhythms of the environment (times of day and seasons, phases of the moon and tides) or regulated by them (biorhythmics, internal clock). A temporal metamerism of fundamental importance for all living beings is the succession of generations. The cyclical return to the simplest initial situation (fertilized egg cell, spore, brood bud, etc.) means that every stage of development in the reproductive (life) cycle is both a consequence and a cause of the initial constellation.

The peculiarities of living systems mean that they have symmetry and pattern forms that mineralogy / crystallography lacks. Additional symmetry as well as stochastic and functional S are important. With these symmetry forms, the condition of the similarity of pattern elements or the equality of their arrangement is partially or completely abandoned. The S. expresses itself here in the fact that the existence and orientation of one or more additional elements can be postulated from the existence of pattern elements. This underlines the systemic character of the organisms. Under Complementary symmetry (Antisymmetry) is to be understood as the regular assignment of dissimilar but complementary units. In organisms, antisymmetric structures are often used for recognition and / or reproduction (enzyme / substrate; receptor / ligand; antigen / antibody). From the macroscopic area, the structure of the corresponding mating organs in male and female animals (lock-and-key principle) or the joints of vertebrates should be mentioned. Supermolecular biostructures are generally similar, but not identically developed (e.g. the leaves of a tree, the cells of a ciliated epithelium or the mitochondria of a single cell). The fluctuations and inequalities result from the fact that the new formation of such pattern elements does not follow a rigid organizational scheme, but results from the interlocking of regulatory processes with corresponding statistical fluctuations. The finished sample elements are variable within limits: stochastic (statistical) S. Instead of real congruence, there is the similarity and the morphological / physiological equivalence of the pattern elements. Functional S. is not morphologically comprehensible; it is a non-illustrative, but particularly typical form of the S. for living (and technical) systems, based on chains and networks of functional antisymmetries. The more elements involved, the more complex the performance of a system can be, but at the same time the lower the morphological S. low degree of symmetry of cell structures, which at the same time have the highest functional S.