Symmetry in biology

For example, the face of a human being has a plane of symmetry down its centre, or a pine cone displays a clear symmetrical spiral pattern.

Typically, this involves repeating a body part 4, 5, 6 or 8 times around the axis – referred to as tetramerism, pentamerism, hexamerism and octamerism, respectively.

Roughly identical floral structures – petals, sepals, and stamens – occur at regular intervals around the axis of the flower, which is often the female reproductive organ containing the carpel, style and stigma.

This radial symmetry is ecologically important in allowing the jellyfish to detect and respond to stimuli (mainly food and danger) from all directions.

[10] Hexamerism is found in the corals and sea anemones (class Anthozoa), which are divided into two groups based on their symmetry.

The most common corals in the subclass Hexacorallia have a hexameric body plan; their polyps have six-fold internal symmetry and a number of tentacles that is a multiple of six.

Icosahedral symmetry occurs in an organism which contains 60 subunits generated by 20 faces, each an equilateral triangle, and 12 corners.

Many viruses, including canine parvovirus, show this form of symmetry due to the presence of an icosahedral viral shell.

Examples include Circoporus octahedrus, Circogonia icosahedra, Lithocubus geometricus and Circorrhegma dodecahedra.

[1] Organisms which show approximate spherical symmetry include the freshwater green alga Volvox.

Bacteria are categorized based on their shapes into three classes: cocci (spherical-shaped), bacillus (rod-shaped) and spirochetes (spiral-shaped) cells.

In reality, this is a severe over-simplification as bacterial cells can be curved, bent, flattened, oblong spheroids and many more shapes.

[13] Due to the huge number of bacteria considered to be cocci (coccus if a single cell), it is unlikely that all of these show true spherical symmetry.

It is important to distinguish between the generalized use of the word 'spherical' to describe organisms at ease, and the true meaning of spherical symmetry.

The AP axis is essential in defining the polarity of bilateria and allowing the development of a front and back to give the organism direction.

The front end encounters the environment before the rest of the body so sensory organs such as eyes tend to be clustered there.

Therefore, a distinct head, with sense organs connected to a central nervous system, tends to develop.

It is also argued that the development of an AP axis is important in locomotion – bilateral symmetry gives the body an intrinsic direction and allows streamlining to reduce drag.

[1] In addition to this group, evidence for biradial symmetry has even been found in the 'perfectly radial' freshwater polyp Hydra (a cnidarian).

The first suggestion is that an ancestral animal had no symmetry (was asymmetric) before cnidarians and bilaterians separated into different evolutionary lineages.

[28] The presence of these asymmetrical features requires a process of symmetry breaking during development, both in plants and animals.

Symmetry breaking occurs at several different levels in order to generate the anatomical asymmetry which we observe.

In a region of the embryo referred to as the node there are small hair-like structures (monocilia) that all rotate together in a particular direction.

For example the direction of helical growth in Arabidopsis, the most commonly studied model plant, shows left-handedness.

In the same way as animals, symmetry breaking in plants can occur at a molecular (genes/proteins), subcellular, cellular, tissue and organ level.

A selection of animals showing a range of possible body symmetries, including asymmetry, radial, and bilateral body plans
Illustration depicting the difference between bilateral ( Drosophila ), radial ( actinomorphic flowers) and spherical ( coccus bacteria) symmetry
Lilium bulbiferum displays hexamerism with repeated parts arranged around the axis of the flower.
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Apple cut horizontally showing that pentamerism also occurs in fruit
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Gastroenteritis viruses have icosahedral symmetry.
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Volvox is a microscopic green freshwater alga with spherical symmetry. Young colonies can be seen inside the larger ones.
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The small emperor moth, Saturnia pavonia , displays a deimatic pattern with bilateral symmetry.
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Flower of bee orchid ( Ophrys apifera ) is bilaterally symmetrical ( zygomorphic ). The lip of the flower resembles the (bilaterally symmetric) abdomen of a female bee; pollination occurs when a male bee attempts to mate with it.
The Ediacaran phylum Trilobozoa possess a wide variety of body shapes, mostly tri-radial symmetry, although their most famous member, Tribrachidium , possesses a triskelion body shape. [ 25 ]
Schematic diagram of signalling pathways on the left and right side of a chick embryo, ultimately leading to the development of asymmetry
Bilateral features in the face and body, such as left and right eyes, ears, lips, wrists and thighs, often show some extent of fluctuating asymmetry. Some individuals show greater asymmetry than others.