User:Charl Hutchings/sandbox

(while bilateral symmetry favours locomotion by generating a streamlined body) radial (rotational) --- in intro

change definition of radial symmetry --> Organisms with radial symmetry show a repeating pattern around a central axis such that they can be separated into several identical pieces when cut through the central point, much like pieces of a pie. Typically this involves a repeating body part 4, 5, 6 or 8 times around the axis - referred to as tetramerism, pentamerism, hexamerism and octomerism, respectively.

Some jellyfish, such as Aureila marginalis, show tetramerism with a four-fold radial symmetry. This is immediately obvious when looking at the jellyfish due to the presence of four gonads visible through its translucent body. This radial symmetry is ecologically important in allowing the jellyfish to detect and respond to stimuli (mainly food and danger) from all directions.

Icosahedral symmetry
Icosahedral symmetry occurs in an organism which contains 60 subunits generated by 20 faces, each an equilateral triangle, and 12 corners. Within the icosahedron there is 2-fold, 3-fold and 5-fold symmetry. Many viruses, including canine parvovirus show this form of radial symmetry due to the presence of an icosahedral viral shell. Such symmetry has evolved because it allows the viral particle to be built up of repetitive subunits consisting of a limited number of structural proteins (encoded by viral genes), thereby saving space in the viral genome. The icosahedral symmetry can still be maintained with more than 60 subunits, but only in factors of 60. For example, the T=3 Tomato bushy stunt virus has 60x3 protein subunits (180 copies of the same structural protein).

Evolution of symmetry
Like all the traits of organisms, symmetry (or indeed asymmetry) evolves due to an advantage to the organism - a process of natural selection. This involves changes in the presence of symmetry-related genes throughout time.

Evolution of symmetry in plants
Early flowering plants had radially symmetric flowers but since then many plants have evolved bilaterally symmetrical flowers. The evolution of bilateral symmetry is due to the expression of CYCLOIDEA genes. Evidence for the role of the CYCLOIDEA gene family comes from mutations in these genes which cause a reversion to radial symmetry. The CYCLOIDEA genes encode transcription factors, proteins which control the expression of other genes. This allows their expression to influence developmental pathways relating to symmetry. For example, in Antirrhinum majus CYCLOIDEA is expressed during early development in the dorsal domain of the flower meristem and continue to be expressed later on in the dorsal petals to control their size and shape. It is believed that the evolution of specialized pollinators may play a part in the transition of radially symmetrical flowers to bilaterally symmetrical flowers.

Evolution of symmetry in animals
The two earliest groups of animals which are considered to have a defined structure and the cniderians and ctenophores. While both groups display biradial symmetry in some cases, it is generally thought that ctenophores are biradial and the majority of cnidarians are radial leading to the argument that the ctenophores represent the intermediate step in the evolution of bilateral symmetry. However, in other cases it is thought that biradial symmetry evolved independently in ctenophora while cnidarians are the closest relatives to the bilaterial animals.

Females of some species select for symmetry, presumed by biologists to be a mark (technically a "cue") of fitness. Facial symmetry influences human judgements of human attractiveness. Female barn swallows, a species where adults have long tail streamers, prefer to mate with males that have the most symmetrical tails.

Biradial symmetry
Biradial symmetry is found in organisms which show morphological features (internal or external) of both bilateral and radial symmetry. This could represent an intermediate stage in the evolution of bilateral symmetry from a radially symmetric ancestor **keep their reference.

The animal group with the most obvious biradial symmetry is the ctenophores. In addition to this group, evidence for biradial symmetry has even been found in the 'perfectly radial'freshwater polyp Hydra (a cnidarian).

Symmetry breaking
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. These levels include asymmetric gene expression, protein expression, and activity of cells.

For example, left-right asymmetry in mammals has been investigated extensively in the embryos of mice. Such studies have led to support for the nodal flow hypothesis. In a region of the embryo referred to as the node there are small hair-like structures (monocilia) which all rotate together in a particular direction. This creates a unidirectional flow of signalling molecules causing these signals to accumulate on one side of the embryo and not the other. This results in the activation of different developmental pathways on each side, and subsequent asymmetry.

Much of the investigation of the genetic basis of symmetry breaking has been done on chick embryos. In chick embryos the left side expresses genes called NODAL and LEFTY2 which activate PITX2 to signal the development of left side structures. Whereas, the right side does not express pitx2 and consequently develops right side structures. A more complete pathway is shown in the image at the side of the page.

For more information about symmetry breaking in animals please refer to the left-right asymmetry page.

Plants also show asymmetry. For example the direction of helical growth in Arabidopsis, the most commonly studied model plant, shows left-handedness. Interestingly, the genes involved in this asymmetry are similar (closely related) to those in animal asymmetry - both LEFTY1 and LEFTY2 play a role. In the same way as animals, symmetry breaking in plants can occur at a molecular (genes/proteins), subcellular, cellular, tissue and organ level.

Bilateral symmetry
Organisms with bilateral symmetry contain a single plane of symmetry, the sagittal plane, which divides the organism into two roughly mirror image left and right halves - approximate reflectional symmetry.

Animals with bilateral symmetry are classified into a large group called the bilateria which contains 99% of all animals (comprising over 32 phyla and 1 million described species). All bilaterians have some asymmetrical features, for example the human heart and liver are positioned asymmetrically despite the body having external bilateral symmetry.

The bilateral symmetry of bilaterians is a complex trait which develops due to the expression of many genes. The bilateria have two axes of polarity. The first is an anterior-posterior (AP) axis which can be visualised as an imaginary axis running from the head or mouth to the tail or other end of an organism. The second is the dorsal-ventral (DV) axis which runs perpendicular to the AP axis.

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. This is also the site where a mouth develops since it is the first part of the body to encounter food. Therefore, distinct head, with sense organs connected to a central nervous system, tends to develop. This pattern of development (with a distinct head and tail) is called cephalization. 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.

In addition to animals, the flowers of some plants also show bilateral symmetry. Such plants are referred to as zygomorphic and include the orchid (Orchidaceae) and pea (Fabaceae) families, and also most of the figwort family (Scrophulariaceae).

Although this is the standard explanation for the evolution of bilateral symmetry, evidence indicating that bilateral symmetry evolved in a sessile (non-mobile) organism has led to some dispute. For example, within the Cnidaria there are species with radial, biradial, and bilateral symmetry, but those with bilateral symmetry tend to be sessile. In this case it has been suggested that rather than the AP and DV axes evolving for locomotive purposes, instead bilateral symmetry may have evolved