Radial symmetry
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Symmetry in biology refers to the symmetry observed in
organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multice ...

organism
s, including plants, animals,
fungi A fungus (plural: fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as ...

fungi
, and
bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a Bacte ...

bacteria
. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a plane of symmetry down its centre, or a pine cone with a clear symmetrical spiral pattern. Internal features can also show symmetry, for example the tubes in the human body (responsible for transporting
gas Gas is one of the four fundamental states of matter (the others being solid, liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an appl ...

gas
es,
nutrients A nutrient is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical composition * Matter, anything that has mass and ta ...
, and waste products) which are
cylindrical A cylinder (from ) has traditionally been a Solid geometry, three-dimensional solid, one of the most basic of curvilinear geometric shapes. Geometrically, it can be considered as a Prism (geometry), prism with a circle as its base. This traditio ...
and have several planes of symmetry. Biological symmetry can be thought of as a balanced distribution of duplicate body parts or shapes within the body of an organism. Importantly, unlike in mathematics, symmetry in biology is always approximate. For example, plant leaves – while considered symmetrical – rarely match up exactly when folded in half. Symmetry is one class of
patterns in nature Patterns of the veiled chameleon, ''Chamaeleo calyptratus'', provide camouflage and signal aposematism, mood as well as sexual selection, breeding condition. Patterns in nature are visible regularities of form found in the natural world. These ...
whereby there is near-repetition of the pattern element, either by
reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal r ...
or
rotation A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation plane'', and the imaginary line extending from the center an ...
. While
sponges Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the Diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

sponges
and
placozoans The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual ...
represent two groups of animals which do not show any symmetry (i.e. are asymmetrical), the
body plan A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to many members of a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ) ...
s of most
multicellular organism A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to a unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few ...
s exhibit, and are defined by, some form of symmetry. There are only a few types of symmetry which are possible in body plans. These are
radial Radial is a geometric term of location which may refer to: Mathematics and Direction * Vector (geometric), a line * Radius, adjective form of * Radial distance, a directional coordinate in a polar coordinate system * Radial set * A bearing from ...
(cylindrical),
bilateral Bilateral may refer to any concept including two sides, in particular: *Bilateria, bilateral animals *Bilateralism, the political and cultural relations between two states *Bilateral, occurring on both sides of an organism (Anatomical terms of loc ...

bilateral
, biradial and
spherical of a sphere A sphere (from Greek language, Greek —, "globe, ball") is a geometrical object in three-dimensional space Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values ...
symmetry. While the classification of
viruses A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecul ...
as an "organism" remains controversial, viruses also contain
icosahedral symmetry A regular icosahedron has 60 rotational (or orientation-preserving) symmetries, and a symmetry order of 120 including transformations that combine a reflection and a rotation. A regular dodecahedron A regular dodecahedron or pentagonal dodec ...
. The importance of symmetry is illustrated by the fact that groups of animals have traditionally been defined by this feature in
taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only about biological ...
groupings. The
Radiata Radiata or Radiates is a historical taxonomic rank that was used to classify animals with Symmetry (biology)#Radial symmetry, radially symmetric body plans. The term Radiata is no longer accepted, as it united several different groupings of anim ...
, animals with radial symmetry, formed one of the four branches of
Georges Cuvier Jean Léopold Nicolas Frédéric, Baron Cuvier (; 23 August 1769 – 13 May 1832), known as Georges Cuvier, was a French naturalist Natural history is a domain of inquiry involving organisms, including animals, fungus, fungi, and plants, in ...

Georges Cuvier
's classification of the
animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...

animal
kingdom Kingdom may refer to: Monarchy * A type of monarchy * A realm ruled by: **A king, during the reign of a male monarch **A queen regnant, during the reign of a female monarch Taxonomy * Kingdom (biology), a category in biological taxonomy Arts an ...
. Meanwhile,
Bilateria The Bilateria or bilaterians are animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material ...
is a taxonomic grouping still used today to represent organisms with
embryo An embryo is the early stage of development of a multicellular organism. In general, in organisms that Sexual reproduction, reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization and continue ...

embryo
nic bilateral symmetry.


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 repeating a body part 4, 5, 6 or 8 times around the axis – referred to as tetramerism, pentamerism, hexamerism and octomerism, respectively. Such organisms exhibits no left or right sides but do have a top and a bottom surface, or a front and a back. George Cuvier classified animals with radial symmetry in the taxon Radiata (''Zoophytes''), which is now generally accepted to be an assemblage of different animal phyla that do not share a single common ancestor (a
polyphyletic File:Monophyly, paraphyly, polyphyly.png, 300px, Cladogram of the primates, showing a monophyly (the simians, in yellow), a paraphyly (the prosimians, in cyan, including the red patch), and a polyphyly (the night-active primates, the lorises and th ...
group). Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth , to the center of the opposite (aboral) end. Animals in the phyla
Cnidaria Image:Sea nettles.jpg, Chrysaora fuscescens, Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in Fresh water, freshwater and Marine habitats, ...

Cnidaria
and
Echinoderm An echinoderm () is any member of the phylum Echinodermata (; ) of marine life, marine animals. The adults are recognizable by their (usually five-point) radial symmetry, and include starfish, sea urchins, sand dollars, and sea cucumbers, as wel ...
ata generally show radial symmetry, although many
sea anemone Sea anemones are the marine, predation, predatory animals of the order (biology), order Actiniaria. They are named after the anemone, a terrestrial flowering plant, because of the colourful appearance of many. Sea anemones are classified in the p ...

sea anemone
s and some
coral Corals are marine invertebrates Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vert ...

coral
s within the Cnidaria have bilateral symmetry defined by a single structure, the siphonoglyph. Radial symmetry is especially suitable for sessile animals such as the sea anemone, floating animals such as
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella-s ...

jellyfish
, and slow moving organisms such as
starfish Starfish or sea stars are star-shaped echinoderm An echinoderm is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values ...

starfish
; whereas bilateral symmetry favours
locomotion Locomotion means the act or ability of an entity or person to transport or move oneself from place to place. Locomotion or Loco-Motion may refer to: Motion * Motion (physics) *Specific types of motion ** Animal locomotion *** Terrestrial locomoti ...
by generating a
streamlined.'') Streamlines, streaklines and pathlines are field lines in a fluid flow In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural s ...
body. Many flowers are also radially symmetric, or "
actinomorphic Floral symmetry describes whether, and how, a flower A flower, sometimes known as a bloom or blossom, is the reproduction, reproductive structure found in flowering plants (plants of the division Magnoliophyta, also called angiosperms). T ...
". Roughly identical floral structures –
petal upright=1.4, Diagram showing the parts of a mature flower. In this example the perianth is separated into a calyx (sepals) and corolla (petals) Petals are modified leaves A leaf (plural leaves) is the principal lateral appendage of the ...

petal
s,
sepal upright=1.4, Diagram showing the parts of a mature flower. In this example the perianth is separated into a calyx (sepals) and corolla (petals) A sepal ( or ) is a part of the flower A flower, sometimes known as a bloom or blossom, is the ...
s, and
stamen The stamen (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full vers ...
s – occur at regular intervals around the axis of the flower, which is often the female
reproductive organ A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process by which new in ...
containing the
carpel Gynoecium (; ) is most commonly used as a collective term for the parts of a flower A flower, sometimes known as a bloom or blossom, is the reproduction, reproductive structure found in flowering plants (plants of the division Magnoliophyta, ...
,
style Style is a manner of doing or presenting things and may refer to: * Architectural style, the features that make a building or structure historically identifiable * Design, the process of creating something * Fashion, a prevailing mode of clothing s ...
and
stigma Stigma or plural stigmata, stigmas may refer to: * Social stigma, the disapproval of a person based on physical or behavioral characteristics that distinguish them from others Symbolism * Stigmata, bodily marks or wounds resembling the crucifix ...
.


Subtypes of radial symmetry

Some jellyfish, such as '' Aurelia 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 A gonad, sex gland, or reproductive gland is a mixed gland that produces the gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ...
, visible through its
translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions are ...
body. This radial symmetry is
ecologically Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Topics of interest include the biodiversity ...
important in allowing the jellyfish to detect and respond to
stimuli A stimulus is something that causes a physiological response. It may refer to: *Stimulation Stimulation is the encouragement of development or the cause of activity generally. For example, "The press provides stimulation of political discourse." ...
(mainly food and danger) from all directions.
Flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλά ...

Flowering plant
s show five-fold symmetry, or pentamerism, in many of their flowers and fruits. This is easily seen through the arrangement of five
carpel Gynoecium (; ) is most commonly used as a collective term for the parts of a flower A flower, sometimes known as a bloom or blossom, is the reproduction, reproductive structure found in flowering plants (plants of the division Magnoliophyta, ...
s (seed pockets) in an
apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple fruit tree, trees are agriculture, cultivated worldwide and are the most widely grown species in the genus ''Malus''. The tree originated in Central Asia, wher ...

apple
when cut transversely. Among animals, only the echinoderms such as
sea star Starfish or sea stars are star-shaped echinoderm An echinoderm is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values ...
s,
sea urchin Sea urchins () are spiny, globular echinoderm An echinoderm () is any member of the phylum Echinodermata (; ) of marine life, marine animals. The adults are recognizable by their (usually five-point) radial symmetry, and include starfish, s ...

sea urchin
s, and
sea lilies Crinoids are marine animals that make up the Class (biology), class Crinoidea, one of the classes of the phylum Echinodermata, which also includes the starfish, brittle stars, sea urchins and sea cucumbers. Those crinoids which, in their adult fo ...
are pentamerous as adults, with five arms arranged around the mouth. Being bilaterian animals, however, they initially develop with mirror symmetry as larvae, then gain pentaradial symmetry later. is found in the
coral Corals are marine invertebrates Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vert ...

coral
s and
sea anemones Sea anemones are the marine, predatory animals of the order Actiniaria. They are named after the anemone ''Anemone'' () is a genus of flowering plants in the buttercup family (biology), family Ranunculaceae. Plants of the genus are commonly ...
(class
Anthozoa Anthozoa is a class of marine invertebrates which includes the sea anemones, stony corals and soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plankton. The basic unit of th ...

Anthozoa
), which are divided into two groups based on their symmetry. The most common corals in the subclass
Hexacorallia Hexacorallia is a class of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyps, generally with 6-fold symmetry. It includes all of the stony coral Corals are marine invertebrates within the class (biology), ...
have a hexameric body plan; their polyps have six-fold internal symmetry and a number of
tentacle with 2 tentacles and 8 arms In zoology Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the animal kingdom, including the anatomy, structure, emb ...

tentacle
s that is a multiple of six. is found in corals of the subclass
Octocorallia Octocorallia (also known as Alcyonaria) is a subclass of Anthozoa comprising around 3,000 species of water-based organisms formed of colonial polyps with 8-fold symmetry. It includes the blue coral, soft corals, sea pens, and gorgonians (sea ...
. These have polyps with eight tentacles and octameric radial symmetry. The
octopus An octopus (pl. octopuses/octopi, see below for variants) is a soft-bodied, eight- limbed mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. The members are known as molluscs or mollusks (). Ar ...

octopus
, however, has bilateral symmetry, despite its eight arms. Triradial symmetry was present in
Trilobozoa Trilobozoa ("three-lobed animals") is a taxon In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular ...
from the Late
Ediacaran The Ediacaran Period ( ) is a geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of d ...
period.


Icosahedral symmetry

Icosahedral symmetry occurs in an organism which contains 60 subunits generated by 20 faces, each an
equilateral triangle In geometry, an equilateral triangle is a triangle in which all three sides have the same length. In the familiar Euclidean geometry, an equilateral triangle is also equiangular polygon, equiangular; that is, all three internal angles are also con ...

equilateral triangle
, and 12 corners. Within the
icosahedron In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non-similarity (geometry), similar shapes of icosahedra, some of them ...

icosahedron
there is 2-fold, 3-fold and 5-fold symmetry. Many viruses, including ''
canine parvovirus Canine parvovirus (also referred to as CPV, CPV2, or parvo) is a contagious virus mainly affecting dog The domestic dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated form of wolf. The dog descended from an ancie ...
'', show this form of symmetry due to the presence of an icosahedral viral shell. Such symmetry has
evolved Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are passed on from parent to offsp ...

evolved
because it allows the viral particle to be built up of repetitive subunits consisting of a limited number of structural
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabol ...

protein
s (encoded by viral
genes In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
), thereby saving space in the viral
genome In the fields of molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, ...

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). Although these viruses are often referred to as 'spherical', they do not show true mathematical spherical symmetry. In the early 20th century,
Ernst Haeckel Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biology that stu ...

Ernst Haeckel
described (Haeckel, 1904) a number of species of
Radiolaria The Radiolaria, also called Radiozoa, are protozoa Protozoa (also protozoan, plural protozoans) is an informal term for a group of single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microor ...

Radiolaria
, some of whose skeletons are shaped like various regular polyhedra. Examples include ''Circoporus octahedrus'', ''Circogonia icosahedra'', ''Lithocubus geometricus'' and ''Circorrhegma dodecahedra''. The shapes of these creatures should be obvious from their names. Tetrahedral symmetry is not present in ''Callimitra agnesae''.


Spherical symmetry

Spherical symmetry is characterised by the ability to draw an endless, or great but finite, number of symmetry axes through the body. This means that spherical symmetry occurs in an organism if it is able to be cut into two identical halves through any cut that runs through the organism's center. True spherical symmetry is not found in animal body plans. Organisms which show approximate spherical symmetry include the freshwater green alga ''
Volvox ''Volvox'' is a Polyphyly, polyphyletic genus of chlorophyte green algae in the family Volvocaceae. It forms spherical colony (biology), colonies of up to 50,000 cells. They live in a variety of freshwater habitats, and were first reported by Anto ...

Volvox
''. Bacteria are often referred to as having a 'spherical' shape. 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. 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 same situation is seen in the description of viruses – 'spherical' viruses do not necessarily show spherical symmetry, being usually icosahedral.


Bilateral symmetry

Organisms with bilateral symmetry contain a single plane of symmetry, the
sagittal plane In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any ind ...

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 of many
genes In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
. The bilateria have two axes of
polarity Polarity may refer to: Science *Polarity (mutual inductance), the relationship between components such as transformer windings *Polarity (projective geometry), in mathematics, a duality of order two *Polarity in embryogenesis, the animal and vegeta ...
. The first is an
anterior Standard anatomical terms of location deal unambiguously with the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient ...
- 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 Dorsal (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Repu ...
-
ventral Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Terms used generally derive from Latin or Greek language, Greek roots and used to describe something in its standard anatomical position. This ...

ventral
(DV) axis which runs
perpendicular In elementary geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with propertie ...

perpendicular
to the AP axis. During development the AP axis is always specified before the DV 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, a 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 Cephalization is an evolutionary trend in which, over many generations, the mouth, sense organs, and nerve ganglia become concentrated at the front end of an animal, producing a head region. This is associated with animal locomotion, movement and bi ...
. 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 Drag or The Drag may refer to: Places * Drag, Norway, a village in Tysfjord municipality, Nordland, Norway * ''Drág'', the Hungarian name for Dragu Commune in Sălaj County, Romania * Drag (Austin, Texas), the portion of Guadalupe Street adja ...
. In addition to animals, the flowers of some plants also show bilateral symmetry. Such plants are referred to as
zygomorphic Floral symmetry describes whether, and how, a flower A flower, sometimes known as a bloom or blossom, is the reproduction, reproductive structure found in flowering plants (plants of the division Magnoliophyta, also called angiosperms). T ...
and include the orchid (''
Orchidaceae Orchidaceae ( ), common name, commonly called the orchid family, is a diverse and widespread family (biology), family of flowering plants, with blooms that are often colourful and fragrant. Along with the Asteraceae, they are one of the two lar ...

Orchidaceae
'') and pea (''
Fabaceae The Fabaceae or Leguminosae,International Code of Nomencl ...

Fabaceae
'') families, and most of the figwort family (''
Scrophulariaceae The Scrophulariaceae are a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maint ...
''). The leaves of plants also commonly show approximate bilateral symmetry.


Biradial symmetry

Biradial symmetry is found in organisms which show morphological features (internal or external) of both bilateral and radial symmetry. Unlike radially symmetrical organisms which can be divided equally along many planes, biradial organisms can only be cut equally along two planes. This could represent an intermediate stage in the evolution of bilateral symmetry from a radially symmetric ancestor. The animal group with the most obvious biradial symmetry is the
ctenophores Ctenophora (; ctenophore ; ) comprise a phylum of marine life, marine invertebrates, commonly known as comb jellies, that marine habitats, inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly r ...
. In ctenophores the two planes of symmetry are (1) the plane of the tentacles and (2) the plane of the pharynx. In addition to this group, evidence for biradial symmetry has even been found in the 'perfectly radial' freshwater polyp '' Hydra'' (a cnidarian). Biradial symmetry, especially when considering both internal and external features, is more common than originally accounted for.


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 Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable Phenotypic trait, traits characteristic of a populatio ...
. This involves changes in the
frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparent ...
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 of ''CYCLOIDEA'' genes. Evidence for the role of the ''CYCLOIDEA'' gene family comes from
mutations Image:Darwin Hybrid Tulip Mutation 2014-05-01.jpg, A red tulip exhibiting a partially yellow petal due to a mutation in its genes In biology, a mutation is an alteration in the base sequence, nucleotide sequence of the genome of an organism, vir ...

mutations
in these genes which cause a reversion to radial symmetry. The ''CYCLOIDEA'' genes encode
transcription factors In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), r ...
, proteins which control the expression of other genes. This allows their expression to influence developmental pathways relating to symmetry. For example, in ''
Antirrhinum majus ''Antirrhinum majus'', the common snapdragon (often - especially in horticulture - simply "snapdragon"), is a species of flowering plant belonging to the genus ''Antirrhinum''. The plant was placed in the family Plantaginaceae following a revisi ...

Antirrhinum majus
'', ''CYCLOIDEA'' is expressed during early development in the dorsal domain of the flower
meristem The meristem is a type of tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dub ...
and continues 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

Symmetry is often selected for in the evolution of animals. This is unsurprising since asymmetry is often an indication of unfitness – either defects during development or injuries throughout a lifetime. This is most apparent during mating during which females of some species select males with highly symmetrical features. For example,
facial symmetry Facial symmetry is one specific measure of bodily symmetry Symmetry (from Greek συμμετρία ''symmetria'' "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful pro ...
influences human judgements of human attractiveness.Grammer, K.; Thornhill, R. (1994). Human (Homo sapiens) facial attractiveness and sexual selection: the role of symmetry and averageness. Journal of comparative psychology (Washington, D.C. : 1983), 108(3), 233–42. Additionally, female
barn swallow The barn swallow (''Hirundo rustica'') is the most widespread species of swallow The swallows, martins, and saw-wings, or Hirundinidae, are a family of passerine bird Birds are a group of warm-blooded vertebrates constituting the c ...

barn swallow
s, a species where adults have long tail streamers, prefer to mate with males that have the most symmetrical tails. While symmetry is known to be under selection, the evolutionary history of different types of symmetry in animals is an area of extensive debate. Traditionally it has been suggested that bilateral animals evolved from a radial
ancestor An ancestor, also known as a forefather, fore-elder or a forebear, is a parent A parent is a caregiver of the offspring In biology, offspring are the young born of living organism, organisms, produced either by a single organism or, in the ...

ancestor
.
Cnidarians Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...
, a phylum containing animals with radial symmetry, are the most closely related group to the bilaterians. Cnidarians are one of two groups of early animals considered to have defined structure, the second being the
ctenophores Ctenophora (; ctenophore ; ) comprise a phylum of marine life, marine invertebrates, commonly known as comb jellies, that marine habitats, inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly r ...
. Ctenophores show biradial symmetry leading to the suggestion that they represent an intermediate step in the evolution of bilateral symmetry from radial symmetry. Interpretations based only on morphology are not sufficient to explain the evolution of symmetry. Two different explanations are proposed for the different symmetries in cnidarians and bilateria. The first suggestion is that an ancestral animal had no symmetry (was asymmetric) before cnidarians and bilaterians separated into different evolutionary lineages. Radial symmetry could have then evolved in cnidarians and bilateral symmetry in bilaterians. Alternatively, the second suggestion is that an ancestor of cnidarians and bilaterians had bilateral symmetry before the cnidarians evolved and became different by having radial symmetry. Both potential explanations are being explored and evidence continues to fuel the debate.


Asymmetry

Although asymmetry is typically associated with being unfit, some species have evolved to be asymmetrical as an important
adaptation In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...

adaptation
. Many members of the phylum Porifera (sponges) have no symmetry, though some are radially symmetric. File:Fi Kreuzschnabel m Kopf.jpg, Head of a male
crossbill The crossbill is a genus, ''Loxia'', of birds in the finch family ( Fringillidae), with six species. These birds are characterised by the mandibles with crossed tips, which gives the group its English name. Adult males tend to be red or orange i ...
showing asymmetrical upper and lower beak File:Pseudopleuronectes americanus.jpg, A
winter flounder The winter flounder (''Pseudopleuronectes americanus''), also known as the black back, is a right-eyed (" dextral") flatfish A flatfish is a member of the ray-finned demersal fish order Pleuronectiformes, also called the Heterosomata, someti ...

winter flounder
, a type of flatfish, with both eyes on the same side of its head File:Diogenes pugilator.jpg, Hermit crabs have different sized claws File:Grapevinesnail 01a.jpg, A and its helical shell File:Chicoreus palmarosae.jpg, ''
Chicoreus palmarosae ''Chicoreus palmarosae'' (rose-branch murex) is a species of predatory sea snail, a marine (ocean), marine gastropod mollusk in the family Muricidae, the murex snails. Description The size of an adult shell varies between 65 mm and 130  ...

Chicoreus palmarosae
'', a sea snail, illustrating asymmetry, which is seen in all
gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived fr ...
in the form of a helical shell File:Orange slug.jpg, A , clearly showing the
pneumostome The pneumostome of ''Triboniophorus graeffei'' is on its dorsal surface. The pneumostome or breathing pore is a respiratory opening of the external body anatomy of an air-breathing land slug or land snail. It is a part of the respiratory system ...

pneumostome
File:Caribou (PSF).jpg, alt=Illustration of adult caribou in profile., Male
caribou The reindeer (''Rangifer tarandus''), also known as the caribou in North America, is a species of deer with circumpolar distribution, native to Arctic, subarctic, tundra, boreal, and mountainous regions of northern Europe, Siberia, and North ...

caribou
usually possess one brow tine flattened into a shovel shape


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
embryo An embryo is the early stage of development of a multicellular organism. In general, in organisms that Sexual reproduction, reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization and continue ...

embryo
s 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 Nodal is a secretory protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including E ...
'' 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 (biology), 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.


See also


Biological structures

* Standard anatomical position * Anatomical terms of motion * Anatomical terms of muscle * Anatomical terms of bone * Anatomical terms of neuroanatomy * Glossary of botanical terms * Glossary of plant morphology * Glossary of leaf morphology * Glossary of entomology terms * Plant morphology


Terms of orientation

* Handedness * Laterality * Proper right and proper left * Reflection symmetry * Sinistral and dextral * Direction (disambiguation) * Symmetry (disambiguation)


References


Citations


Sources

* Philip Ball, Ball, Philip (2009). ''Shapes''. Oxford University Press. * Ian Stewart (mathematician), Stewart, Ian (2007). ''What Shape is a Snowflake? Magical Numbers in Nature''. Weidenfeld and Nicolson. * D'Arcy Wentworth Thompson, Thompson, D'Arcy (1942). ''On Growth and Form''. Cambridge University Press. *Ernst Haeckel, Haeckel, Ernst, E. (1904). ''Kunstformen der Natur''. Available as Haeckel, E. (1998); ''Art forms in nature'', Prestel USA. . {{DEFAULTSORT:Symmetry In Biology Symmetry Developmental biology Animal anatomy Evolutionary biology pt:Simetria#Simetria na biologia